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144-455: Lunar distance is commonly used to express the distance to near-Earth object encounters. Lunar semi-major axis is an important astronomical datum; the few-millimeter precision of the range measurements determines semi-major axis to a few decimeters; it has implications for testing gravitational theories such as general relativity and and for refining other astronomical values, such as the mass , radius , and rotation of Earth. The measurement

288-504: A binary planet , its centre of gravity is within Earth, about 4,671 km (2,902 miles) or 73.3% of the Earth's radius from the centre of the Earth. This centre of gravity remains on the line between the centres of the Earth and Moon as the Earth completes its diurnal rotation. The path of the Earth–Moon system in its solar orbit is defined as the movement of this mutual centre of gravity around

432-416: A diurnal libration , which allows one to view an additional one degree's worth of lunar longitude. For the same reason, observers at both of Earth's geographical poles would be able to see one additional degree's worth of libration in latitude. Besides these "optical librations" caused by the change in perspective for an observer on Earth, there are also "physical librations" which are actual nutations of

576-495: A 1.6% chance of Earth impact in April 2029. As observations were collected over the next three days, the calculated chance of impact increased to as high as 2.7%, then fell back to zero, as the uncertainty zone for this close approach no longer included the Earth. There was still some uncertainty about potential impacts during later close approaches, however, as the precision of orbital calculations improved due to additional observations,

720-522: A global catastrophe, was met by 2011. In later years, the survey effort was expanded to include smaller objects which have the potential for large-scale, though not global, damage. NEOs have low surface gravity, and many have Earth-like orbits that make them easy targets for spacecraft. As of April 2024 , five near-Earth comets and six near-Earth asteroids, one of them with a moon, have been visited by spacecraft. Samples of three have been returned to Earth, and one successful deflection test

864-439: A lunar eclipse, the lunar distance can be calculated using trigonometry . The earliest accounts of attempts to measure the lunar distance using this technique were by Greek astronomer and mathematician Aristarchus of Samos in the 4th century BC and later by Hipparchus , whose calculations produced a result of 59–67  R 🜨 ( 376 000 –427 000  km or 233 000 –265 000  mi ). This method later found its way into

1008-484: A maximal apogee, separated by two weeks, and a maximal perigee and a minimal apogee, also separated by two weeks. The distance to the Moon can be measured to an accuracy of 2 mm over a 1-hour sampling period, which results in an overall uncertainty of a decimeter for the semi-major axis. However, due to its elliptical orbit with varying eccentricity, the instantaneous distance varies with monthly periodicity. Furthermore,

1152-540: A network of infrasound sensors designed to detect the detonation of nuclear devices. Asteroid impact prediction remains in its infancy and successfully predicted asteroid impacts are rare. The vast majority of impacts recorded by IMS are not predicted. Observed impacts aren't restricted to the surface and atmosphere of Earth. Dust-sized NEOs have impacted man-made spacecraft, including the space probe Long Duration Exposure Facility , which collected interplanetary dust in low Earth orbit for six years from 1984. Impacts on

1296-582: A new crater 40 m (130 ft) across, was the largest ever observed as of July 2019 . Through human history, the risk that any near-Earth object poses has been viewed having regard to both the culture and the technology of human society . Through history, humans have associated NEOs with changing risks, based on religious, philosophical or scientific views, as well as humanity's technological or economical capability to deal with such risks. Thus, NEOs have been seen as omens of natural disasters or wars; harmless spectacles in an unchanging universe;

1440-402: A perspective effect which allows us to see up to eight degrees of longitude of its eastern (right) far side . Conversely, when the Moon reaches its apogee, its orbital motion is slower than its rotation, revealing eight degrees of longitude of its western (left) far side. This is referred to as optical libration in longitude . The Moon's axis of rotation is inclined by in total 6.7° relative to

1584-642: A plan to deflect the asteroid with rockets in case it was found to be on a collision course with Earth. Project Icarus received wide media coverage, and inspired the 1979 disaster movie Meteor , in which the US and the USSR join forces to blow up an Earth-bound fragment of an asteroid hit by a comet. The first astronomical program dedicated to the discovery of near-Earth asteroids was the Palomar Planet-Crossing Asteroid Survey . The link to impact hazard,

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1728-451: A potential 2028 close approach 0.00031 AU (46,000 km) from the Earth, well within the orbit of the Moon, but with a large error margin allowing for a direct hit. Further data allowed a revision of the 2028 approach distance to 0.0064 AU (960,000 km), with no chance of collision. By that time, inaccurate reports of a potential impact had caused a media storm. In 1998, the movies Deep Impact and Armageddon popularised

1872-399: A re-accumulation of fragments at an initial distance of 3.8  R 🜨 (24,000 km or 15,000 mi). This theory assumes the initial impact to have occurred 4.5 billion years ago. Until the late 1950s all measurements of lunar distance were based on optical angular measurements : the earliest accurate measurement was by Hipparchus in the 2nd century BC. The space age marked

2016-523: A restriction that applies to comets in particular, but this approach is not universal. Some authors further restrict the definition to orbits that are at least partly further than 0.983 AU away from the Sun. NEOs are thus not necessarily currently near the Earth, but they can potentially approach the Earth relatively closely. Many NEOs have complex orbits due to constant perturbation by the Earth's gravity, and some of them can temporarily change from an orbit around

2160-411: A result, the ratio of the known and the estimated total number of near-Earth asteroids larger than 1 km in diameter rose from about 20% in 1998 to 65% in 2004, 80% in 2006, and 93% in 2011. The original Spaceguard goal has thus been met, only three years late. As of March 2024 , 861 NEAs larger than 1 km have been discovered. In 2005, the original USA Spaceguard mandate was extended by

2304-496: A semi-major axis of 384 402 ± 1.2 km (238,856 ± 0.75 mi), which was the most precise measurement of the lunar distance at the time. An experiment which measured the round-trip time of flight of laser pulses reflected directly off the surface of the Moon was performed in 1962, by a team from Massachusetts Institute of Technology , and a Soviet team at the Crimean Astrophysical Observatory . During

2448-516: A space mission to avert the threat. REP. STEWART: ... are we technologically capable of launching something that could intercept [an asteroid]? ... DR. A'HEARN: No. If we had spacecraft plans on the books already, that would take a year ... I mean a typical small mission ... takes four years from approval to start to launch ... The ATLAS project, by contrast, aims to find impacting asteroids shortly before impact, much too late for deflection maneuvers but still in time to evacuate and otherwise prepare

2592-545: A theory that Noah's flood in the Bible was caused by a comet impact. Human perception of near-Earth asteroids as benign objects of fascination or killer objects with high risk to human society has ebbed and flowed during the short time that NEAs have been scientifically observed. The 1937 close approach of Hermes and the 1968 close approach of Icarus first raised impact concerns among scientists. Icarus earned significant public attention due to alarmist news reports. while Hermes

2736-410: A turning point when the precision of this value was much improved. During the 1950s and 1960s, there were experiments using radar, lasers, and spacecraft, conducted with the benefit of computer processing and modeling. Some historically significant or otherwise interesting methods of determining the lunar distance: The oldest method of determining the lunar distance involved measuring the angle between

2880-418: A well-defined geometric model of epicycles and evection . Isaac Newton was the first to develop a complete theory of motion, Newtonian mechanics . The observations of the lunar motion were the main test of his theory. There are several different periods associated with the lunar orbit. The sidereal month is the time it takes to make one complete orbit around Earth with respect to the fixed stars. It

3024-848: A workshop at Vulcano , Italy in 1995, and set up The Spaceguard Foundation also in Italy a year later. In 1998, the United States Congress gave NASA a mandate to detect 90% of near-earth asteroids over 1 km (0.62 mi) diameter (that threaten global devastation) by 2008. Several surveys have undertaken " Spaceguard " activities (an umbrella term), including Lincoln Near-Earth Asteroid Research (LINEAR), Spacewatch , Near-Earth Asteroid Tracking (NEAT), Lowell Observatory Near-Earth-Object Search (LONEOS), Catalina Sky Survey (CSS), Campo Imperatore Near-Earth Object Survey (CINEOS), Japanese Spaceguard Association , Asiago-DLR Asteroid Survey (ADAS) and Near-Earth Object WISE (NEOWISE). As

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3168-447: Is 383,397 km. The actual distance varies over the course of the orbit of the Moon . Values at closest approach ( perigee ) or at farthest ( apogee ) are rarer the more extreme they are. The graph at right shows the distribution of perigee and apogee over six thousand years. Jean Meeus gives the following extreme values for 1500 BC to AD 8000: The instantaneous lunar distance is constantly changing. The actual distance between

3312-446: Is about 27.32 days. The synodic month is the time it takes the Moon to reach the same visual phase . This varies notably throughout the year, but averages around 29.53 days. The synodic period is longer than the sidereal period because the Earth–Moon system moves in its orbit around the Sun during each sidereal month, hence a longer period is required to achieve a similar alignment of Earth,

3456-574: Is also believed that this anomalously high rate of recession may continue to accelerate. Theoretically, the lunar distance will continue to increase until the Earth and Moon become tidally locked , as are Pluto and Charon . This would occur when the duration of the lunar orbital period equals the rotational period of Earth, which is estimated to be 47 Earth days. The two bodies would then be at equilibrium, and no further rotational energy would be exchanged. However, models predict that 50 billion years would be required to achieve this configuration, which

3600-505: Is also useful in expressing the lunar radius , as well as the distance to the Sun . Millimeter- precision measurements of the lunar distance are made by measuring the time taken for laser beam light to travel between stations on Earth and retroreflectors placed on the Moon. The Moon is spiraling away from Earth at an average rate of 3.8 cm (1.5 in) per year, as detected by the Lunar Laser Ranging experiment . Because of

3744-473: Is any small Solar System body orbiting the Sun whose closest approach to the Sun ( perihelion ) is less than 1.3 times the Earth–Sun distance ( astronomical unit , AU). This definition applies to the object's orbit around the Sun, rather than its current position, thus an object with such an orbit is considered an NEO even at times when it is far from making a close approach of Earth . If an NEO's orbit crosses

3888-529: Is assessed at 1 in 34,000. The corresponding Palermo scale value of −2.05 is still the second highest for all objects on the Sentry List Table. On December 24, 2004, 370 m (1,210 ft) asteroid 99942 Apophis (at the time known only by its provisional designation 2004 MN 4 ) was assigned a 4 on the Torino scale, the highest rating given to date, as the information available at the time translated to

4032-466: Is called major lunar standstill . Around this time, the Moon's declination will vary from −28°36′ to +28°36′. Conversely, 9.3 years later, the angle between the Moon's orbit and Earth's equator reaches its minimum of 18°20′. This is called a minor lunar standstill . The last lunar standstill was a minor standstill in October 2015. At that time the descending node was lined up with the equinox (the point in

4176-442: Is closer to the ecliptic plane instead of its primary 's (in this case, Earth's) equatorial plane . The Moon's orbital plane is inclined by about 5.1° with respect to the ecliptic plane, whereas Earth's equatorial plane is tilted by about 23° with respect to the ecliptic plane . The properties of the orbit described in this section are approximations. The Moon's orbit around Earth has many variations ( perturbations ) due to

4320-478: Is detected, like all other small Solar System bodies, its positions and brightness are submitted to the (IAU's) Minor Planet Center (MPC) for cataloging. The MPC maintains separate lists of confirmed NEOs and potential NEOs. The MPC maintains a separate list for the potentially hazardous asteroids (PHAs). NEOs are also catalogued by two separate units of the Jet Propulsion Laboratory (JPL) of NASA :

4464-520: Is determined to be 384,399.0 km. Due to the modern accessibility of accurate timing devices, high resolution digital cameras, GPS receivers, powerful computers and near-instantaneous communication, it has become possible for amateur astronomers to make high accuracy measurements of the lunar distance. On May 23, 2007, digital photographs of the Moon during a near-occultation of Regulus were taken from two locations, in Greece and England. By measuring

Lunar distance - Misplaced Pages Continue

4608-465: Is distinct from the nodal precession of its orbital plane and axial precession of the moon itself. The mean inclination of the lunar orbit to the ecliptic plane is 5.145°. Theoretical considerations show that the present inclination relative to the ecliptic plane arose by tidal evolution from an earlier near-Earth orbit with a fairly constant inclination relative to Earth's equator. It would require an inclination of this earlier orbit of about 10° to

4752-403: Is known as lunar parallax . For increased accuracy, the measured angle can be adjusted to account for refraction and distortion of light passing through the atmosphere. Early attempts to measure the distance to the Moon exploited observations of a lunar eclipse combined with knowledge of Earth's radius and an understanding that the Sun is much further than the Moon. By observing the geometry of

4896-426: Is minor compared to the difference between lunar night and lunar day. At the lunar poles, instead of usual lunar days and nights of about 15 Earth days, the Sun will be "up" for 173 days as it will be "down"; polar sunrise and sunset takes 18 days each year. "Up" here means that the centre of the Sun is above the horizon. Lunar polar sunrises and sunsets occur around the time of eclipses (solar or lunar). For example, at

5040-485: Is nearly constant throughout the night, but an observer on the surface of Earth is actually 1 Earth radius from the center of Earth. This offset brings them closest to the Moon when it is overhead. Modern cameras have achieved a resolution capable of capturing the Moon with enough precision to detect and measure this tiny variation in apparent size. The results of this experiment were calculated as LD = 60.51 +3.91 −4.19   R 🜨 . The accepted value for that night

5184-426: Is not occurring fast enough for the rotation to lengthen to a month before other effects change the situation: approximately 2.3 billion years from now, the increase of the Sun's radiation will have caused Earth's oceans to evaporate, removing the bulk of the tidal friction and acceleration. The Moon is in synchronous rotation , meaning that it keeps the same face toward Earth at all times. This synchronous rotation

5328-418: Is now widely accepted that collisions in the past have had a significant role in shaping the geological and biological history of Earth. Asteroids as small as 20 metres (66 ft) in diameter can cause significant damage to the local environment and human populations. Larger asteroids penetrate the atmosphere to the surface of the Earth, producing craters if they impact a continent or tsunamis if they impact

5472-430: Is only true on average because the Moon's orbit has a definite eccentricity. As a result, the angular velocity of the Moon varies as it orbits Earth and hence is not always equal to the Moon's rotational velocity which is more constant. When the Moon is at its perigee, its orbital motion is faster than its rotation. At that time the Moon is a bit ahead in its orbit with respect to its rotation about its axis, and this creates

5616-514: Is significantly longer than the expected lifetime of the Solar System . Laser measurements show that the average lunar distance is increasing, which implies that the Moon was closer in the past, and that Earth's days were shorter. Fossil studies of mollusk shells from the Campanian era (80 million years ago) show that there were 372 days (of 23 h 33 min) per year during that time, which implies that

5760-494: Is slowly being transferred to the Moon's orbit. The result is that Earth's rate of spin is gradually decreasing (at a rate of 2.4 milliseconds/century ), and the lunar orbit is gradually expanding. The rate of recession is 3.830 ± 0.008 cm per year . However, it is believed that this rate has recently increased, as a rate of 3.8 cm/year would imply that the Moon is only 1.5 billion years old, whereas scientific consensus supports an age of about 4 billion years. It

5904-464: Is that in the past the Earth rotated much faster, a day possibly lasting only 9 hours on the early Earth. The resulting tidal waves in the ocean would have then been much shorter and it would have been more difficult for the long wavelength tidal forcing to excite the short wavelength tides. The Moon is gradually receding from Earth into a higher orbit, and calculations suggest that this would continue for about 50 billion years. By that time, Earth and

Lunar distance - Misplaced Pages Continue

6048-455: Is the mean anomaly (more or less how moon has moved from perigee) and D {\displaystyle D} is the mean elongation (more or less how far it has moved from conjunction with the Sun at new moon). They can be calculated from G M = 134.963 411 38° + 13.064 992 953 630°/d · t D = 297.850 204 20° + 12.190 749 117 502°/d · t where t is the time (in days) since January 1, 2000 (see Epoch (astronomy) ). This shows that

6192-464: The George E. Brown, Jr. Near-Earth Object Survey Act, which calls for NASA to detect 90% of NEOs with diameters of 140 m (460 ft) or greater, by 2020. In January 2020, it was estimated that less than half of these have been found, but objects of this size hit the earth only about once in 2000 years. In December 2023, the ratio of discovered NEOs with diameters of 140 m (460 ft) or greater

6336-692: The Nubian Desert in Sudan. It was the first time that an asteroid was observed and its impact was predicted prior to its entry into the atmosphere as a meteor . 10.7 kg of meteorites were recovered after the impact. As of September 2024 , nine impacts have been predicted, all of them small bodies that produced meteor explosions, with some impacts in remote areas only detected by the Comprehensive Nuclear-Test-Ban Treaty Organization 's International Monitoring System (IMS) ,

6480-468: The Rocky Mountains from the U.S. Southwest to Canada. It passed within 58 km (36 mi) of the Earth's surface. On October 13, 1990, Earth-grazing meteoroid EN131090 was observed above Czechoslovakia and Poland, moving at 41.74 km/s (25.94 mi/s) along a 409 km (254 mi) trajectory from south to north. The closest approach to the Earth was 98.67 km (61.31 mi) above

6624-450: The Solar eclipse of March 9, 2016 , the Moon was near its descending node, and the Sun was near the point in the sky where the equator of the Moon crosses the ecliptic. When the Sun reaches that point, the centre of the Sun sets at the lunar north pole and rises at the lunar south pole. The solar eclipse of September 1 of the same year , the Moon was near its ascending node, and the Sun was near

6768-530: The Vernal Equinox and the stars in about 27.32 days (a tropical month and sidereal month ) and one revolution relative to the Sun in about 29.53 days (a synodic month ). Earth and the Moon orbit about their barycentre (common centre of mass ), which lies about 4,670 km (2,900 miles) from Earth's centre (about 73% of its radius), forming a satellite system called the Earth–Moon system . On average,

6912-498: The distance to the Moon is about 384,400 km (238,900 mi) from Earth's centre, which corresponds to about 60 Earth radii or 1.282 light-seconds. With a mean orbital velocity around the barycentre between the Earth and the Moon, of 1.022 km/s (0.635 miles/s, 2,286 miles/h), the Moon covers a distance approximately its diameter, or about half a degree on the celestial sphere , each hour. The Moon differs from most regular satellites of other planets in that its orbit

7056-409: The parallax between the Moon and the chosen background star, the lunar distance was calculated. A more ambitious project called the "Aristarchus Campaign" was conducted during the lunar eclipse of 15 April 2014. During this event, participants were invited to record a series of five digital photographs from moonrise until culmination (the point of greatest altitude). The method took advantage of

7200-469: The seasonal behaviour of the Sun, but with a period of 27.2 days instead of 365 days. Note that a point on the Moon can actually be visible when it is about 34 arc minutes below the horizon, due to atmospheric refraction . Because of the inclination of the Moon's orbit with respect to the Earth's equator, the Moon is above the horizon at the North and South Pole for almost two weeks every month, even though

7344-473: The 1980s, with mounting evidence for the theory that the Cretaceous–Paleogene extinction event (in which the non-avian dinosaurs died out) 65 million years ago was caused by a large asteroid impact . On March 23, 1989, the 300 m (980 ft) diameter Apollo asteroid 4581 Asclepius (1989 FC) missed the Earth by 700,000 km (430,000 mi). If the asteroid had impacted it would have created

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7488-400: The 2010s, each year, several mostly small NEOs pass Earth closer than the distance of the Moon. As astronomers became able to discover ever smaller and fainter and ever more numerous near-Earth objects, they began to routinely observe and catalogue close approaches. As of April 2024 , the closest approach without impact ever detected, other than meteors or fireballs that went through

7632-433: The 30 m (98 ft) asteroid 367943 Duende ( 2012 DA 14 ) passed approximately 27,700 km (17,200 mi) above the surface of Earth, closer than satellites in geosynchronous orbit. The asteroid was not visible to the unaided eye. This was the first sub-lunar close passage of an object discovered during a previous passage, and was thus the first to be predicted well in advance. Some small asteroids that enter

7776-479: The Apollo missions in 1969, astronauts placed retroreflectors on the surface of the Moon for the purpose of refining the accuracy and precision of this technique. The measurements are ongoing and involve multiple laser facilities. The instantaneous precision of the Lunar Laser Ranging experiments can achieve small millimeter resolution, and is the most reliable method of determining the lunar distance. The semi-major axis

7920-665: The Center for Near Earth Object Studies (CNEOS) and the Solar System Dynamics Group. CNEOS's catalog of near-Earth objects includes the approach distances of asteroids and comets. NEOs are also catalogued by a unit of ESA , the Near-Earth Objects Coordination Centre (NEOCC). Orbit of the Moon The Moon orbits Earth in the prograde direction and completes one revolution relative to

8064-403: The Earth and accelerates the Moon, but because the ocean dissipates so much tidal energy, the present ocean tides have an order of magnitude greater effect than the solid Earth tides. Because of the tidal torque, caused by the ellipsoids, some of Earth's angular (or rotational) momentum is gradually being transferred to the rotation of the Earth–Moon pair around their mutual centre of mass, called

8208-651: The Earth dangerously closely and the estimated consequences that an impact would have if it occurs. Objects with both an Earth minimum orbit intersection distance (MOID) of 0.05 AU or less and an absolute magnitude of 22.0 or brighter (a rough indicator of large size) are considered PHAs. Objects that either cannot approach closer to the Earth than 0.05  AU (7,500,000 km; 4,600,000 mi), or which are fainter than H = 22.0 (about 140 m (460 ft) in diameter with assumed albedo of 14%), are not considered PHAs. The first near-Earth objects to be observed by humans were comets. Their extraterrestrial nature

8352-451: The Earth surface, while larger objects hit the water surface, forming tsunami waves, or the solid surface, forming impact craters . The frequency of impacts of objects of various sizes is estimated on the basis of orbit simulations of NEO populations, the frequency of impact craters on the Earth and the Moon, and the frequency of close encounters. The study of impact craters indicates that impact frequency has been more or less steady for

8496-441: The Earth's orbit, and the object is larger than 140 meters (460 ft) across, it is considered a potentially hazardous object (PHO). Most known PHOs and NEOs are asteroids , but about 0.35% are comets . There are over 34,000 known near-Earth asteroids (NEAs) and over 120 known short-period near-Earth comets (NECs). A number of solar-orbiting meteoroids were large enough to be tracked in space before striking Earth. It

8640-411: The Moon across and the maximum Earth tide. As the Earth rotates faster than the Moon travels around its orbit, this small angle produces a gravitational torque which slows the Earth and accelerates the Moon in its orbit. In the case of the ocean tides, the speed of tidal waves in the ocean is far slower than the speed of the Moon's tidal forcing. As a result, the ocean is never in near equilibrium with

8784-400: The Moon and Earth can change as quickly as 75 meters per second , or more than 1,000 km (620 mi) in just 6 hours, due to its non-circular orbit. There are other effects that also influence the lunar distance. Some factors include: The formula of Chapront and Touzé for the distance in kilometres begins with the terms: where G M {\displaystyle G_{M}}

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8928-443: The Moon and a chosen reference point from multiple locations, simultaneously. The synchronization can be coordinated by making measurements at a pre-determined time, or during an event which is observable to all parties. Before accurate mechanical chronometers, the synchronization event was typically a lunar eclipse , or the moment when the Moon crossed the meridian (if the observers shared the same longitude). This measurement technique

9072-400: The Moon can be observed as flashes of light with a typical duration of a fraction of a second. The first lunar impacts were recorded during the 1999 Leonid storm. Subsequently, several continuous monitoring programs were launched. A lunar impact that was observed on September 11, 2013, lasted 8 seconds, was likely caused by an object 0.6–1.4 m (2.0–4.6 ft) in diameter, and created

9216-413: The Moon is not perpendicular to its orbital plane, so the lunar equator is not in the plane of its orbit, but is inclined to it by a constant value of 6.688° (this is the obliquity ). As was discovered by Jacques Cassini in 1722, the rotational axis of the Moon precesses with the same rate as its orbital plane, but is 180° out of phase (see Cassini's Laws ) . Therefore, the angle between the ecliptic and

9360-472: The Moon would be in a mutual spin–orbit resonance or tidal locking , in which the Moon will orbit Earth in about 47 days (currently 27 days), and both the Moon and Earth would rotate around their axes in the same time, always facing each other with the same side. This has already happened to the Moon—the same side always faces Earth—and is also slowly happening to the Earth. However, the slowdown of Earth's rotation

9504-410: The Moon's apparent size changes as it moves toward and away from an observer on Earth. An event called a " supermoon " occurs when the full Moon is closest to Earth (perigee). The largest possible apparent diameter of the Moon is the same 12% larger (as perigee versus apogee distances) than the smallest; the apparent area is 25% more and so is the amount of light it reflects toward Earth. The variance in

9648-420: The Moon's orbital distance corresponds with changes in its tangential and angular speeds, per Kepler's second law . The mean angular movement relative to an imaginary observer at the Earth–Moon barycentre is 13.176 ° per day to the east ( J2000.0 epoch). The Moon's elongation is its angular distance east of the Sun at any time. At new moon, it is zero and the Moon is said to be in conjunction . At full moon,

9792-421: The Moon. During this approach, Icarus became the first minor planet to be observed using radar . This was the first close approach predicted years in advance, since Icarus had been discovered in 1949. The first near-Earth asteroid known to have passed Earth closer than the distance of the Moon was 1991 BA , a 5–10 m (16–33 ft) body which passed at a distance of 170,000 km (110,000 mi). By

9936-532: The Palermo Scale. Observations during the August 2022 close approach were expected to ascertain whether the asteroid will impact or miss Earth in 2095. As of April 2024 , the risk of the 2095 impact was put at 1 in 10, still the highest, with a Palermo Scale rating of −2.98. A year before the 1968 close approach of asteroid Icarus, Massachusetts Institute of Technology students launched Project Icarus, devising

10080-679: The South Pole. The Moon's light is used by zooplankton in the Arctic when the Sun is below the horizon for months and must have been helpful to the animals that lived in Arctic and Antarctic regions when the climate was warmer. About 1000 BC , the Babylonians were the first human civilization known to have kept a consistent record of lunar observations. Clay tablets from that period, which have been found in Iraq, are inscribed with cuneiform writing recording

10224-583: The Sun is below the horizon for six months at a time. The period from moonrise to moonrise at the poles is a tropical month , about 27.3 days, quite close to the sidereal period. When the Sun is the furthest below the horizon ( winter solstice ), the Moon will be full when it is at its highest point. When the Moon is in Gemini it will be above the horizon at the North Pole, and when it is in Sagittarius it will be up at

10368-436: The Sun to one around the Earth, but the term is applied flexibly for these objects, too. The orbits of some NEOs intersect that of the Earth, so they pose a collision danger. These are considered potentially hazardous objects (PHOs) if their estimated diameter is above 140 meters. PHOs include potentially hazardous asteroids (PHAs). PHAs are defined based on two parameters relating to respectively their potential to approach

10512-413: The Sun, passed Earth undetected at a distance of 0.0120 AU (4.65 LD) on June 12, 1999. In 1937, 800 m (2,600 ft) asteroid 69230 Hermes was discovered when it passed the Earth at twice the distance of the Moon . On June 14, 1968, the 1.4 km (0.87 mi) diameter asteroid 1566 Icarus passed Earth at a distance of 0.042 AU (6,300,000 km), or 16 times the distance of

10656-431: The Sun, Earth, and Moon align in three dimensions. In effect, this means that the " tropical year " on the Moon is only 347 days long. This is called the draconic year or eclipse year. The "seasons" on the Moon fit into this period. For about half of this draconic year, the Sun is north of the lunar equator (but at most 1.543°), and for the other half, it is south of the lunar equator. The effect of these seasons, however,

10800-408: The Sun, and the Moon. The anomalistic month is the time between perigees and is about 27.55 days. The Earth–Moon separation determines the strength of the lunar tide raising force. The draconic month is the time from ascending node to ascending node. The time between two successive passes of the same ecliptic longitude is called the tropical month . The latter periods are slightly different from

10944-465: The Sun. This definition excludes larger bodies such as planets , like Venus ; natural satellites which orbit bodies other than the Sun, like Earth's Moon ; and artificial bodies orbiting the Sun. A small Solar System body can be an asteroid or a comet , thus an NEO is either a near-Earth asteroid (NEA) or a near-Earth comet (NEC). The organisations cataloging NEOs further limit their definition of NEO to objects with an orbital period under 200 years,

11088-399: The Sun. Consequently, Earth's centre veers inside and outside the solar orbital path during each synodic month as the Moon moves in its orbit around the common centre of gravity. The Sun's gravitational effect on the Moon is more than twice that of Earth's on the Moon; consequently, the Moon's trajectory is always convex (as seen when looking Sunward at the entire Sun–Earth–Moon system from

11232-526: The affected Earth region. Another project, the Zwicky Transient Facility (ZTF), which surveys for objects that change their brightness rapidly, also detects asteroids passing close to Earth. Scientists involved in NEO research have also considered options for actively averting the threat if an object is found to be on a collision course with Earth. All viable methods aim to deflect rather than destroy

11376-480: The angular velocity. If the thumb of the right hand points to the north celestial pole, its fingers curl in the direction that the Moon orbits Earth, Earth orbits the Sun, and the Moon and Earth rotate on their own axes. In representations of the Solar System , it is common to draw the trajectory of Earth from the point of view of the Sun, and the trajectory of the Moon from the point of view of Earth. This could give

11520-522: The asteroid was removed from the Sentry Risk Table entirely in February 2008. In 2021, 2010 RF 12 was listed with the highest chance of impacting Earth, at 1 in 22 on September 5, 2095. At only 7 m (23 ft) across, the asteroid however is much too small to be considered a potentially hazardous asteroid and it poses no serious threat: the possible 2095 impact therefore rated only −3.32 on

11664-605: The atmosphere (see #Earth-grazers below), was an encounter with asteroid 2020 VT 4 on November 14, 2020. The 5–11 m (16–36 ft) NEA was detected receding from Earth; calculations showed that on the day before, it had a close approach at about 6,750 km (4,190 mi) from the Earth's centre, or about 380 km (240 mi) above its surface. On November 8, 2011, asteroid (308635) 2005 YU 55 , relatively large at about 400 m (1,300 ft) in diameter, passed within 324,930 km (201,900 mi) (0.845 lunar distances ) of Earth. On February 15, 2013,

11808-562: The atomic bomb dropped on Hiroshima , approximately 15 kilotonnes of TNT) at five years, for asteroids 60 m (200 ft) across (an impact energy of 10 megatons , comparable to the Tunguska event in 1908) at 1,300 years, for asteroids 1 km (0.62 mi) across at 440 thousand years, and for asteroids 5 km (3.1 mi) across at 18 million years. Some other models estimate similar impact frequencies, while others calculate higher frequencies. For Tunguska-sized (10 megaton) impacts,

11952-473: The average value of this is the inverse of 384,399 km (238,854 mi). On the other hand, the time-averaged distance (rather than the inverse of the average inverse distance) between the centers of Earth and the Moon is 385,000.6 km (239,228.3 mi). One can also model the orbit as an ellipse that is constantly changing, and in this case one can find a formula for the semi-major axis, again involving trigonometric terms. The average value by this method

12096-444: The barycentre. See tidal acceleration for a more detailed description. This slightly greater orbital angular momentum causes the Earth–Moon distance to increase at approximately 38 millimetres per year. Conservation of angular momentum means that Earth's axial rotation is gradually slowing, and because of this its day lengthens by approximately 24 microseconds every year (excluding glacial rebound ). Both figures are valid only for

12240-544: The consequences of such an impact would be. Some NEOs have had temporarily positive Torino or Palermo scale ratings after their discovery. Since 1998, the United States, the European Union, and other nations have been scanning the sky for NEOs in an effort called Spaceguard . The initial US Congress mandate to NASA to catalog at least 90% of NEOs that are at least 1 kilometre (0.62 mi) in diameter, sufficient to cause

12384-633: The current configuration of the continents. Tidal rhythmites from 620 million years ago show that, over hundreds of millions of years, the Moon receded at an average rate of 22 mm (0.87 in) per year (2200 km or 0.56% or the Earth-moon distance per hundred million years) and the day lengthened at an average rate of 12 microseconds per year (or 20 minutes per hundred million years), both about half of their current values. The present high rate may be due to near resonance between natural ocean frequencies and tidal frequencies. Another explanation

12528-401: The direction of the pole of rotation of the Moon in space: but these are very small. When viewed from the north celestial pole (that is, from the approximate direction of the star Polaris ) the Moon orbits Earth anticlockwise and Earth orbits the Sun anticlockwise, and the Moon and Earth rotate on their own axes anticlockwise. The right-hand rule can be used to indicate the direction of

12672-411: The distance is perturbed by the gravitational effects of various astronomical bodies – most significantly the Sun and less so Venus and Jupiter. Other forces responsible for minute perturbations are: gravitational attraction to other planets in the Solar System and to asteroids; tidal forces; and relativistic effects. The effect of radiation pressure from the Sun contributes an amount of ± 3.6 mm to

12816-418: The ecliptic with a period of 18.6 years or 19.3549° per year. When viewed from the celestial north, the nodes move clockwise around Earth, opposite to Earth's own spin and its revolution around the Sun. An eclipse of the Moon or Sun can occur when the nodes align with the Sun, roughly every 173.3 days. Lunar orbit inclination also determines eclipses; shadows cross when nodes coincide with full and new moon when

12960-410: The elongation is 180° and it is said to be in opposition . In both cases, the Moon is in syzygy , that is, the Sun, Moon and Earth are nearly aligned. When elongation is either 90° or 270°, the Moon is said to be in quadrature . The orientation of the orbit is not fixed in space but rotates over time. This orbital precession is called apsidal precession and is the rotation of the Moon's orbit within

13104-419: The equator and the north pole is visible. This is called libration in latitude . The nodes are points at which the Moon's orbit crosses the ecliptic. The Moon crosses the same node every 27.2122 days, an interval called the draconic month or draconitic month . The line of nodes, the intersection between the two respective planes, has a retrograde motion : for an observer on Earth, it rotates westward along

13248-420: The equator to produce a present inclination of 5° to the ecliptic. It is thought that originally the inclination to the equator was near zero, but it could have been increased to 10° through the influence of planetesimals passing near the Moon while falling to the Earth. If this had not happened, the Moon would now lie much closer to the ecliptic and eclipses would be much more frequent. The rotational axis of

13392-520: The estimates range from one event every 2,000–3,000 years to one event every 300 years. The second-largest observed event after the Tunguska meteor was a 1.1 megaton air blast in 1963 near the Prince Edward Islands between South Africa and Antarctica, which was detected only by infrasound sensors. However this may have been a nuclear test . The third-largest, but by far best-observed impact,

13536-449: The fact that the Moon is actually closest to an observer when it is at its highest point in the sky, compared to when it is on the horizon. Although it appears that the Moon is biggest when it is near the horizon, the opposite is true. This phenomenon is known as the Moon illusion . The reason for the difference in distance is that the distance from the center of the Moon to the center of the Earth

13680-482: The first scientific study in human history. However, the Babylonians seem to have lacked any geometric or physical interpretation of their data, and they could not predict future lunar eclipses (though "warnings" were issued before likely eclipse times). Ancient Greek astronomers were the first to introduce and analyze mathematical models of the motion of objects in the sky. Ptolemy described lunar motion by using

13824-544: The first asteroid with a temporarily positive rating on the Torino Scale, with about a 1 in 9,300 chance of an impact in 2049. Additional observations reduced the estimated risk to zero, and the asteroid was removed from the Sentry Risk Table in April 2002. It is now known that within the next two centuries, 2002 CU 11 will pass the Earth at a safe closest distance (perigee) of 0.00425 AU (636,000 km; 395,000 mi) on August 31, 2080. Asteroid (29075) 1950 DA

13968-468: The general public. The simple Torino scale was established at an IAU workshop in Torino in June 1999, in the wake of the public confusion about the impact risk of 1997 XF 11 . It rates the risks of impacts in the next 100 years according to impact energy and impact probability, using integer numbers between 0 and 10: The more complex Palermo Technical Impact Hazard Scale , established in 2002, compares

14112-536: The gravitational attraction of the Sun and planets, the study of which ( lunar theory ) has a long history. The orbit of the Moon is a nearly circular ellipse about Earth (the semimajor and semiminor axes are 384,400 km and 383,800 km, respectively: a difference of only 0.16%). The equation of the ellipse yields an eccentricity of 0.0549 and perigee and apogee distances of 362,600 km (225,300 mi) and 405,400 km (251,900 mi) respectively (a difference of 12%). Since nearer objects appear larger,

14256-404: The impression that the Moon orbits Earth in such a way that sometimes it goes backwards when viewed from the Sun's perspective. However, because the orbital velocity of the Moon around Earth (1 km/s) is small compared to the orbital velocity of Earth about the Sun (30 km/s), this never happens. There are no rearward loops in the Moon's solar orbit. Considering the Earth–Moon system as

14400-411: The inclination is at its maximum of 28°36', the centre of the Moon's disk will be above the horizon every day only from latitudes less than 60°27' (90° − 28°36' – 57' parallax) north or south. At higher latitudes , there will be a period of at least one day each month when the Moon does not rise, but there will also be a period of at least one day each month when the Moon does not set. This is similar to

14544-406: The influence of the Sun and other perturbations, the Moon's orbit around the Earth is not a precise ellipse. Nevertheless, different methods have been used to define a semi-major axis . Ernest William Brown provided a formula for the parallax of the Moon as viewed from opposite sides of the Earth, involving trigonometric terms. This is equivalent to a formula for the inverse of the distance, and

14688-607: The largest explosion in recorded history, equivalent to 20,000 megatons of TNT . It attracted widespread attention because it was discovered only after the closest approach. From the 1990s, a typical frame of reference in searches for NEOs has been the scientific concept of risk . The awareness of the wider public of the impact risk rose after the observation of the impact of the fragments of Comet Shoemaker–Levy 9 into Jupiter in July 1994. In March 1998, early orbit calculations for recently discovered asteroid (35396) 1997 XF 11 showed

14832-600: The likelihood of an impact at a certain date to the probable number of impacts of a similar energy or greater until the possible impact, and takes the logarithm of this ratio. Thus, a Palermo scale rating can be any positive or negative real number, and risks of any concern are indicated by values above zero. Unlike the Torino scale, the Palermo scale is not sensitive to newly discovered small objects with an orbit known with low confidence. The National Aeronautics and Space Administration NASA maintains an automated system to evaluate

14976-440: The local meridian, from stations at Greenwich and at Cape of Good Hope . A distance was calculated with an uncertainty of 30 km , and this remained the definitive lunar distance value for the next half century. By recording the instant when the Moon occults a background star, (or similarly, measuring the angle between the Moon and a background star at a predetermined moment) the lunar distance can be determined, as long as

15120-501: The lunar distance was about 60.05  R 🜨 (383,000 km or 238,000 mi). There is geological evidence that the average lunar distance was about 52  R 🜨 (332,000 km or 205,000 mi) during the Precambrian Era ; 2500 million years BP . The widely accepted giant impact hypothesis states that the Moon was created as a result of a catastrophic impact between Earth and another planet, resulting in

15264-411: The lunar distance. Although the instantaneous uncertainty is a few millimeters, the measured lunar distance can change by more than 30,000 km (19,000 mi) from the mean value throughout a typical month. These perturbations are well understood and the lunar distance can be accurately modeled over thousands of years. Through the action of tidal forces , the angular momentum of Earth's rotation

15408-403: The lunar equator is always 1.543°, even though the rotational axis of the Moon is not fixed with respect to the stars. It also means that when the Moon is farthest north of the ecliptic, the centre of the part seen from Earth is about 6.7° south of the lunar equator and the south pole is visible, whereas when the Moon is farthest south of the ecliptic the centre of the visible part is 6.7° north of

15552-471: The measurements are taken from multiple locations of known separation. Astronomers O'Keefe and Anderson calculated the lunar distance by observing four occultations from nine locations in 1952. They calculated a semi-major axis of 384 407 .6 ± 4.7 km (238,859.8 ± 2.9 mi). This value was refined in 1962 by Irene Fischer , who incorporated updated geodetic data to produce a value of 384 403 .7 ± 2 km (238,857.4 ± 1 mi). The distance to

15696-465: The moon was measured by means of radar first in 1946 as part of Project Diana . Later, an experiment was conducted in 1957 at the U.S. Naval Research Laboratory that used the echo from radar signals to determine the Earth-Moon distance. Radar pulses lasting 2 μs were broadcast from a 50-foot (15 m) diameter radio dish. After the radio waves echoed off the surface of the Moon, the return signal

15840-613: The need for dedicated survey telescopes and options to head off an eventual impact were first discussed at a 1981 interdisciplinary conference in Snowmass, Colorado . Plans for a more comprehensive survey, named the Spaceguard Survey, were developed by NASA from 1992, under a mandate from the United States Congress . To promote the survey on an international level, the International Astronomical Union (IAU) organised

15984-451: The newly discovered comet 55P/Tempel–Tuttle has the same orbit as the Leonids. The first near-Earth asteroid to be discovered was 433 Eros in 1898. The asteroid was subject to several extensive observation campaigns, primarily because measurements of its orbit enabled a precise determination of the then imperfectly known distance of the Earth from the Sun. If a near-Earth object is near

16128-464: The normal to the plane of the ecliptic. This leads to a similar perspective effect in the north–south direction that is referred to as optical libration in latitude , which allows one to see almost 7° of latitude beyond the pole on the far side. Finally, because the Moon is only about 60 Earth radii away from Earth's centre of mass, an observer at the equator who observes the Moon throughout the night moves laterally by one Earth diameter. This gives rise to

16272-400: The notion that near-Earth objects could cause catastrophic impacts. Also at that time, a scare arose about a supposed 2003 impact of a planet called Nibiru with Earth, which persisted on the internet as the predicted impact date was moved to 2012 and then 2017. There are two schemes for the scientific classification of impact hazards from NEOs, as a way to communicate the risk of impacts to

16416-437: The orbital plane, i.e. the axes of the ellipse change direction. The lunar orbit's major axis – the longest diameter of the orbit, joining its nearest and farthest points, the perigee and apogee , respectively – makes one complete revolution every 8.85 Earth years, or 3,232.6054 days, as it rotates slowly in the same direction as the Moon itself (direct motion) – meaning precesses eastward by 360°. The Moon's apsidal precession

16560-598: The part of its orbit closest to Earth's at the same time Earth is at the part of its orbit closest to the near-Earth object's orbit, the object has a close approach, or, if the orbits intersect, could even impact the Earth or its atmosphere. As of May 2019 , only 23 comets have been observed to pass within 0.1 AU (15,000,000 km; 9,300,000 mi) of Earth, including 10 which are or have been short-period comets. Two of these near-Earth comets, Halley's Comet and 73P/Schwassmann–Wachmann , have been observed during multiple close approaches. The closest observed approach

16704-407: The past 3.5 billion years, which requires a steady replenishment of the NEO population from the asteroid main belt . One impact model based on widely accepted NEO population models estimates the average time between the impact of two stony asteroids with a diameter of at least 4 m (13 ft) at about one year; for asteroids 7 m (23 ft) across (which impacts with as much energy as

16848-412: The point in the sky where the equator of the Moon crosses the ecliptic. When the Sun reaches that point, the centre of the Sun rises at the lunar north pole and sets at the lunar south pole. Every 18.6 years, the angle between the Moon's orbit and Earth's equator reaches a maximum of 28°36′, the sum of Earth's equatorial tilt (23°27′) and the Moon's orbital inclination (5°09′) to the ecliptic . This

16992-492: The radio waves echoed off the surface of the Moon, the return signal was detected and the delay time measured. Multiple signals were added together to obtain a reliable signal by superimposing oscilloscope traces onto photographic film. From the measurements, the distance was calculated with an uncertainty of 1.25 km (0.777 mi). These initial experiments were intended to be proof-of-concept experiments and only lasted one day. Follow-on experiments lasting one month produced

17136-604: The ratio to 76%. Given the rarity of impacts by objects this big mentioned above, there are probably no objects of 140 metres or larger that will hit the earth in the next few centuries. In January 2016, NASA announced the creation of the Planetary Defense Coordination Office (PDCO) to track NEOs larger than about 30–50 m (98–164 ft) in diameter and coordinate an effective threat response and mitigation effort. Survey programs aim to identify threats years in advance, giving humanity time to prepare

17280-509: The risk of impact at any date was completely eliminated by 2021. Consequently, Apophis was removed from the Sentry Risk Table. In February 2006, (144898) 2004 VD 17 , having a diameter around 300 metres, was assigned a Torino Scale rating of 2 due to a close encounter predicted for May 4, 2102. After additional observations allowed increasingly precise predictions, the Torino rating was lowered first to 1 in May 2006, then to 0 in October 2006, and

17424-454: The sea. Interest in NEOs has increased since the 1980s because of greater awareness of this risk. Asteroid impact avoidance by deflection is possible in principle, and methods of mitigation are being researched. Two scales, the simple Torino scale and the more complex Palermo scale , rate the risk presented by an identified NEO based on the probability of it impacting the Earth and on how severe

17568-426: The sidereal month. The average length of a calendar month (a twelfth of a year) is about 30.4 days. This is not a lunar period, though the calendar month is historically related to the visible lunar phase. The gravitational attraction that the Moon exerts on Earth is the cause of tides in both the ocean and the solid Earth; the Sun has a smaller tidal influence. The solid Earth responds quickly to any change in

17712-419: The sky having right ascension zero and declination zero). The nodes are moving west by about 19° per year. The Sun crosses a given node about 20 days earlier each year. When the inclination of the Moon's orbit to the Earth's equator is at its minimum of 18°20′, the centre of the Moon's disk will be above the horizon every day from latitudes less than 70°43' (90° − 18°20' – 57' parallax) north or south. When

17856-407: The smallest perigee occurs at either new moon or full moon (ca 356870 km), as does the greatest apogee (ca 406079 km), whereas the greatest perigee will be around half-moon (ca 370180 km), as will be the smallest apogee (ca 404593 km). The exact values will be slightly different due to other terms. Twice in every full moon cycle of about 411 days there will be a minimal perigee and

18000-461: The source of era-changing cataclysms or potentially poisonous fumes (during Earth's passage through the tail of Halley's Comet in 1910); and finally as a possible cause of a crater-forming impact that could even cause extinction of humans and other life on Earth. The potential of catastrophic impacts by near-Earth comets was recognised as soon as the first orbit calculations provided an understanding of their orbits: in 1694, Edmond Halley presented

18144-483: The surface. It was captured by two all-sky cameras of the European Fireball Network , which for the first time enabled geometric calculations of the orbit of such a body. When a near-Earth object impacts Earth, objects up to a few tens of metres across ordinarily explode in the upper atmosphere (usually harmlessly), with most or all of the solids vaporized and only small amounts of meteorites arriving to

18288-532: The threat from known NEOs over the next 100 years, which generates the continuously updated Sentry Risk Table . All or nearly all of the objects are highly likely to drop off the list eventually as more observations come in, reducing the uncertainties and enabling more accurate orbital predictions. A similar table is maintained on NEODyS (Near Earth Objects Dynamic Site) by the European Space Agency (ESA). In March 2002, (163132) 2002 CU 11 became

18432-525: The threatening NEO, because the fragments would still cause widespread destruction. Deflection, which means a change in the object's orbit months to years prior to the predicted impact , also requires orders of magnitude less energy. For a given amount of energy, a greater effect on the momentum of the object can be had by causing some of it to be blasted off it, as was done in the Double Asteroid Redirection Test (see below). When an NEO

18576-420: The tidal forcing, the distortion taking the form of an ellipsoid with the high points roughly beneath the Moon and on the opposite side of Earth. This is a result of the high speed of seismic waves within the solid Earth. However the speed of seismic waves is not infinite and, together with the effect of energy loss within the Earth, this causes a slight delay between the passage of the maximum forcing due to

18720-462: The tidal forcing. Instead, the forcing generates the long ocean waves which propagate around the ocean basins until eventually losing their energy through turbulence, either in the deep ocean or on shallow continental shelves. Although the ocean's response is the more complex of the two, it is possible to split the ocean tides into a small ellipsoid term which affects the Moon plus a second term which has no effect. The ocean's ellipsoid term also slows

18864-481: The times and dates of moonrises and moonsets, the stars that the Moon passed close by, and the time differences between rising and setting of both the Sun and the Moon around the time of a full moon . Babylonian astronomy discovered the three main periods of the Moon's motion and used data analysis to build lunar calendars that extended well into the future. This use of detailed, systematic observations to make predictions based on experimental data may be classified as

19008-419: The upper atmosphere of Earth at a shallow angle remain intact and leave the atmosphere again, continuing on a solar orbit. During the passage through the atmosphere, due to the burning of its surface, such an object can be observed as an Earth-grazing fireball . On August 10, 1972, a meteor that became known as the 1972 Great Daylight Fireball was witnessed by many people and even filmed as it moved north over

19152-427: The work of Ptolemy , who produced a result of 64 + 1 ⁄ 6   R 🜨 ( 409 000  km or 253 000  mi ) at its farthest point. An expedition by French astronomer A.C.D. Crommelin observed lunar meridian transits on the same night from two different locations. Careful measurements from 1905 to 1910 measured the angle of elevation at the moment when a specific lunar crater ( Mösting A ) crossed

19296-460: Was 0.0151 AU (5.88 LD) for Lexell's Comet on July 1, 1770. After an orbit change due to a close approach of Jupiter in 1779, this object is no longer an NEC. The closest approach ever observed for a current short-period NEC is 0.0229 AU (8.92 LD) for Comet Tempel–Tuttle in 1366. Orbital calculations show that P/1999 J6 (SOHO) , a faint sungrazing comet and confirmed short-period NEC observed only during its close approaches to

19440-450: Was 60.61  R 🜨 , which implied a 3% accuracy. The benefit of this method is that the only measuring equipment needed is a modern digital camera (equipped with an accurate clock, and a GPS receiver). Other experimental methods of measuring the lunar distance that can be performed by amateur astronomers involve: Near-Earth object 34,000+ known NEOs, divided into several orbital subgroups A near-Earth object ( NEO )

19584-452: Was added to the Sentry list in April 2002 as the first object with a Palermo scale value greater than zero. The then-calculated 1 in 300 maximum chance of impact and +0.17 Palermo scale value was roughly 50% greater than the background risk of impact by all similarly large objects until 2880. After additional radar and optical observations, as of April 2024 , the probability of this impact

19728-514: Was conducted. Similar missions are in progress. Preliminary plans for commercial asteroid mining have been drafted by private startup companies, but few of these plans were pursued. Near-Earth objects (NEOs) are formally defined by the International Astronomical Union (IAU) as all small Solar System bodies with orbits around the Sun that are at least partially closer than 1.3 astronomical units (AU; Sun–Earth distance) from

19872-447: Was considered a threat because it was lost after its discovery; thus its orbit and potential for collision with Earth were not known precisely. Hermes, having a period of 2.13 years, was only re-discovered in 2003, and it is now known to be no threat for at least the next century. Scientists have recognised the threat of impacts that create craters much bigger than the impacting bodies and have indirect effects on an even wider area since

20016-565: Was detected and the delay time measured. From that measurement, the distance could be calculated. In practice, however, the signal-to-noise ratio was so low that an accurate measurement could not be reliably produced. The experiment was repeated in 1958 at the Royal Radar Establishment , in England. Radar pulses lasting 5 μs were transmitted with a peak power of 2 megawatts, at a repetition rate of 260 pulses per second. After

20160-510: Was estimated at 38%. The Chile-based Vera C. Rubin Observatory , which will survey the southern sky for transient events from 2025, is expected to increase the number of known asteroids by a factor of 10 to 100 and increase the ratio of known NEOs with diameters of 140 m (460 ft) or greater to at least 60%, while the NEO Surveyor satellite, to be launched in 2027, is expected to push

20304-560: Was lost after its 1950 discovery, since its observations over just 17 days were insufficient to precisely determine its orbit. It was rediscovered in December 2000 prior to a close approach the next year, when new observations, including radar imaging, allowed much more precise orbit calculations. It has a diameter of about a kilometer (0.6 miles), and an impact would therefore be globally catastrophic. Although this asteroid will not strike for at least 800 years and thus has no Torino scale rating, it

20448-403: Was recognised and confirmed only after Tycho Brahe tried to measure the distance of a comet through its parallax in 1577 and the lower limit he obtained was well above the Earth diameter; the periodicity of some comets was first recognised in 1705, when Edmond Halley published his orbit calculations for the returning object now known as Halley's Comet . The 1758–1759 return of Halley's Comet

20592-612: Was the Chelyabinsk meteor of 15 February 2013. A previously unknown 20 m (66 ft) asteroid exploded above this Russian city with an equivalent blast yield of 400–500 kilotons. The calculated orbit of the pre-impact asteroid is similar to that of Apollo asteroid 2011 EO 40 , making the latter the meteor's possible parent body. On October 7, 2008, 20 hours after it was first observed and 11 hours after its trajectory has been calculated and announced, 4 m (13 ft) asteroid 2008 TC 3 blew up 37 km (23 mi) above

20736-412: Was the first comet appearance predicted. The extraterrestrial origin of meteors (shooting stars) was only recognised on the basis of the analysis of the 1833 Leonid meteor shower by astronomer Denison Olmsted . The 33-year period of the Leonids led astronomers to suspect that they originate from a comet that would today be classified as an NEO, which was confirmed in 1867, when astronomers found that

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