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46-610: Lodestar is an archaic word for a star that guides, especially the northern pole star . Lodestar may also refer to: Pole star Currently, Earth's pole stars are Polaris (Alpha Ursae Minoris), a bright magnitude 2 star aligned approximately with its northern axis that serves as a pre-eminent star in celestial navigation , and a much dimmer magnitude 5.5 star on its southern axis, Polaris Australis (Sigma Octantis). From around 1700 BC until just after 300 AD, Kochab (Beta Ursae Minoris) and Pherkad (Gamma Ursae Minoris) were twin northern pole stars, though neither

92-475: A declination of –82°, meaning it will rise and set daily for latitudes between 8°S and 8°N, and will not rise to viewers north of this latter 8th parallel north . Precession and proper motion mean that Sirius will be a future southern pole star: at 88.4° S declination in the year 66,270 AD; and 87.7° S declination in the year 93,830 AD. Pole stars of other planets are defined analogously: they are stars (brighter than 6th magnitude, i.e. , visible to

138-497: A degree away from the positions charted by the ancient Greek astronomer Hipparchus roughly 1850 years earlier. The lesser meaning of "proper" used is arguably dated English (but neither historic, nor obsolete when used as a postpositive , as in "the city proper") meaning "belonging to" or "own". "Improper motion" would refer to perceived motion that is nothing to do with an object's inherent course, such as due to Earth's axial precession , and minor deviations, nutations well within

184-541: A distance of 5° from celestial north. Precession will eventually point the north celestial pole nearer the stars in the constellation Hercules , pointing towards Tau Herculis around 18,400 AD. The celestial pole will then return to the stars in constellation Draco (Thuban, mentioned above) before returning to the current constellation, Ursa Minor. When Polaris becomes the North Star again around 27,800 AD, due to its proper motion it then will be farther away from

230-436: A distant 7° from the pole, never close enough to be taken as marking the pole, while third-magnitude Delta Cygni will be a more helpful pole star, at a distance of 3° from celestial north, around 11,250 AD. Precession will then point the north celestial pole nearer the constellation Lyra , where the second brightest star in the northern celestial hemisphere , Vega , will be a pole star around 14,500 AD, though at

276-415: A given epoch , often J2000.0 ) are given in the direction of right ascension ( μ α ) and of declination ( μ δ ). Their combined value is computed as the total proper motion ( μ ). It has dimensions of angle per time , typically arcseconds per year or milliarcseconds per year. Knowledge of the proper motion, distance, and radial velocity allows calculations of an object's motion from

322-476: Is 90 km/s and its radial velocity is 111 km/s (perpendicular (at a right, 90° angle), which gives a true or "space" motion of 142 km/s. True or absolute motion is more difficult to measure than the proper motion, because the true transverse velocity involves the product of the proper motion times the distance. As shown by this formula, true velocity measurements depend on distance measurements, which are difficult in general. In 1992 Rho Aquilae became

368-404: Is barely visible on a clear night , making it less useful for casual navigational or astronomy alignment purposes. It is a yellow giant 294 light years from Earth. Its angular separation from the pole is about 1° (as of 2000 ). The Southern Cross constellation functions as an approximate southern pole constellation, by pointing to where a southern pole star would be. At the equator , it

414-543: Is called stella maris , the sterre of the see, for he ledeth in the see men that saylle and have shyppemannes crafte. Polaris was associated with Marian veneration from an early time, Our Lady, Star of the Sea being a title of the Blessed Virgin. This tradition goes back to a misreading of Saint Jerome 's translation of Eusebius ' Onomasticon , De nominibus hebraicis (written ca. 390). Jerome gave stilla maris "drop of

460-547: Is designated μ α* . For example, the proper motion results in right ascension in the Hipparcos Catalogue (HIP) have already been converted. Hence, the individual proper motions in right ascension and declination are made equivalent for straightforward calculations of various other stellar motions. The position angle θ is related to these components by: Motions in equatorial coordinates can be converted to motions in galactic coordinates . For most stars seen in

506-459: Is given by the Pythagorean theorem : where δ is the declination. The factor in cos δ accounts for the widening of the lines (hours) of right ascension away from the poles, cos δ , being zero for a hypothetical object fixed at a celestial pole in declination. Thus, a co-efficient is given to negate the misleadingly greater east or west velocity (angular change in α ) in hours of Right Ascension

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552-528: Is invisible in light-polluted urban skies. During the 1st millennium BC, Beta Ursae Minoris (Kochab) was the bright star closest to the celestial pole, but it was never close enough to be taken as marking the pole, and the Greek navigator Pytheas in ca. 320 BC described the celestial pole as devoid of stars. In the Roman era , the celestial pole was about equally distant between Polaris and Kochab. The precession of

598-590: Is one source of such images. In the past, searches for high proper motion objects were undertaken using blink comparators to examine the images by eye. More modern techniques such as image differencing can scan digitized images, or comparisons to star catalogs obtained by satellites. As any selection biases of these surveys are well understood and quantifiable, studies have confirmed more and inferred approximate quantities of unseen stars – revealing and confirming more by studying them further, regardless of brightness, for instance. Studies of this kind show most of

644-508: Is possible to see both Polaris and the Southern Cross. The celestial south pole is moving toward the Southern Cross, which has pointed to the south pole for the last 2000 years or so. As a consequence, the constellation is no longer visible from subtropical northern latitudes, as it was in the time of the ancient Greeks . Around 200 BC, the star Beta Hydri was the nearest bright star to the celestial south pole. Around 2800 BC, Achernar

690-448: Is the direction of the proper motion on the celestial sphere (with 0 degrees meaning the motion is north, 90 degrees meaning the motion is east, (left on most sky maps and space telescope images) and so on), and the second is its magnitude, typically expressed in arcseconds per year (symbols: arcsec/yr, as/yr, ″/yr, ″ yr ) or milliarcseconds per year (symbols: mas/yr, mas yr ). Proper motion may alternatively be defined by

736-771: The Triangulum Galaxy M33, the third largest and only ordinary spiral galaxy in the Local Group, located 0.860 ± 0.028 Mpc beyond the Milky Way. The motion of the Andromeda Galaxy was measured in 2012, and an Andromeda–Milky Way collision is predicted in about 4.5 billion years. Proper motion of the NGC 4258 (M106) galaxy in the M106 group of galaxies was used in 1999 to find an accurate distance to this object. Measurements were made of

782-501: The "Star of the Sea" metaphor, saying that Mary is the "Star of the Sea" to be followed on the way to Christ, "lest we capsize amid the storm-tossed waves of the sea." In Mandaean cosmology , the Pole Star is considered to be auspicious and is associated with the World of Light ("heaven"). Mandaeans face north when praying, and temples are also oriented towards the north. On the contrary,

828-512: The 5th century, when it was still removed from the celestial pole by about 8°. It was known as scip-steorra ("ship-star") in 10th-century Anglo-Saxon England , reflecting its use in navigation. In the Vishnu Purana , it is personified under the name Dhruva ("immovable, fixed"). The name stella polaris was coined in the Renaissance, even though at that time it was well recognized that it

874-570: The Solar System's frame of reference and its motion from the galactic frame of reference – that is motion in respect to the Sun, and by coordinate transformation , that in respect to the Milky Way . Over the course of centuries, stars appear to maintain nearly fixed positions with respect to each other, so that they form the same constellations over historical time. As examples, both Ursa Major in

920-398: The angular changes per year in the star's right ascension ( μ α ) and declination ( μ δ ) with respect to a constant epoch . The components of proper motion by convention are arrived at as follows. Suppose an object moves from coordinates (α 1 , δ 1 ) to coordinates (α 2 , δ 2 ) in a time Δ t . The proper motions are given by: The magnitude of the proper motion μ

966-410: The celestial north pole, on 24 March 2100. Precession will next point the north celestial pole at stars in the northern constellation Cepheus . The pole will drift to space equidistant between Polaris and Gamma Cephei ("Errai") by 3000 AD, with Errai reaching its closest alignment with the northern celestial pole around 4200 AD. Iota Cephei and Beta Cephei will stand on either side of

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1012-437: The center of the galaxy at a speed of about 220 km/s at a radius of 8,000 parsecs (26,000 ly) from Sagittarius A* which can be taken as the rate of rotation of the Milky Way itself at this radius. Any proper motion is a two-dimensional vector (as it excludes the component as to the direction of the line of sight) and it bears two quantities or characteristics: its position angle and its magnitude . The first

1058-519: The cluster. Stellar proper motions have been used to infer the presence of a super-massive black hole at the center of the Milky Way. This now confirmed to exist black hole is called Sgr A* , and has a mass of 4.3 × 10   M ☉ (solar masses). Proper motions of the galaxies in the Local Group are discussed in detail in Röser. In 2005, the first measurement was made of the proper motion of

1104-430: The current star, with stars that will be "near-north" indicators when no North Star exists during the cycle, including each star's average brightness and closest alignment to the north celestial pole during the cycle: Currently, there is no South Pole Star like Polaris , the so-called North Star . Sigma Octantis is the closest near naked-eye star to the south celestial pole, but at apparent magnitude 5.47 it

1150-461: The equinoxes takes about 25,770 years to complete a cycle. Polaris' mean position (taking account of precession and proper motion ) will reach a maximum declination of +89°32'23", which translates to 1657" (or 0.4603°) from the celestial north pole, in February 2102. Its maximum apparent declination (taking account of nutation and aberration ) will be +89°32'50.62", which is 1629" (or 0.4526°) from

1196-545: The first star to have its Bayer designation invalidated by moving to a neighbouring constellation – it is now in Delphinus . Stars with large proper motions tend to be nearby; most stars are far enough away that their proper motions are very small, on the order of a few thousandths of an arcsecond per year. It is possible to construct nearly complete samples of high proper motion stars by comparing photographic sky survey images taken many years apart. The Palomar Sky Survey

1242-407: The further it is towards the imaginary infinite poles, above and below the earth's axis of rotation, in the sky. The change μ α , which must be multiplied by cos δ to become a component of the proper motion, is sometimes called the "proper motion in right ascension", and μ δ the "proper motion in declination". If the proper motion in right ascension has been converted by cos δ , the result

1288-467: The largest proper motion of all stars, moving at 10.3″ yr . Large proper motion usually strongly indicates an object is close to the Sun. This is so for Barnard's Star, about 6 light-years away. After the Sun and the Alpha Centauri system, it is the nearest known star. Being a red dwarf with an apparent magnitude of 9.54, it is too faint to see without a telescope or powerful binoculars. Of

1334-459: The naked eye limit needed to serve as a useful indicator of north to an Earth-based observer, resulting in periods of time during the cycle when there is no clearly defined North Star. There will also be periods during the cycle when bright stars give only an approximate guide to "north", as they may be greater than 5° of angular diameter removed from direct alignment with the north celestial pole. The 26,000 year cycle of North Stars, starting with

1380-447: The naked eye under ideal conditions) that most closely coincide with the projection of the planet's axis of rotation onto the celestial sphere. Different planets have different pole stars because their axes are oriented differently. (See Poles of astronomical bodies .) In the medieval period, Polaris was also known as stella maris ("star of the sea", from its use for navigation at sea), as in e.g. Bartholomaeus Anglicus (d. 1272), in

1426-463: The nearest stars are intrinsically faint and angularly small, such as red dwarfs . Measurement of the proper motions of a large sample of stars in a distant stellar system, like a globular cluster, can be used to compute the cluster's total mass via the Leonard-Merritt mass estimator . Coupled with measurements of the stars' radial velocities , proper motions can be used to compute the distance to

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1472-430: The northern celestial pole some time around 5200 AD, before moving to closer alignment with the brighter star Alpha Cephei ("Alderamin") around 7500 AD. Precession will then point the north celestial pole at stars in the northern constellation Cygnus . Like Beta Ursae Minoris during the 1st millennium BC, the bright star closest to the celestial pole in the 10th millennium AD, first-magnitude Deneb , will be

1518-472: The northern sky and Crux in the southern sky, look nearly the same now as they did hundreds of years ago. However, precise long-term observations show that such constellations change shape, albeit very slowly, and that each star has an independent motion . This motion is caused by the movement of the stars relative to the Sun and Solar System . The Sun travels in a nearly circular orbit (the solar circle ) about

1564-433: The pole than it is now, while in 23,600 BC it was closer to the pole. Over the course of Earth's 26,000-year axial precession cycle, a series of bright naked eye stars (an apparent magnitude up to +6; a full moon is −12.9) in the northern hemisphere will hold the transitory title of North Star. While other stars might line up with the north celestial pole during the 26,000 year cycle, they do not necessarily meet

1610-471: The radial motion of objects in that galaxy moving directly toward and away from Earth, and assuming this same motion to apply to objects with only a proper motion, the observed proper motion predicts a distance to the galaxy of 7.2 ± 0.5 Mpc . Proper motion was suspected by early astronomers (according to Macrobius , c. AD 400) but a proof was not provided until 1718 by Edmund Halley , who noticed that Sirius , Arcturus and Aldebaran were over half

1656-494: The sea" as a (false) Hebrew etymology of the name Maria . This stilla maris was later misread as stella maris ; the misreading is also found in the manuscript tradition of Isidore 's Etymologiae (7th century); it probably arises in the Carolingian era ; a late 9th-century manuscript of Jerome's text still has stilla , not stella , but Paschasius Radbertus , also writing in the 9th century, makes an explicit reference to

1702-638: The sky, the observed proper motions are small and unremarkable. Such stars are often either faint or are significantly distant, have changes of below 0.01″ per year, and do not appear to move appreciably over many millennia. A few do have significant motions, and are usually called high-proper motion stars. Motions can also be in almost seemingly random directions. Two or more stars, double stars or open star clusters , which are moving in similar directions, exhibit so-called shared or common proper motion (or cpm.), suggesting they may be gravitationally attached or share similar motion in space. Barnard's Star has

1748-479: The south is associated with the World of Darkness . Proper motion Proper motion is the astrometric measure of the observed changes in the apparent places of stars or other celestial objects in the sky, as seen from the center of mass of the Solar System , compared to the abstract background of the more distant stars . The components for proper motion in the equatorial coordinate system (of

1794-426: The stars visible to the naked eye (conservatively limiting unaided visual magnitude to 6.0), 61 Cygni A (magnitude V= 5.20) has the highest proper motion at 5.281″ yr , discounting Groombridge 1830 (magnitude V= 6.42), proper motion: 7.058″ yr . A proper motion of 1 arcsec per year 1 light-year away corresponds to a relative transverse speed of 1.45 km/s. Barnard's Star's transverse speed

1840-441: The stars' proper motions), the role of North Star has passed from one star to another in the remote past, and will pass in the remote future. In 3000 BC, the faint star Thuban in the constellation Draco was the North Star, aligning within 0.1° distance from the celestial pole, the closest of any of the visible pole stars. However, at magnitude 3.67 (fourth magnitude) it is only one-fifth as bright as Polaris, and today it

1886-473: The translation of John Trevisa (1397): by the place of this sterre place and stedes and boundes of the other sterres and of cercles of heven ben knowen: therefore astronomers beholde mooste this sterre. Then this ster is dyscryved of the moste shorte cercle; for he is ferre from the place that we ben in; he hydeth the hugenesse of his quantite for unmevablenes of his place, and he doth cerfifie men moste certenly, that beholde and take hede therof; and therfore he

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1932-412: Was 89.35 degrees North; (at epoch J2000 it was 89.26 degrees N). So it appears due north in the sky to a precision better than one degree, and the angle it makes with respect to the true horizon (after correcting for refraction and other factors) is within a degree of the latitude of the observer. The celestial pole will be nearest Polaris in 2100. Due to the precession of the equinoxes (as well as

1978-433: Was as close to the pole as Polaris is now. In classical antiquity , Beta Ursae Minoris (Kochab) was closer to the celestial north pole than Alpha Ursae Minoris. While there was no naked-eye star close to the pole, the midpoint between Alpha and Beta Ursae Minoris was reasonably close to the pole, and it appears that the entire constellation of Ursa Minor , in antiquity known as Cynosura (Greek Κυνόσουρα "dog's tail"),

2024-496: Was only 8 degrees from the south pole. In the next 7500 years, the south celestial pole will pass close to the stars Gamma Chamaeleontis (4200 AD), I Carinae , Omega Carinae (5800 AD), Upsilon Carinae , Iota Carinae (Aspidiske, 8100 AD) and Delta Velorum (Alsephina, 9200 AD). From the eightieth to the ninetieth centuries, the south celestial pole will travel through the False Cross . Around 14,000 AD Canopus will have

2070-509: Was several degrees away from the celestial pole; Gemma Frisius in the year 1547 determined this distance as 3°8'. An explicit identification of Mary as stella maris with the North Star ( Polaris ) becomes evident in the title Cynosura seu Mariana Stella Polaris (i.e. "Cynosure, or the Marian Polar Star"), a collection of Marian poetry published by Nicolaus Lucensis (Niccolo Barsotti de Lucca) in 1655. In 2022 Polaris' mean declination

2116-471: Was used as indicating the northern direction for the purposes of navigation by the Phoenicians . The ancient name of Ursa Minor, anglicized as cynosure , has since itself become a term for "guiding principle" after the constellation's use in navigation. Alpha Ursae Minoris (Polaris) was described as ἀειφανής (transliterated as aeiphanes ) meaning "always above the horizon", "ever-shining" by Stobaeus in

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