A pole star or polar star is a star, preferably bright, nearly aligned with the axis of a rotating astronomical body.
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 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") 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 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 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.
Precession of the equinoxes
In October 2012, Polaris had the declination +89°19′8″ (at epoch J2000 it was +89°15′51.2″). Therefore, it always 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 equal to the latitude of the observer to better than one degree. The celestial pole will be nearest Polaris in 2100 and will thereafter become more distant.
Due to the precession of the equinoxes (as well as the stars' proper motions), the role of North Star has passed (and will pass) from one star to another in the remote past (and 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 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 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 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 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 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 11,500 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 13,700 AD, though at 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 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 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 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:
|Alpha Ursae Minoris||Polaris||1.98||Ursa Minor||within 0.5°||the current North Star|
|Gamma Cephei||Errai||3.21||Cepheus||within 3°||will become the North Star at about 3,100 AD|
|Iota Cephei||3.51||Cepheus||within 5°||shares timing with Beta Cephei|
|Beta Cephei||Alfirk||3.51||Cepheus||within 5°||will become the North Star at about 5,900 AD|
|Alpha Cephei||Alderamin||2.51||Cepheus||within 3°||will become the North Star at about 7,600 AD|
|Alpha Cygni||Deneb||1.25||Cygnus||within 7°||will become the North Star at about 10,200 AD|
|Delta Cygni||Fawaris||2.87||Cygnus||within 3°||will become the North Star at about 11,600 AD|
|Alpha Lyrae||Vega||0.026||Lyra||within 5°||used to be the North Star at about 11,500 BC; and will become the North Star at 13,700 AD|
|Iota Herculis||3.75||Hercules||within 4°|
|Tau Herculis||3.89||Hercules||within 1°|
|Alpha Draconis||Thuban||3.65||Draco||within 0.2°||used to be the North Star at about 3,000 BC|
|Iota Draconis||Edasich||3.29||Draco||within 5°|
|Kappa Draconis||3.82||Draco||within 6°||a near-north star, shares timing with Kochab|
|Beta Ursae Minoris||Kochab||2.08||Ursa Minor||within 7°||used to be the North Star at about 1,100 BC|
Southern pole star (South Star)
Currently, there is no South Star as useful as Polaris, the so-called North Star. Polaris Australis (Sigma Octantis) is the closest naked-eye star to the south celestial pole, but at apparent magnitude 5.47 it is barely visible on a clear night, making it unusable for navigational 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 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 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 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.
Pole stars of other planets are defined analogously: they are stars (brighter than 6th magnitude, i.e., visible to 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.)
- Alpha Pictoris is the south pole star of Mercury while Omicron Draconis is its north star.
- 42 Draconis is the closest star to the north pole of Venus. Eta¹ Doradus is the closest to the south pole. (Note: The IAU uses the right-hand rule to define a positive pole for the purpose of determining orientation. Using this convention, Venus is tilted 177° ("upside down").)
- The lunar south pole star is Delta Doradus, and the north pole star[note 1] Omicron Draconis.
- Kappa Velorum is only a couple of degrees from the south celestial pole of Mars. The top two stars in the Northern Cross, Sadr and Deneb, point to the north celestial pole of Mars.
- The north pole of Jupiter is a little over two degrees away from Zeta Draconis, while its south pole is about two degrees north of Delta Doradus.
- Delta Octantis is the south pole star of Saturn. Its north pole is in the far northern region of Cepheus, about six degrees from Polaris.
- Eta Ophiuchi is the north pole star of Uranus, and 15 Orionis is its south pole star.
- The north pole of Neptune points to a spot midway between Gamma and Delta Cygni. Its south pole star is Gamma Velorum.
In religion and mythology
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. Bartholomeus Anglicus (d. 1272), in 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 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 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 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, the south is associated with the World of Darkness.
In Japan, the Pole Star was represented by Myōken Bosatsu (妙見菩薩).
In the Greek Magical Papyri, the Pole Star was identified with Set-Typhon.
In Chinese mythology, Emperor Zhuanxu is mentioned as a god of the Pole Star.
- Astronomy on Mars § Celestial poles and ecliptic
- Celestial equator
- Guide star
- Lists of stars
- Due to axial precession, the lunar pole describes a small circle on the celestial sphere every 18.6 years. e.g.Moore, Patrick (1983), The Guinness Book of Astronomy Facts & Feats, p. 29,
In 1968 the north pole star of the Moon was Omega Draconis; by 1977 it was 36 Draconis. The south pole star is Delta Doradus.
- κυνόσουρα. Liddell, Henry George; Scott, Robert; A Greek–English Lexicon at the Perseus Project.
- implied by Johannes Kepler (cynosurae septem stellas consideravit quibus cursum navigationis dirigebant Phoenices): "Notae ad Scaligeri Diatribam de Aequinoctiis" in Kepleri Opera Omnia ed. Ch. Frisch, vol. 8.1 (1870) p. 290
- ἀειφανής in Liddell and Scott.
- Gemmae Frisii de astrolabo catholico liber: quo latissime patentis instrumenti multiplex usus explicatur, & quicquid uspiam rerum mathematicarum tradi possit continetur, Steelsius (1556), p. 20
- Ridpath, Ian (1988). "Chapter Three: The celestial eighty-eight – Ursa Minor". Star Tales. Cambridge: The Lutterworth Press. ISBN 978-0-7188-2695-6.
...in the early 16th century ... Polaris was still around three and a half degrees from the celestial pole ...will reach its closest to the north celestial pole around AD 2100, when the separation will be less than half a degree
- Jean Meeus, Mathematical Astronomy Morsels Ch. 50; Willmann-Bell 1997
- Ridpath, Ian, ed. (2004). Norton's Star Atlas. New York: Pearson Education. p. 5. ISBN 0-13-145164-2.
Around 4800 years ago Thuban (α Draconis) lay a mere 0°.1 from the pole. Deneb (α Cygni) will be the brightest star near the pole in about 8000 years' time, at a distance of 7°
- Moore, Patrick (2005). The Observer's Year: 366 Nights in the Universe. p. 283.
- Kaler, James B., "KOCHAB (Beta Ursae Minoris)", Stars, University of Illinois, retrieved 2018-04-28
- Our Monthly, vol. 4, Presbyterian Magazine Company, 1871, p. 53.
- McClure, Bruce; Deborah, Byrd (2017-09-29). "Gamma Cephei: A future Pole Star". EarthSky. Retrieved 2018-04-25.
- Kaler, James B., "ALDERAMIN (Alpha Cephei)", Stars, University of Illinois, retrieved 2018-04-28
- Kaler, James B., "TAU HER (Tau Herculis)", Stars, University of Illinois, retrieved 2018-04-27
- "Sigma Octantis". Jumk.De. 6 August 2013.
- "The North Star: Polaris". Space.com. May 7, 2012. Retrieved 6 August 2013.
- Hobbs, Trace (May 21, 2013). "Night Sky Near the Equator". Wordpress. Retrieved 6 August 2013.
- "Beta Hydri".
- "Precession". moonkmft.co.uk. Retrieved 24 September 2018.
- Kieron Taylor (1 March 1994). "Precession". Sheffield Astronomical Society. Retrieved 2018-09-24.
- Bruce McClure. "Sirius, future South Pole Star". EarthSky. Retrieved 2018-01-03.
- 2004. Starry Night Pro, Version 5.8.4. Imaginova.ISBN 978-0-07-333666-4. www.starrynight.com
- Archinal, Brent A.; A'Hearn, Michael F.; Bowell, Edward G.; Conrad, Albert R.; Consolmagno, Guy J.; et al. (2010). "Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2009" (PDF). Celestial Mechanics and Dynamical Astronomy. 109 (2): 101–135. Bibcode:2011CeMDA.109..101A. doi:10.1007/s10569-010-9320-4. S2CID 189842666. Archived from the original (PDF) on 2016-03-04. Retrieved 2018-09-06.
- cited after J. O. Halliwell, (ed.), The Works of William Shakespeare vol. 5 (1856), p. 40.]
- Conversations-Lexicon Für Bildende Kunst vol. 7 (1857), 141f.
- A. Maas,"The Name of Mary", The Catholic Encyclopedia (1912)
- stella maris, sive illuminatrix Maria, inter fluctivagas undas pelagi, fide ac moribus sequenda est, ne mergamur undis diluvii PL vol. 120, p. 94.
- Bhayro, Siam (2020-02-10). "Cosmology in Mandaean Texts". Hellenistic Astronomy. Brill. pp. 572–579. doi:10.1163/9789004400566_046. ISBN 9789004243361. S2CID 213438712. Retrieved 2021-09-03.
|Look up pole star in Wiktionary, the free dictionary.|
|Look up Pole Star in Wiktionary, the free dictionary.|
Media files used on this page
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Artist's impression of "the oldest star of our Galaxy": HE 1523-0901
- About 13.2 billion years old
- Approximately 7500 light years far from Earth
- Published as part of Hamburg/ESO Survey in the May 10 2007 issue of The Astrophysical Journal
This is a mosaic image, one of the largest ever taken by NASA's Hubble Space Telescope, of the Crab Nebula, a six-light-year-wide expanding remnant of a star's supernova explosion. Japanese and Chinese astronomers recorded this violent event in 1054 CE, as did, almost certainly, Native Americans.
The orange filaments are the tattered remains of the star and consist mostly of hydrogen. The rapidly spinning neutron star embedded in the center of the nebula is the dynamo powering the nebula's eerie interior bluish glow. The blue light comes from electrons whirling at nearly the speed of light around magnetic field lines from the neutron star. The neutron star, like a lighthouse, ejects twin beams of radiation that appear to pulse 30 times a second due to the neutron star's rotation. A neutron star is the crushed ultra-dense core of the exploded star.
The Crab Nebula derived its name from its appearance in a drawing made by Irish astronomer Lord Rosse in 1844, using a 36-inch telescope. When viewed by Hubble, as well as by large ground-based telescopes such as the European Southern Observatory's Very Large Telescope, the Crab Nebula takes on a more detailed appearance that yields clues into the spectacular demise of a star, 6,500 light-years away.The newly composed image was assembled from 24 individual Wide Field and Planetary Camera 2 exposures taken in October 1999, January 2000, and December 2000. The colors in the image indicate the different elements that were expelled during the explosion. Blue in the filaments in the outer part of the nebula represents neutral oxygen, green is singly-ionized sulfur, and red indicates doubly-ionized oxygen.
Author/Creator: Me, Licence: Copyrighted free use
SVG replacement for File:Spaceship and the Sun.jpg. A stylized illustration of a spaceship and the sun, based on the description of the emblem of the fictional Galactic Empire in Isaac Asimov's Foundation series ("The golden globe with its conventionalized rays, and the oblique cigar shape that was a space vessel"). This image could be used as a icon for science-fiction related articles.
This view of the rising Earth greeted the Apollo 8 astronauts as they came from behind the Moon after the fourth nearside orbit. Earth is about five degrees above the horizon in the photo. The unnamed surface features in the foreground are near the eastern limb of the Moon as viewed from Earth. The lunar horizon is approximately 780 kilometers from the spacecraft. Width of the photographed area at the horizon is about 175 kilometers. On the Earth 240,000 miles away, the sunset terminator bisects Africa.
This is a montage of planetary images taken by spacecraft managed by the Jet Propulsion Laboratory in Pasadena, CA. Included are (from top to bottom) images of Mercury, Venus, Earth (and Moon), Mars, Jupiter, Saturn, Uranus and Neptune. The spacecraft responsible for these images are as follows:
- the Mercury image was taken by Mariner 10,
- the Venus image by Magellan,
- the Earth and Moon images by Galileo,
- the Mars image by Mars Global Surveyor,
- the Jupiter image by Cassini, and
- the Saturn, Uranus and Neptune images by Voyager.
- Pluto is not shown as it is no longer a planet. The inner planets (Mercury, Venus, Earth, Moon, and Mars) are roughly to scale to each other; the outer planets (Jupiter, Saturn, Uranus, and Neptune) are roughly to scale to each other. PIA 00545 is the same montage with Neptune shown larger in the foreground. Actual diameters are given below:
- Sun (to photosphere) 1,392,684 km
- Mercury 4,879.4 km
- Venus 12,103.7 km
- Earth 12,756.28 km
- Moon 3,476.2 km
- Mars 6,804.9 km
- Jupiter 142,984 km
- Saturn 120,536 km
- Uranus 51,118 km
- Neptune 49,528 km
Author/Creator: A. Duro/ESO, Licence: CC BY 4.0
Science and art unite in this beautiful photograph, taken in Chile’s Atacama Desert by ESO Photo Ambassador Adhemar M. Duro Jr. To create this visual masterpiece Adhemar pointed his camera at the sky’s south pole, the point at the centre of all the bright arcs and circles. All the stars in the night sky revolve around this point. Over a period of several hours, this motion creates star trails, with each individual star tracing out a circle on the sky. These trails display the various brightnesses and colours of each star, creating a captivating scene! Towards the top left of the image, you can see a short, bright streak of light cutting across the trails — this is caused by a meteor, burning up in a flash of light as it enters Earth’s atmosphere. The desert’s harsh and arid landscape, illuminated here by the light from the stars themselves, is the perfect place to view the night sky. Because of the location’s favourable conditions several telescopes are hosted here, including ESO’s Very Large Telescope, ESO’s Visible and Infrared Survey Telescope for Astronomy (VISTA), and the forthcoming European Extremely Large Telescope, which is currently under construction atop Cerro Armazones.
Nachtaufnahme vom sommerlichem nördlichen Sternenhimmel. Standort: Ehrenbürg (Walberla), Belichtung ca. 45 min. Aufgenommen auf Farbfilm, mit 50mm
Author/Creator: Fernando da Rosa (Fedaro), Licence: CC BY-SA 3.0
The south celestial pole, using photos taken with the intervalometer. Every shot had an exposure time of 25 seconds, between 22:53 January 24, 2014, until 02:59 of January 25, 2014. Series of shots where you can see the rotation of the Earth's axis relative to the south celestial pole, clearly see the two Magellanic Clouds in the West of the Milky Way, and the Southern Cross. Near the end of the video you can see the rise of the moon that illuminates the scene. The shots were perform with a Nikon D7000 camera with a 10.5 mm lens nikkor f/2.8 stopped down to f: 3.5 exposure 25 seconds at ISO 800
On black field aurora borealis, above of which accompanied by four-spiked star; both silver.