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71-446: Solar time is a calculation of the passage of time based on the position of the Sun in the sky . The fundamental unit of solar time is the day , based on the synodic rotation period . Traditionally, there are three types of time reckoning based on astronomical observations: apparent solar time and mean solar time (discussed in this article), and sidereal time , which is based on

142-546: A causal relation . General relativity does not address the nature of time for extremely small intervals where quantum mechanics holds. In quantum mechanics, time is treated as a universal and absolute parameter, differing from general relativity's notion of independent clocks. The problem of time consists of reconciling these two theories. As of 2024, there is no generally accepted theory of quantum general relativity. Generally speaking, historical methods of temporal measurement, or chronometry , have taken two distinct forms:

213-520: A clock dial or calendar) that marks the occurrence of a specified event (as to hour or date) is obtained by counting from certain starting date ( epoch ), and relevant to a certain time zone (including daylight saving time ). Precise measurements, as in astronomy , use a fiducial epoch – a central reference point. Artifacts from the Paleolithic suggest that the moon was used to reckon time as early as 6,000 years ago. Lunar calendars were among

284-563: A circle) or in degrees (360 degrees to a circle)—one or the other, not both. Negative hour angles (−180° < LHA object < 0°) indicate the object is approaching the meridian, positive hour angles (0° < LHA object < 180°) indicate the object is moving away from the meridian; an hour angle of zero means the object is on the meridian. Right ascension is frequently given in sexagesimal hours-minutes-seconds format (HH:MM:SS) in astronomy, though may be given in decimal hours, sexagesimal degrees (DDD:MM:SS), or, decimal degrees. Observing

355-620: A dimension. Isaac Newton said that we are merely occupying time, he also says that humans can only understand relative time . Relative time is a measurement of objects in motion. The anti-realists believed that time is merely a convenient intellectual concept for humans to understand events. This means that time was useless unless there were objects that it could interact with, this was called relational time . René Descartes , John Locke , and David Hume said that one's mind needs to acknowledge time, in order to understand what time is. Immanuel Kant believed that we can not know what something

426-483: A few years and significantly in thousands of years. Mean solar time is the hour angle of the mean Sun plus 12 hours. This 12 hour offset comes from the decision to make each day start at midnight for civil purposes, whereas the hour angle or the mean sun is measured from the local meridian. As of 2009, this is realized with the UT1 time scale, constructed mathematically from very-long-baseline interferometry observations of

497-403: A head in the famous Leibniz–Clarke correspondence . Philosophers in the 17th and 18th century questioned if time was real and absolute, or if it was an intellectual concept that humans use to understand and sequence events. These questions lead to realism vs anti-realism; the realists believed that time is a fundamental part of the universe, and be perceived by events happening in a sequence, in

568-407: A hundred minutes of a hundred seconds, which marked a deviation from the base 12 ( duodecimal ) system used in many other devices by many cultures. The system was abolished in 1806. A large variety of devices have been invented to measure time. The study of these devices is called horology . An Egyptian device that dates to c.  1500 BC , similar in shape to a bent T-square , measured

639-577: A number or calendar date to an instant (point in time), quantifying the duration of a time interval, and establishing a chronology (ordering of events). In modern times, several time specifications have been officially recognized as standards, where formerly they were matters of custom and practice. The invention in 1955 of the caesium atomic clock has led to the replacement of older and purely astronomical time standards such as sidereal time and ephemeris time , for most practical purposes, by newer time standards based wholly or partly on atomic time using

710-482: A recurring pattern of ages or cycles, where events and phenomena repeated themselves in a predictable manner. One of the most famous examples of this concept is found in Hindu philosophy , where time is depicted as a wheel called the " Kalachakra " or "Wheel of Time." According to this belief, the universe undergoes endless cycles of creation, preservation, and destruction. Similarly, in other ancient cultures such as those of

781-417: A solar day varies through the year, and the accumulated effect produces seasonal deviations of up to 16 minutes from the mean. The effect has two main causes. First, due to the eccentricity of Earth's orbit , Earth moves faster when it is nearest the Sun ( perihelion ) and slower when it is farthest from the Sun ( aphelion ) (see Kepler's laws of planetary motion ). Second, due to Earth's axial tilt (known as

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852-516: A year and 20 days in a month, plus five epagomenal days at the end of the year. The reforms of Julius Caesar in 45 BC put the Roman world on a solar calendar . This Julian calendar was faulty in that its intercalation still allowed the astronomical solstices and equinoxes to advance against it by about 11 minutes per year. Pope Gregory XIII introduced a correction in 1582; the Gregorian calendar

923-765: Is a fundamental concept to define other quantities, such as velocity . To avoid a circular definition, time in physics is operationally defined as "what a clock reads", specifically a count of repeating events such as the SI second . Although this aids in practical measurements, it does not address the essence of time. Physicists developed the concept of the spacetime continuum, where events are assigned four coordinates: three for space and one for time. Events like particle collisions , supernovas , or rocket launches have coordinates that may vary for different observers, making concepts like "now" and "here" relative. In general relativity , these coordinates do not directly correspond to

994-454: Is a theoretical ideal scale realized by TAI. Geocentric Coordinate Time and Barycentric Coordinate Time are scales defined as coordinate times in the context of the general theory of relativity. Barycentric Dynamical Time is an older relativistic scale that is still in use. Many ancient cultures, particularly in the East, had a cyclical view of time. In these traditions, time was often seen as

1065-426: Is about 86,400.002 SI seconds, i.e., about 24.0000006 hours. The apparent sun is the true sun as seen by an observer on Earth. Apparent solar time or true solar time is based on the apparent motion of the actual Sun . It is based on the apparent solar day , the interval between two successive returns of the Sun to the local meridian . Apparent solar time can be crudely measured by a sundial . The length of

1136-418: Is ahead of apparent time by about 14 minutes near February 6, and behind apparent time by about 16 minutes near November 3. The equation of time is this difference, which is cyclical and does not accumulate from year to year. Mean time follows the mean sun. Jean Meeus describes the mean sun as follows: Consider a first fictitious Sun travelling along the ecliptic with a constant speed and coinciding with

1207-430: Is credited to Egyptians because of their sundials, which operated on a duodecimal system. The importance of the number 12 is due to the number of lunar cycles in a year and the number of stars used to count the passage of night. The most precise timekeeping device of the ancient world was the water clock , or clepsydra , one of which was found in the tomb of Egyptian pharaoh Amenhotep I . They could be used to measure

1278-500: Is dominated by temporality ( kala ), everything within time is subject to change and decay. Overcoming pain and death requires knowledge that transcends temporal existence and reveals its eternal foundation. Two contrasting viewpoints on time divide prominent philosophers. One view is that time is part of the fundamental structure of the universe  – a dimension independent of events, in which events occur in sequence . Isaac Newton subscribed to this realist view, and hence it

1349-498: Is in Byrhtferth 's Enchiridion (a science text) of 1010–1012, where it was defined as 1/564 of a momentum (1 1 ⁄ 2 minutes), and thus equal to 15/94 of a second. It was used in the computus , the process of calculating the date of Easter. As of May 2010 , the smallest time interval uncertainty in direct measurements is on the order of 12 attoseconds (1.2 × 10 seconds), about 3.7 × 10 Planck times . The second (s)

1420-812: Is kept within 0.9 second of UT1 by the introduction of one-second steps to UTC, the leap second . The Global Positioning System broadcasts a very precise time signal based on UTC time. The surface of the Earth is split into a number of time zones . Standard time or civil time in a time zone deviates a fixed, round amount, usually a whole number of hours, from some form of Universal Time, usually UTC. Most time zones are exactly one hour apart, and by convention compute their local time as an offset from UTC. For example, time zones at sea are based on UTC. In many locations (but not at sea) these offsets vary twice yearly due to daylight saving time transitions. Some other time standards are used mainly for scientific work. Terrestrial Time

1491-411: Is neither an event nor a thing, and thus is not itself measurable nor can it be travelled. Furthermore, it may be that there is a subjective component to time, but whether or not time itself is "felt", as a sensation, or is a judgment, is a matter of debate. In Philosophy, time was questioned throughout the centuries; what time is and if it is real or not. Ancient Greek philosophers asked if time

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1562-483: Is not rather than what it is, an approach similar to that taken in other negative definitions . However, Augustine ends up calling time a "distention" of the mind (Confessions 11.26) by which we simultaneously grasp the past in memory, the present by attention, and the future by expectation. Isaac Newton believed in absolute space and absolute time; Leibniz believed that time and space are relational. The differences between Leibniz's and Newton's interpretations came to

1633-655: Is qualitative, as opposed to quantitative. In Greek mythology, Chronos (ancient Greek: Χρόνος) is identified as the Personification of Time. His name in Greek means "time" and is alternatively spelled Chronus (Latin spelling) or Khronos. Chronos is usually portrayed as an old, wise man with a long, gray beard, such as "Father Time". Some English words whose etymological root is khronos/chronos include chronology , chronometer , chronic , anachronism , synchronise , and chronicle . Rabbis sometimes saw time like "an accordion that

1704-476: Is quantified by the equation of time , and is due to the eccentricity of Earth's orbit (as in, Earth's orbit is not perfectly circular, meaning that the Earth–Sun distance varies throughout the year), and the fact that Earth's axis is not perpendicular to the plane of its orbit (the so-called obliquity of the ecliptic ). The effect of this is that a clock running at a constant rate – e.g. completing

1775-588: Is seen as progressing in a straight line from past to future without repetition. In general, the Islamic and Judeo-Christian world-view regards time as linear and directional , beginning with the act of creation by God. The traditional Christian view sees time ending, teleologically, with the eschatological end of the present order of things, the " end time ". In the Old Testament book Ecclesiastes , traditionally ascribed to Solomon (970–928 BC), time (as

1846-447: Is sometimes referred to as Newtonian time . The opposing view is that time does not refer to any kind of "container" that events and objects "move through", nor to any entity that "flows", but that it is instead part of a fundamental intellectual structure (together with space and number) within which humans sequence and compare events. This second view, in the tradition of Gottfried Leibniz and Immanuel Kant , holds that time

1917-402: Is the SI base unit. A minute (min) is 60 seconds in length (or, rarely, 59 or 61 seconds when leap seconds are employed), and an hour is 60 minutes or 3600 seconds in length. A day is usually 24 hours or 86,400 seconds in length; however, the duration of a calendar day can vary due to Daylight saving time and Leap seconds . A time standard is a specification for measuring time: assigning

1988-461: Is the local hour angle of the object, LST is the local sidereal time , α object {\displaystyle \alpha _{\text{object}}} is the object's right ascension , GST is Greenwich sidereal time and λ observer {\displaystyle \lambda _{\text{observer}}} is the observer's longitude (positive east from the prime meridian ). These angles can be measured in time (24 hours to

2059-489: Is the rotation of the Earth with respect to the sun and hence is mean solar time. However, UT1, the version in common use since 1955, uses a slightly different definition of rotation that corrects for the motion of Earth's poles as it rotates. The difference between this corrected mean solar time and Coordinated Universal Time (UTC) determines whether a leap second is needed. (Since 1972 the UTC time scale has run on SI seconds , and

2130-413: Is to measure in degrees westward from the prime meridian ( Greenwich hour angle , GHA ), from the local meridian ( local hour angle , LHA ) or from the first point of Aries ( sidereal hour angle , SHA ). The hour angle is paired with the declination to fully specify the location of a point on the celestial sphere in the equatorial coordinate system . The local hour angle (LHA) of an object in

2201-402: Is unless we experience it first hand. Hour angle It may be given in degrees, time, or rotations depending on the application. The angle may be expressed as negative east of the meridian plane and positive west of the meridian plane, or as positive westward from 0° to 360°. The angle may be measured in degrees or in time, with 24 = 360° exactly. In celestial navigation , the convention

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2272-569: The Clock of the Long Now . They can be driven by a variety of means, including gravity, springs, and various forms of electrical power, and regulated by a variety of means such as a pendulum . Alarm clocks first appeared in ancient Greece around 250 BC with a water clock that would set off a whistle. This idea was later mechanized by Levi Hutchins and Seth E. Thomas . A chronometer is a portable timekeeper that meets certain precision standards. Initially,

2343-409: The calendar , a mathematical tool for organising long intervals of time, and the clock (e.g., watch ), a physical mechanism that counts the passage of time. In day-to-day life, a clock was consulted for periods less than a day, whereas a calendar was consulted for periods longer than a day. Increasingly, personal electronic devices display both calendars and clocks simultaneously. The number (as on

2414-408: The diurnal motions of radio sources located in other galaxies, and other observations. The duration of daylight varies during the year but the length of a mean solar day is nearly constant, unlike that of an apparent solar day. An apparent solar day can be 20 seconds shorter or 30 seconds longer than a mean solar day. Long or short days occur in succession, so the difference builds up until mean time

2485-412: The obliquity of the ecliptic ), the Sun's annual motion is along a great circle (the ecliptic ) that is tilted to Earth's celestial equator . When the Sun crosses the equator at both equinoxes , the Sun's daily shift (relative to the background stars) is at an angle to the equator, so the projection of this shift onto the equator is less than its average for the year; when the Sun is farthest from

2556-665: The philosophy of eternalism views the subject from a different angle. Time is one of the seven fundamental physical quantities in both the International System of Units (SI) and International System of Quantities . The SI base unit of time is the second , which is defined by measuring the electronic transition frequency of caesium atoms. General relativity is the primary framework for understanding how spacetime works. Through advances in both theoretical and experimental investigations of spacetime, it has been shown that time can be distorted and dilated , particularly at

2627-668: The Hebrew word עידן, זמן iddan (age, as in "Ice age") zĕman(time) is often translated) is a medium for the passage of predestined events. (Another word, زمان" זמן" zamān , meant time fit for an event , and is used as the modern Arabic , Persian , and Hebrew equivalent to the English word "time".) The Greek language denotes two distinct principles, Chronos and Kairos . The former refers to numeric, or chronological, time. The latter, literally "the right or opportune moment", relates specifically to metaphysical or Divine time. In theology, Kairos

2698-520: The Mayans, Aztecs, and Chinese, there were also beliefs in cyclical time, often associated with astronomical observations and calendars. These cultures developed complex systems to track time, seasons, and celestial movements, reflecting their understanding of cyclical patterns in nature and the universe. The cyclical view of time contrasts with the linear concept of time more common in Western thought, where time

2769-474: The Middle Dutch word klocke which, in turn, derives from the medieval Latin word clocca , which ultimately derives from Celtic and is cognate with French, Latin, and German words that mean bell . The passage of the hours at sea was marked by bells and denoted the time (see ship's bell ). The hours were marked by bells in abbeys as well as at sea. Clocks can range from watches to more exotic varieties such as

2840-456: The SI second, when adopted, was already a little shorter than the current value of the second of mean solar time.) Time Time is the continuous progression of our changing existence that occurs in an apparently irreversible succession from the past , through the present , and into the future . It is a component quantity of various measurements used to sequence events , to compare

2911-469: The SI second. International Atomic Time (TAI) is the primary international time standard from which other time standards are calculated. Universal Time (UT1) is mean solar time at 0° longitude, computed from astronomical observations. It varies from TAI because of the irregularities in Earth's rotation. Coordinated Universal Time (UTC) is an atomic time scale designed to approximate Universal Time. UTC differs from TAI by an integral number of seconds. UTC

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2982-447: The Sun from Earth, the solar hour angle is an expression of time, expressed in angular measurement, usually degrees, from solar noon . At solar noon the hour angle is zero degrees, with the time before solar noon expressed as negative degrees, and the local time after solar noon expressed as positive degrees. For example, at 10:30 AM local apparent time the hour angle is −22.5° (15° per hour times 1.5 hours before noon). The cosine of

3053-650: The Sun seeming to have covered a 360-degree arc around Earth's axis. When the Sun has covered exactly 15 degrees (1/24 of a circle, both angles being measured in a plane perpendicular to Earth's axis), local apparent time is 13:00 exactly; after 15 more degrees it will be 14:00 exactly. The problem is that in September the Sun takes less time (as measured by an accurate clock) to make an apparent revolution than it does in December; 24 "hours" of solar time can be 21 seconds less or 29 seconds more than 24 hours of clock time. This change

3124-404: The apparent motions of stars other than the Sun. A tall pole vertically fixed in the ground casts a shadow on any sunny day. At one moment during the day, the shadow will point exactly north or south (or disappear when and if the Sun moves directly overhead). That instant is called local apparent noon , or 12:00 local apparent time. About 24 hours later the shadow will again point north–south,

3195-523: The causal structure of events. Instead, the spacetime interval is calculated and classified as either space-like or time-like, depending on whether an observer exists that would say the events are separated by space or by time. Since the time required for light to travel a specific distance is the same for all observers—a fact first publicly demonstrated by the Michelson–Morley experiment —all observers will consistently agree on this definition of time as

3266-523: The corresponding slowing of Earth's rotation by the Moon. The sun has always been visible in the sky, and its position forms the basis of apparent solar time, the timekeeping method used in antiquity. An Egyptian obelisk constructed c. 3500 BC, a gnomon in China dated 2300 BC, and an Egyptian sundial dated 1500 BC are some of the earliest methods for measuring the sun's position. Babylonian astronomers knew that

3337-475: The duration of events (or the intervals between them), and to quantify rates of change of quantities in material reality or in the conscious experience . Time is often referred to as a fourth dimension , along with three spatial dimensions . Scientists have theorized a beginning of time in our universe (the Big Bang ) and an end ( heat death ). The cyclic model describes a cyclical nature, whereas

3408-404: The edges of black holes . Throughout history, time has been an important subject of study in religion, philosophy, and science. Temporal measurement has occupied scientists and technologists and has been a prime motivation in navigation and astronomy . Time is also of significant social importance, having economic value (" time is money ") as well as personal value, due to an awareness of

3479-654: The equation of time in his Handy Tables . Apparent solar time grew less useful as commerce increased and mechanical clocks improved. Mean solar time was introduced in almanacs in England in 1834 and in France in 1835. Because the sun was difficult to observe directly due to its large size in the sky, mean solar time was determined as a fixed ratio of time as observed by the stars, which used point-like observations. A specific standard for measuring "mean solar time" from midnight came to be called Universal Time. Conceptually Universal Time

3550-604: The equator at both solstices , the Sun's shift in position from one day to the next is parallel to the equator, so the projection onto the equator of this shift is larger than the average for the year (see tropical year ). In June and December when the sun is farthest from the celestial equator, a given shift along the ecliptic corresponds to a large shift at the equator. Therefore, apparent solar days are shorter in March and September than in June or December. These lengths will change slightly in

3621-517: The events of the abbeys and monasteries of the Middle Ages. Richard of Wallingford (1292–1336), abbot of St. Alban's abbey, famously built a mechanical clock as an astronomical orrery about 1330. Great advances in accurate time-keeping were made by Galileo Galilei and especially Christiaan Huygens with the invention of pendulum-driven clocks along with the invention of the minute hand by Jost Burgi. The English word clock probably comes from

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3692-439: The first mechanical clocks driven by an escapement mechanism. The hourglass uses the flow of sand to measure the flow of time. They were used in navigation. Ferdinand Magellan used 18 glasses on each ship for his circumnavigation of the globe (1522). Incense sticks and candles were, and are, commonly used to measure time in temples and churches across the globe. Water clocks, and, later, mechanical clocks, were used to mark

3763-789: The first to appear, with years of either 12 or 13 lunar months (either 354 or 384 days). Without intercalation to add days or months to some years, seasons quickly drift in a calendar based solely on twelve lunar months. Lunisolar calendars have a thirteenth month added to some years to make up for the difference between a full year (now known to be about 365.24 days) and a year of just twelve lunar months. The numbers twelve and thirteen came to feature prominently in many cultures, at least partly due to this relationship of months to years. Other early forms of calendars originated in Mesoamerica, particularly in ancient Mayan civilization. These calendars were religiously and astronomically based, with 18 months in

3834-472: The frequency of electronic transitions in certain atoms to measure the second. One of the atoms used is caesium ; most modern atomic clocks probe caesium with microwaves to determine the frequency of these electron vibrations. Since 1967, the International System of Measurements bases its unit of time, the second, on the properties of caesium atoms. SI defines the second as 9,192,631,770 cycles of

3905-416: The hour angle (cos( h )) is used to calculate the solar zenith angle . At solar noon, h = 0.000 so cos( h ) = 1 , and before and after solar noon the cos(±  h ) term = the same value for morning (negative hour angle) or afternoon (positive hour angle), so that the Sun is at the same altitude in the sky at 11:00AM and 1:00PM solar time. The sidereal hour angle (SHA) of a body on the celestial sphere

3976-438: The hours even at night but required manual upkeep to replenish the flow of water. The ancient Greeks and the people from Chaldea (southeastern Mesopotamia) regularly maintained timekeeping records as an essential part of their astronomical observations. Arab inventors and engineers, in particular, made improvements on the use of water clocks up to the Middle Ages. In the 11th century, Chinese inventors and engineers invented

4047-449: The hours of daylight varied throughout the year. A tablet from 649 BC shows that they used a 2:1 ratio for the longest day to the shortest day, and estimated the variation using a linear zigzag function. It is not clear if they knew of the variation in the length of the solar day and the corresponding equation of time . Ptolemy clearly distinguishes the mean solar day and apparent solar day in his Almagest (2nd century), and he tabulated

4118-443: The limited time in each day and in human life spans . Cultural attitudes towards the human use of time are apparent in the verbs used—from "kill" to "waste" to "pass"—and sayings (like carpe diem ). The concept of time can be complex. Multiple notions exist and defining time in a manner applicable to all fields without circularity has consistently eluded scholars. Nevertheless, diverse fields such as business, industry, sports,

4189-573: The nature of time. Plato , in the Timaeus , identified time with the period of motion of the heavenly bodies. Aristotle , in Book IV of his Physica defined time as 'number of movement in respect of the before and after'. In Book 11 of his Confessions , St. Augustine of Hippo ruminates on the nature of time, asking, "What then is time? If no one asks me, I know: if I wish to explain it to one that asketh, I know not." He begins to define time by what it

4260-494: The observer's sky is LHA object = LST − α object {\displaystyle {\text{LHA}}_{\text{object}}={\text{LST}}-\alpha _{\text{object}}} or LHA object = GST + λ observer − α object {\displaystyle {\text{LHA}}_{\text{object}}={\text{GST}}+\lambda _{\text{observer}}-\alpha _{\text{object}}} where LHA object

4331-419: The passage of time from the shadow cast by its crossbar on a nonlinear rule. The T was oriented eastward in the mornings. At noon, the device was turned around so that it could cast its shadow in the evening direction. A sundial uses a gnomon to cast a shadow on a set of markings calibrated to the hour. The position of the shadow marks the hour in local time . The idea to separate the day into smaller parts

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4402-618: The radiation that corresponds to the transition between two electron spin energy levels of the ground state of the Cs atom. Today, the Global Positioning System in coordination with the Network Time Protocol can be used to synchronize timekeeping systems across the globe. In medieval philosophical writings, the atom was a unit of time referred to as the smallest possible division of time. The earliest known occurrence in English

4473-415: The same number of pendulum swings in each hour – cannot follow the actual Sun; instead it follows an imaginary " mean Sun " that moves along the celestial equator at a constant rate that matches the real Sun's average rate over the year. This is "mean solar time", which is still not perfectly constant from one century to the next but is close enough for most purposes. As of 2008, a mean solar day

4544-576: The sciences, and the performing arts all incorporate some notion of time into their respective measuring systems . Traditional definitions of time involved the observation of periodic motion such as the apparent motion of the sun across the sky, the phases of the moon, and the passage of a free-swinging pendulum. More modern systems include the Global Positioning System , other satellite systems, Coordinated Universal Time and mean solar time . Although these systems differ from one another, with careful measurements they can be synchronized. In physics, time

4615-580: The term was used to refer to the marine chronometer , a timepiece used to determine longitude by means of celestial navigation , a precision first achieved by John Harrison . More recently, the term has also been applied to the chronometer watch , a watch that meets precision standards set by the Swiss agency COSC . The most accurate timekeeping devices are atomic clocks , which are accurate to seconds in many millions of years, and are used to calibrate other clocks and timekeeping instruments. Atomic clocks use

4686-420: The true sun at the perigee and apogee (when the Earth is in perihelion and aphelion, respectively). Then consider a second fictitious Sun travelling along the celestial equator at a constant speed and coinciding with the first fictitious Sun at the equinoxes. This second fictitious sun is the mean Sun . The length of the mean solar day is slowly increasing due to the tidal acceleration of the Moon by Earth and

4757-480: Was an illusion to humans. Plato believed that time was made by the Creator at the same instant as the heavens. He also says that time is a period of motion of the heavenly bodies . Aristotle believed that time correlated to movement, that time did not exist on its own but was relative to motion of objects. He also believed that time was related to the motion of celestial bodies ; the reason that humans can tell time

4828-466: Was because of orbital periods and therefore there was a duration on time. The Vedas , the earliest texts on Indian philosophy and Hindu philosophy dating to the late 2nd millennium BC , describe ancient Hindu cosmology , in which the universe goes through repeated cycles of creation, destruction and rebirth, with each cycle lasting 4,320 million years. Ancient Greek philosophers , including Parmenides and Heraclitus , wrote essays on

4899-480: Was expanded and collapsed at will." According to Kabbalists , "time" is a paradox and an illusion . According to Advaita Vedanta , time is integral to the phenomenal world, which lacks independent reality. Time and the phenomenal world are products of maya , influenced by our senses, concepts, and imaginations. The phenomenal world, including time, is seen as impermanent and characterized by plurality, suffering, conflict, and division. Since phenomenal existence

4970-462: Was linear or cyclical and if time was endless or finite . These philosophers had different ways of explaining time; for instance, ancient Indian philosophers had something called the Wheel of Time. It is believed that there was repeating ages over the lifespan of the universe. This led to beliefs like cycles of rebirth and reincarnation . The Greek philosophers believe that the universe was infinite, and

5041-520: Was only slowly adopted by different nations over a period of centuries, but it is now by far the most commonly used calendar around the world. During the French Revolution , a new clock and calendar were invented as part of the dechristianization of France and to create a more rational system in order to replace the Gregorian calendar. The French Republican Calendar 's days consisted of ten hours of

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