FACTOID # 11: Oklahoma has the highest rate of women in State or Federal correctional facilities.

 Home Encyclopedia Statistics States A-Z Flags Maps FAQ About

 WHAT'S NEW

SEARCH ALL

Search encyclopedia, statistics and forums:

(* = Graphable)

Encyclopedia > Precession (astronomy)

The precession of Earth's axis of rotation with respect to inertial space is also called the precession of the equinoxes. Like a wobbling top, the direction of the Earth's axis is changing; while today, the North Pole points roughly to Polaris, over time it will change. Because of this wobble, the position of the earth in its orbit around the sun at the moment of the equinoxes and solstices will also change. Precession of a gyroscope Precession refers to a change in the direction of the axis of a rotating object. ... This article is about Earth as a planet. ... In physics, the expression inertial space refers to the background reference that is provided by the phenomenon of inertia. ... The two celestial poles are the imaginary points where the Earths spin axis intersects the imaginary rotating sphere of gigantic radius, called the celestial sphere. ... Polaris (Î± UMi / Î± Ursae Minoris / Alpha Ursae Minoris), more commonly known as The North Star or simply North Star, is the brightest star in the constellation Ursa Minor. ... Illumination of the Earth by the Sun on the day of equinox, (ignoring twilight). ... -1...

The term precession typically refers only to the largest periodic motion. Other changes of Earth's axis are nutation and polar motion; their magnitude is very much smaller. Rotation (green), Precession (blue) and Nutation (red) of the Earth Nutation is a slight irregular motion (etymologically a nodding) in the axis of rotation of a largely axially symmetric object, such as a gyroscope or a planet. ... This article needs to be wikified. ...

Currently, this annual motion is about 50.3 seconds of arc per year or 1 degree every 71.6 years. The process is slow, but cumulative. A complete precession cycle covers a period of approximately 25,765 years, the so called Platonic year, during which time the equinox regresses a full 360° through all twelve constellations of the zodiac. Precessional movement is also the determining factor in the length of an astrological age. A Great year (also known as a Platonic year or Equinoctial cycle) is the time required for one complete cycle of the precession of the equinoxes: about 25700 years. ... Position of vernal equinox occurring in Pisces after leaving Aries constellation (through the precession of the equinoxes backward motion). ...

In ancient times the precession of the equinox referred to the motion of the equinox relative to the background stars in the zodiac; this is equivalent to the modern understanding. It acted as a method of keeping time in the Great Year.[citation needed]Hipparchus is credited with discovering that the positions of the equinoxes move westward along the ecliptic compared to the fixed stars on the celestial sphere. The exact dates of his life are not known, but astronomical observations attributed to him date from 147 BC to 127 BC and were described in his writings, none of which survive to date. The fixed stars (from the Latin stellae fixae) are celestial objects that do not seem to move in relation to the other stars of the night sky. ... A Great year (also known as a Platonic year or Equinoctial cycle) is the time required for one complete cycle of the precession of the equinoxes: about 25700 years. ... For the Athenian tyrant, see Hipparchus (son of Pisistratus). ... The plane of the ecliptic is well seen in this picture from the 1994 lunar prospecting Clementine spacecraft. ... A fixed star is a celestial object that does not seem to move (in comparison to the other stars of the night sky). ... The celestial sphere is divided by the celestial equator. ... Astronomy, which etymologically means law of the stars, (from Greek: &#945;&#963;&#964;&#961;&#959;&#957;&#959;&#956;&#943;&#945; = &#940;&#963;&#964;&#961;&#959;&#957; + &#957;&#972;&#956;&#959;&#962;) is a science involving the observation and explanation of events occurring outside Earth and its atmosphere. ... Observation basically means watching something and taking note of anything it does. ... Centuries: 3rd century BC - 2nd century BC - 1st century BC Decades: 190s BC 180s BC 170s BC 160s BC 150s BC - 140s BC - 130s BC 120s BC 110s BC 100s BC 90s BC Years: 152 BC 151 BC 150 BC 149 BC 148 BC - 147 BC - 146 BC 145 BC... Centuries: 3rd century BC - 2nd century BC - 1st century BC Decades: 170s BC 160s BC 150s BC 140s BC 130s BC - 120s BC - 110s BC 100s BC 90s BC 80s BC 70s BC Years: 132 BC 131 BC 130 BC 129 BC 128 BC - 127 BC - 126 BC 125 BC...

Though there is still-controversial evidence that Aristarchus of Samos possessed distinct values for the sidereal and tropical years as early as ca. 280 BC, the discovery of precession is usually attributed to Hipparchus of Rhodes or Nicaea, a Greek astronomer who was active in the 2nd century BC. Virtually all Hipparchus' writings are lost, including his work on precession. They are mentioned in Ptolemy's Almagest, where precession is explained as the rotation of the celestial sphere around a motionless Earth. It is reasonable to assume that Hipparchus, like Ptolemy, thought of precession in geocentric terms as a motion of the heavens. The first definite reference to precession as the result of a motion of the Earth's axis is Nicolaus Copernicus's De revolutionibus orbium coelestium (1543). He called precession the third motion of the earth. Over a century later it was explained in Isaac Newton's Philosophiae Naturalis Principia Mathematica (1687) to be a consequence of gravitation (Evans 1998, p. 246). However, Newton's original precession equations did not work and were revised considerably by Jean le Rond d'Alembert and subsequent scientists. Statue of Aristarchus at Aristotle University in Thessalonica, Greece Aristarchus (310 BC - ca. ... For the Athenian tyrant, see Hipparchus (son of Pisistratus). ... Deer statues in Mandraki harbor, where the Colossus of Rhodes once stood This article is about the Greek island of Rhodes. ... Ä°znik (which derives from the former Greek name ÎÎ¯ÎºÎ±Î¹Î±, Nicaea) is a city in Turkey which is known primarily as the site of the First and Second Councils of Nicaea, the first and seventh Ecumenical councils in the early history of the Christian church, the Nicene Creed, and as the capital... A recreation of the famous Library of Alexandria Greek astronomy is the astronomy of those who spoke Greek in classical antiquity. ... A medieval artists rendition of Claudius Ptolemaeus Claudius Ptolemaeus (Greek: ; ca. ... Almagest is the Latin form of the Arabic name (al-kitabu-l-mijisti, i. ... The celestial sphere is divided by the celestial equator. ... The geocentric model (in Greek: geo = earth and centron = centre) of the universe is a paradigm which places the Earth at its center. ... â€œCopernicusâ€ redirects here. ... Nicolai Copernici Torinensis De Revolutionibus Orbium Coelestium, Libri VI - On the Revolutions of the Heavenly Spheres, by Nicolaus Copernicus of Torin, Six Books (title page of 2nd edition, Basel, 1566) De revolutionibus orbium coelestium (English: ), first printed in 1543 in Nuremberg, is the seminal work on heliocentric theory and the... Sir Isaac Newton (4 January 1643 â€“ 31 March 1727) [ OS: 25 December 1642 â€“ 20 March 1726][1] was an English physicist, mathematician, astronomer, natural philosopher, and alchemist. ... Newtons own copy of his Principia, with handwritten corrections for the second edition. ... â€œGravityâ€ redirects here. ... Jean le Rond dAlembert, pastel by Maurice Quentin de La Tour Jean le Rond dAlembert (November 16, 1717 â€“ October 29, 1783) was a French mathematician, mechanician, physicist and philosopher. ...

Similar claims have been made that precession was known in Ancient Egypt prior to the time of Hipparchus. Some buildings in the Karnak temple complex, for instance, were allegedly oriented towards the point on the horizon where certain stars rose or set at key times of the year. A few centuries later, when precession made the orientations obsolete, the temples would be rebuilt. Note however that the observation that a stellar alignment has grown wrong does not necessarily mean that the Egyptians understood that the stars moved across the sky at the rate of about one degree per 72 years. Nonetheless, they kept accurate calendars and if they recorded the date of the temple reconstructions it would be a fairly simple matter to plot the rough precession rate. The Dendera Zodiac, a star-map from the Hathor temple at Dendera from a late (Ptolemaic) age, supposedly records precession of the equinoxes (Tompkins 1971). In any case, if the ancient Egyptians knew of precession, their knowledge is not recorded in surviving astronomical texts. Khafres Pyramid (4th dynasty) and Great Sphinx of Giza (c. ... Map of Karnak, showing major temple complexes Interior of Temple First pylon of precinct of Amun viewed from the west Al-Karnak (Arabic Ø§Ù„ÙƒØ±Ù†Ùƒ, in Ancient Egypt was named Ipet Sut, the most venerated place) is a small village in Egypt, located on the banks of the River Nile some 2. ... Entrance to the Dendera Temple Complex Dendera Temple complex, (Ancient Egyptian: Iunet or Tantere). ... Hathor Temple is the main temple in the Dendera Temple Complex, built around 1st century BC. Hathor Temple, photographed 23rd December 2003 Categories: Buildings and structures stubs | Ancient Egypt stubs ... Entrance to the Dendera Temple Complex, photographed 23rd December 2003 Dendera (also spelled Denderah), is a little town in Egypt. ...

The former professor of the history of science at MIT, Giorgio de Santillana, argues in his book, Hamlet's Mill, that many ancient cultures may have known of the slow movement of the stars across the sky; the observable result of the precession of the equinox. This 700 page book, co-authored by Hertha von Dechend, makes reference to approximately 200 myths from over 30 ancient cultures that hinted at the motion of the heavens, some of which are thought to date to the neolithic period. MapÃºa Institute of Technology (MIT, MapÃºaTech or simply MapÃºa) is a private, non-sectarian, Filipino tertiary institute located in Intramuros, Manila. ... Hamlets Mill, by Giorgio de Santillana and Hertha von Dechend, is a nonfiction work of history and comparative mythology (particularly the subfield of archaeoastronomy), similar to Joseph Campbells The Masks of God; its essential premise is that much mythology and ancient literature has been badly misinterpreted and that... An array of Neolithic artifacts, including bracelets, axe heads, chisels, and polishing tools. ...

Identifying alignments of monuments with solar, lunar, and stellar phenomena is a major part of archaeoastronomy. Stonehenge is the most famous of many megalithic structures that indicate the direction of celestial objects at rising or setting. Precession complicates the attempt to find stellar alignments, especially for very old sites. Many archaeological sites cannot be dated exactly, making it difficult or impossible to know whether a proposed alignment would have worked when the site was founded. The sun rising over Stonehenge at the 2005 Summer Solstice. ... For other uses, see Stonehenge (disambiguation). ...

Yu Xi (fourth century CE) was the first Chinese astronomer to mention precession. He estimated the rate of precession as 1° in 50 years (Pannekoek 1961, p. 92). (3rd century - 4th century - 5th century - other centuries) As a means of recording the passage of time, the 4th century was that century which lasted from 301 to 400. ... This article or section is in need of attention from an expert on the subject. ...

## Hipparchus' discovery

Hipparchus gave an account of his discovery in On the Displacement of the Solsticial and Equinoctial Points (described in Almagest III.1 and VII.2). He measured the ecliptic longitude of the star Spica during lunar eclipses and found that it was about 6° west of the autumnal equinox. By comparing his own measurements with those of Timocharis of Alexandria (a contemporary of Euclid who worked with Aristillus early in the 3rd century BC), he found that Spica's longitude had decreased by about 2° in about 150 years. He also noticed this motion in other stars. He speculated that only the stars near the zodiac shifted over time. Ptolemy called this his "first hypothesis" (Almagest VII.1), but did not report any later hypothesis Hipparchus might have devised. Hipparchus apparently limited his speculations because he had only a few older observations, which were not very reliable. Longitude is the east-west geographic coordinate measurement most commonly utilized in cartography and global navigation. ... Spica (Î± Vir / Î± Virginis / Alpha Virginis) is the brightest star in the constellation Virgo, and one of the brightest stars in the nighttime sky. ... Illumination of Earth by Sun on the day of equinox The autumnal equinox (or fall equinox) marks the beginning of astronomical autumn. ... Timocharis of Alexandria (ca. ... Euclid (Greek: ), also known as Euclid of Alexandria, was a Greek mathematician of the Hellenistic period who flourished in Alexandria, Egypt, almost certainly during the reign of Ptolemy I (323 BC-283 BC). ... For the crater, see Aristillus (crater). ...

Why did Hipparchus need a lunar eclipse to measure the position of a star? The equinoctial points are not marked in the sky, so he needed the Moon as a reference point. Hipparchus had already developed a way to calculate the longitude of the Sun at any moment. A lunar eclipse happens during Full moon, when the Moon is in opposition. At the midpoint of the eclipse, the Moon is precisely 180° from the Sun. Hipparchus is thought to have measured the longitudinal arc separating Spica from the Moon. To this value, he added the calculated longitude of the Sun, plus 180° for the longitude of the Moon. He did the same procedure with Timocharis' data (Evans 1998, p. 251). Observations like these eclipses, incidentally, are the main source of data about when Hipparchus worked, since other biographical information about him is minimal. The lunar eclipses he observed, for instance, took place on April 21, 146 BC, and March 21, 135 BC (Toomer 1984, p. 135 n. 14). Time lapse movie of the 3 March 2007 lunar eclipse A lunar eclipse occurs whenever the Moon passes through some portion of the Earths shadow. ... Composite image of the Moon as taken by the Galileo spacecraft on 7 December 1992. ... Opposition is a term used in positional astronomy and astrology to indicate when one celestial body is on the opposite side of the sky when viewed from a particular place (usually the Earth). ...

Hipparchus also studied precession in On the Length of the Year. Two kinds of year are relevant to understanding his work. The tropical year is the length of time that the Sun, as viewed from the Earth, takes to return to the same position along the ecliptic (its path among the stars on the celestial sphere). The sidereal year is the length of time that the Sun takes to return to the same position with respect to the stars of the celestial sphere. Precession causes the stars to change their longitude slightly each year, so the sidereal year is longer than the tropical year. Using observations of the equinoxes and solstices, Hipparchus found that the length of the tropical year was 365+1/4−1/300 days, or 365.24667 days (Evans 1998, p. 209). Comparing this with the length of the sidereal year, he calculated that the rate of precession was not less than 1° in a century. From this information, it is possible to calculate that his value for the sidereal year was 365+1/4+1/144 days (Toomer 1978, p. 218). By giving a minimum rate he may have been allowing for errors in observation. A tropical year is the length of time that the Sun, as viewed from the Earth, takes to return to the same position along the ecliptic (its path among the stars on the celestial sphere). ... The Sun (Latin: Sol) is the star at the center of the Solar System. ... The sidereal year is the time for the Sun to return to the same position in respect to the stars of the celestial sphere. ...

To approximate his tropical year Hipparchus created his own lunisolar calendar by modifying those of Meton and Callippus in On Intercalary Months and Days (now lost), as described by Ptolemy in the Almagest III.1 (Toomer 1984, p. 139). The Babylonian calendar used a cycle of 235 lunar months in 19 years since 499 BC (with only three exceptions before 380 BC), but it did not use a specified number of days. The Metonic cycle (432 BC) assigned 6,940 days to these 19 years producing an average year of 365+1/4+1/76 or 365.26316 days. The Callippic cycle (330 BC) dropped one day from four Metonic cycles (76 years) for an average year of 365+1/4 or 365.25 days. Hipparchus dropped one more day from four Callipic cycles (304 years), creating the Hipparchic cycle with an average year of 365+1/4−1/304 or 365.24671 days, which was close to his tropical year of 365+1/4−1/300 or 365.24667 days. The three Greek cycles were never used to regulate any civil calendar—they only appear in the Almagest in an astronomical context. A lunisolar calendar is a calendar whose date indicates both the moon phase and the time of the solar year. ... Meton of Athens was a mathematician, astronomer and engineer who lived in Athens in the 5th century BCE. He is best known for the 19-year Metonic Cycle which he introduced into the Athenian luni-solar calendar as a method of calculating dates. ... Calippus of Syracuse Callippus (or Calippus) (ca. ... A medieval artists rendition of Claudius Ptolemaeus Claudius Ptolemaeus (Greek: ; ca. ... In the Babylonian calendar a year consisted of 12 lunar months, each beginning when a new crescent moon was first sighted low on the western horizon at sunset. ... The Metonic cycle or Enneadecaeteris in astronomy and calendar studies is a particular approximate common multiple of the year (specifically, the seasonal tropical year) and the synodic month. ... Eclipses may occur repeatedly, separated by some specific interval of time: this interval is called an eclipse cycle. ... Eclipses may occur repeatedly, separated by some specific interval of time: this interval is called an eclipse cycle. ...

### Mithraic question

Mithraism was a mystery religion based on the worship of the god Mithras. It was popular in the Roman Empire from about the 1st century BC to the 5th century CE. Understanding Mithraism has been made difficult by the near-total lack of written descriptions or scripture; the religion must be reconstructed from iconography found in mithraea (a mithraeum was a cave or underground temple sacred to Mithras). Until the 1970s most scholars followed Franz Cumont in identifying Mithras with the Persian god Mithra. Cumont's thesis was re-examined in 1971, and Mithras is now believed to be a syncretic deity only slightly influenced by Persian religion. This article or section is in need of attention from an expert on the subject. ... This does not cite any references or sources. ... Mithras and the Bull: fresco from the mithraeum at Marino, Italy, (3rd century AD) Mithras was the central god of Mithraism, a syncretic Hellenistic mystery religion of male initiates that developed in the Eastern Mediterranean in the 2nd and 1st centuries BC and was practiced in the Roman Empire from... Motto Senatus Populusque Romanus (SPQR) The Roman Empire at its greatest extent. ... A mithraeum found in the ruins of Ostia Antica, Italy. ... Franz-ValÃ©ry-Marie Cumont (Aalst, Belgium, January 3, 1868 - Brussels, August 25, 1947) was a Belgian archaeologist and historian, a philologist and student of epigraphy, who brought these often isolated specialties to bear on the syncretic mystery religions of Late Antiquity, notably Mithraism. ... Mithra (Avestan MiÎ¸ra, modern Persian Ù…Ù‡Ø± Mihr, Mehr, Meher) is an important deity or divine concept (so called Yazata) in Zoroastrianism and later Persian mythology and culture. ... Syncretism is the attempt to reconcile disparate, even opposing, beliefs and to meld practices of various schools of thought. ...

The iconography of Mithraism is now recognized as having pronounced astrological elements, but the details are debated. One scholar of Mithraism, David Ulansey, has proposed that the cult was inspired by Hipparchus' discovery of precession. The centerpiece of his analysis is the tauroctony an image of Mithras sacrificing a bull. According to Ulansey, the tauroctony is a star chart. Mithras is the constellation Perseus, and the bull is Taurus, a constellation of the zodiac. In an earlier astrological age, the vernal equinox had taken place when the Sun was in Taurus. The tauroctony, by this reasoning, commemorated Mithras-Perseus ending the "Age of Taurus" about 2000 BC. Hand-coloured version of the anonymous Flammarion woodcut (1888). ... A tauroctony was the depiction of Mithras ritually slaying a bull, that is a taurobolium. ... A star chart is a map of the night sky. ... Perseus is a northern constellation, named after the Greek hero who slew the monster Medusa. ... Taurus (IPA: , Latin: , symbol , ) is one of the constellations of the zodiac. ... Position of vernal equinox occurring in Pisces after leaving Aries constellation (through the precession of the equinoxes backward motion). ...

## Later studies of precession

The first astronomer known to have continued Hipparchus' work on precession is Ptolemy in the 2nd century. Ptolemy measured the longitudes of Regulus, Spica, and other bright stars with a variation of Hipparchus' lunar method that did not require eclipses. Before sunset, he measured the longitudinal arc separating the Moon from the Sun. Then, after sunset, he measured the arc from the Moon to the star. He used Hipparchus' model to calculate the Sun's longitude, and made corrections for the Moon's motion and its parallax (Evans 1998, pp. 251-255). Ptolemy compared his own observations with those made by Hipparchus, Menelaus of Alexandria, Timocharis, and Agrippa. He found that between Hipparchus' time and his own (about 265 years), the stars had moved 2°40', or 1° in 100 years (36" per year; the rate accepted today is about 50" per year or 1° in 72 years). He also confirmed that precession affected all fixed stars, not just those near the ecliptic. Regulus (Î± Leo / Î± Leonis / Alpha Leonis) is the brightest star in the constellation Leo and one of the brightest stars in the nighttime sky. ... This does not adequately cite its references or sources. ... Menelaus of Alexandria (c. ... Timocharis of Alexandria (ca. ... For other people named Agrippa, see Agrippa. ...

Most ancient authors did not mention precession and perhaps did not know of it. Besides Ptolemy, the list includes Proclus, who rejected precession, and Theon of Alexandria, a commentator on Ptolemy in the 4th century, who accepted Ptolemy's explanation. Theon also reports an alternate theory: This article is about Proclus Diadochus, the Neoplatonist philosopher. ... Theon (c. ...

According to certain opinions ancient astrologers believe that from a certain epoch the solstitial signs have a motion of 8° in the order of the signs, after which they go back the same amount. . . . (Dreyer 1958, p. 204)

Instead of proceeding through the entire sequence of the zodiac, the equinoxes "trepidated" back and forth over an arc of 8°. The theory of trepidation is presented by Theon as an alternative to precession. In the Middle Ages, Islamic and Latin Christian astronomers treated it as a motion of the fixed stars to be added to precession. This theory is commonly attributed to the Arab astronomer Thabit ibn Qurra, but the attribution has been contested in modern times. Nicolaus Copernicus published a different account of trepidation in De revolutionibus orbium coelestium (1543). Trepidation (from Lat. ... This is a sub-article of Islamic science and astronomy. ... Languages Arabic other minority languages Religions Predominantly Sunni Islam, as well as Shia Islam, Greek Orthodoxy, Greek Catholicism, Roman Catholicism, Alawite Islam, Druzism, Ibadi Islam, and Judaism Footnotes a Mainly in Antakya. ... Abul Hasan Thabit ibn Qurra ibn Marwan al-Sabi al-Harrani, (826 â€“ February 18, 901) was an Arab astronomer and mathematician. ... â€œCopernicusâ€ redirects here. ... Nicolai Copernici Torinensis De Revolutionibus Orbium Coelestium, Libri VI - On the Revolutions of the Heavenly Spheres, by Nicolaus Copernicus of Torin, Six Books (title page of 2nd edition, Basel, 1566) De revolutionibus orbium coelestium (English: ), first printed in 1543 in Nuremberg, is the seminal work on heliocentric theory and the...

## Changing pole stars

Precession of Earth's axis around the north ecliptical pole
Precession of Earth's axis around the south ecliptical pole

On the other hand, Thuban in the constellation Draco, which was the pole star in 3000 BC, is much less conspicuous at magnitude 3.67 (one-fifth as bright as Polaris); today it is invisible in light-polluted urban skies. Thuban (Î± Dra / Î± Draconis / Alpha Draconis) is a star (or star system) in the constellation of Draco. ... This article does not cite any references or sources. ... Draco (IPA: , Latin: ) is a far northern constellation that is circumpolar for many northern hemisphere observers. ... // Ceremonial temple butcher knife made of flint, with the Horus name of the pharaoh Djer inscribed on its gold handle. ...

The brilliant Vega in the constellation Lyra is often touted as the best north star (it fulfilled that role around 12000 BC and will do so again around the year AD 14000), however it never comes closer than 5° to the pole. Vega (Î± Lyr / Î± Lyrae / Alpha Lyrae) is the brightest star in the constellation Lyra, the fifth brightest star in the sky and the second brightest star in the Northern celestial hemisphere, after Arcturus. ... For other uses, see Lyra (disambiguation). ...

When Polaris becomes the north star again around 27800 AD, due to its proper motion it then will be farther away from the pole than it is now, while in 23600 BC it came closer to the pole. The proper motion of a star is the motion of the position of the star in the sky (the change in direction in which we see it, as opposed to the radial velocity) after eliminating the improper motions of the stars, which affect their measured coordinates but are not real...

It is more difficult to find the south celestial pole in the sky at this moment, as that area is a particularly bland portion of the sky, and the nominal south pole star is Sigma Octantis, which with magnitude 5.5 is barely visible to the naked eye even under ideal conditions. That will change from the eightieth to the ninetieth centuries, however, when the south celestial pole travels through the False Cross. Sigma Octantis (Ïƒ Oct / Ïƒ Octantis) is a magnitude 5. ... Vela (IPA: , Latin: ) is a southern constellation, one of the four parts into which Argo Navis was split (the others being Carina, Puppis and Pyxis). ...

This situation also is seen on a star map. The orientation of the south pole is moving toward the Southern Cross constellation. For the last 2,000 years or so, the southern Cross has nicely pointed to by the south pole. By consequence, the constellation is no longer visible from subtropical northern latitudes, as it was in the time of the ancient Greeks. Ancient Greece is the term used to describe the Greek_speaking world in ancient times. ...

## Polar shift and equinoxes shift

Precessional movement as seen from 'outside' the celestial sphere
Same picture as above, but now from (near) Earth perspective

The rotation axis of the Earth describes, over a period of 25,700 years, a small circle (blue) among the stars, centered around the ecliptic north pole (the blue E) and with an angular radius of about 23.4°, an angle known as the obliquity of the ecliptic. The direction of precession is opposite to the daily rotation of the Earth on its axis. The orange axis was the Earth's rotation axis 5,000 years ago, when it pointed to the star Thuban. The yellow axis, pointing to Polaris, marks the axis now. The ecliptic coordinate system is a celestial coordinate system that uses the ecliptic for its fundamental plane. ... The Obliquity of the ecliptic is the angle between the plane of the Earths equator and the ecliptic plane in which the Earth rotates around the Sun. ...

The equinoxes occur where the celestial equator intersects the ecliptic (red line), that is, where the Earth's axis is perpendicular to the line connecting the centers of the Sun and Earth. When the axis precesses from one orientation to another, the equatorial plane of the Earth (indicated by the circular grid around the equator) moves. The celestial equator is just the Earth's equator projected onto the celestial sphere, so it moves as the Earth's equatorial plane moves, and the intersection with the ecliptic moves with it. The positions of the poles and equator on Earth do not change, only the orientation of the Earth against the fixed stars.

As seen from the orange grid, 5,000 years ago, the vernal equinox was close to the star Aldebaran of Taurus. Now, as seen from the yellow grid, it has shifted (indicated by the red arrow) to somewhere in the constellation of Pisces. Illumination of Earth by Sun on the day of equinox The vernal equinox (or spring equinox) marks the beginning of astronomical spring. ... Aldebaran from the Arabic (Ø§Ù„Ø¯Ø¨Ø±Ø§Ù† al-dabarÄn) meaning the follower, (Î± Tau / Î± Tauri / Alpha Tauri) is the brightest star in the constellation Taurus and one of the brightest stars in the nighttime sky. ... Taurus (IPA: , Latin: , symbol , ) is one of the constellations of the zodiac. ... For other uses, see Pisces. ...

Still pictures like these are only first approximations as they do not take into account the variable speed of the precession, the variable obliquity of the ecliptic, the planetary precession (whose center lies on a circle about 6° away from the poles) and the proper motions of the stars.

## Explanation

The precession as a consequence of the torque exerted on Earth by differential gravitation

The precession of the equinoxes is caused by the differential gravitational forces of the Sun and the Moon on the Earth. Image File history File links Size of this preview: 800 Ã— 600 pixels Full resolution (2000 Ã— 1500 pixel, file size: 206 KB, MIME type: image/jpeg) File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Precession (astronomy) ... Image File history File links Size of this preview: 800 Ã— 600 pixels Full resolution (2000 Ã— 1500 pixel, file size: 206 KB, MIME type: image/jpeg) File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Precession (astronomy) ... The Sun (Latin: Sol) is the star at the center of the Solar System. ... Apparent magnitude: up to -12. ...

In popular science books, precession is often explained with the example of a spinning top. While the physical effect is the same, some crucial details differ. For a spinning top, gravity causes the top to wobble, which in turn causes precession. The applied force in this case is parallel to the rotation axis. For the Earth, however, the applied forces of the Sun and the Moon are perpendicular to the axis of rotation.

The Sun and the Moon pull on the equatorial bulge; due to its own rotation, the Earth is not a perfect sphere but an oblate spheroid, with an equatorial diameter about 43 kilometers larger than its polar diameter. If the Earth were a perfect sphere, there would be no precession. An oblate spheroid is ellipsoid having a shorter axis and two equal longer axes. ...

The figure below explains how this process works. (Viewing the diagram at its maximum resolution is recommended.) The Earth is given as a perfect sphere with the mass of the bulge approximated by a blue torus around its equator. The green arrows indicate the gravitational forces from the Sun on some extreme points. These forces are not parallel, as they all point toward the center of the Sun. Therefore, the forces working on the northernmost and southernmost parts of the equatorial bulge have a component perpendicular to the ecliptical plane and a component directed parallel to it. The parallel component is centripetal force for the Earth in its orbit around the Sun. The perpendicular components are shown as cyan arrows tangential to the Earth's surface. These tangential forces create a torque (orange), and this torque, added to the rotation (magenta), shifts the rotational axis to a slightly new position (yellow). Over time, the axis precesses along the white circle, which is centered around the ecliptic pole. Torque applied via an adjustable end wrench Relationship between force, torque, and momentum vectors in a rotating system In physics, torque (or often called a moment) can informally be thought of as rotational force or angular force which causes a change in rotational motion. ...

This torque is always in the same direction, perpendicular to the direction in which the rotation axis is tilted away from the ecliptic pole, so that it does not change the axial tilt itself. The magnitude of the torque from the sun (or the moon) varies with the gravitational object's alignment with the earth's spin axis and approaches zero when it is orthogonal.

Although the above explanation involved the Sun, the same explanation holds true for any object moving around the Earth, along or close to the ecliptic, notably, the Moon. The combined action of the Sun and the Moon is called the lunisolar precession. In addition to the steady progressive motion (resulting in a full circle in 25,700 years) the Sun and Moon also cause small periodic variations, due to their changing positions. These oscillations, in both precessional speed and axial tilt, are known as the nutation. The most important term has a period of 18.6 years and an amplitude of less than 20 seconds of arc. Rotation (green), Precession (blue) and Nutation (red) of the Earth Nutation is a slight irregular motion (etymologically a nodding) in the axis of rotation of a largely axially symmetric object, such as a gyroscope or a planet. ...

In addition to lunisolar precession, the actions of the other planets of the solar system cause the whole ecliptic to rotate slowly around an axis which has an ecliptic longitude of about 174° measured on the instantaneous ecliptic. This planetary precession shift is only 0.47 seconds of arc per year (more than a hundred times smaller than lunisolar precession), and takes place along the instantaneous equator.

The sum of the two precessions is known as the general precession.

## History

Precession causes the cycle of seasons (tropical year) to be about 20.4 minutes less than the time for the Earth to return to the same position with respect to the stars. This results in a slow change (one day every 71 calendar years) in the position of the Sun with respect to the stars at an equinox. A tropical year is the length of time that the Sun, as viewed from the Earth, takes to return to the same position along the ecliptic (its path among the stars on the celestial sphere). ...

The steady westward shift of the vernal equinox among the stars is evident over the millennia

Hipparchus estimated the Earth's precession around 130 BC, adding his own observations to those of Babylonian astronomers in the preceding centuries. In particular, they measured the distance of stars such as Spica to the Moon and the Sun during lunar eclipses, and because he could compute the distance of the Moon and the Sun from the equinox at these moments, he noticed that Spica and other stars appeared to have moved over the centuries. Image File history File links Equinox_positions. ... Image File history File links Equinox_positions. ... Centuries: 3rd century BC - 2nd century BC - 1st century BC Decades: 180s BC 170s BC 160s BC 150s BC 140s BC - 130s BC - 120s BC 110s BC 100s BC 90s BC 80s BC Years: 135 BC 134 BC 133 BC 132 BC 131 BC - 130 BC - 129 BC 128 BC... STARS can mean: Shock Trauma Air Rescue Society Special Tactics And Rescue Service, a fictional task force that appears in Capcoms Resident Evil video game franchise. ... Spica (Î± Vir / Î± Virginis / Alpha Virginis) is the brightest star in the constellation Virgo, and one of the brightest stars in the nighttime sky. ... An eclipse occurs whenever the Sun, Earth and Moon line up exactly. ...

It remains controversial as to whether the ancient Egyptians knew of the Precession or not. Michael Rice wrote in his Egypt's Legacy, "Whether or not the ancients knew of the mechanics of the Precession before its definition by Hipparchos the Bithynian in the second century BC is uncertain, but as dedicated watchers of the night sky they could not fail to be aware of its effects." (p. 128) Rice believes that "the Precession is fundamental to an understanding of what powered the development of Egypt" (p. 10), to the extent that "in a sense Egypt as a nation-state and the king of Egypt as a living god are the products of the realisation by the Egyptians of the astronomical changes effected by the immense apparent movement of the heavenly bodies which the Precession implies." (p. 56) Following Carl Gustav Jung, Rice says that "the evidence that the most refined astronomical observation was practised in Egypt in the third millennium BC (and probably even before that date) is clear from the precision with which the Pyramids at Giza are aligned to the cardinal points, a precision which could only have been achieved by their alignment with the stars. This fact alone makes Jung's belief in the Egyptians' knowledge of the Precession a good deal less speculative than once it seemed." (p. 31) The Egyptians also, says Rice, were "to alter the orientation of a temple when the star on whose position it had originally been set moved its position as a consequence of the Precession, something which seems to have happened several times during the New Kingdom." (p. 170) see also Royal Arch and the Precession of the Equinoxes Map of Ancient Egypt Ancient Egypt was the civilization of the Nile Valley between about 3000 BC and the conquest of Egypt by Alexander the Great in 332 BC. As a civilization based on irrigation it is the quintessential example of an hydraulic empire. ... Carl Gustav Jung Carl Gustav Jung (July 26, 1875 &#8211; June 6, 1961) was a Swiss psychiatrist and founder of the neopsychoanalytic school of psychology. ...

## Anomalistic precession

Effects of axial precession on the seasons (source)

Because the anomalistic year is longer than the sidereal year while the tropical year (which calendars attempt to track) is shorter, the two forms of precession add. It takes about 21,000 years for the ellipse to revolve once relative to the vernal equinox, that is, for the perihelion to return to the same date (given a calendar that tracks the seasons perfectly). The dates of perihelion and of aphelion advance each year on this cycle, an average of 1 day per 58 years.

This interaction between the anomalistic and tropical cycle is important in the long-term climate variations on Earth, called the Milankovitch cycles. An equivalent is also known on Mars. Variations in CO2, temperature and dust from the Vostok ice core over the last 400 000 years For the animated movie, see Ice Age (movie). ... Milankovitch cycles are the collective effect of changes in the Earths movements upon its climate, named after Serbian civil engineer and mathematician Milutin MilankoviÄ‡. The eccentricity, axial tilt, and precession of the Earths orbit vary in several patterns, resulting in 100,000 year ice age cycles of the... This article presents information and images about viewing astronomical phenomena from the planet Mars. ...

Notice in the above figure that the areas swept during a specific season changes through time. Orbital mechanics require that the length of the seasons be proportional to the swept areas of the seasonal quadrants, so when the orbital eccentricity is extreme, the seasons on the far side of the orbit may be substantially longer in duration. (This page refers to eccitricity in astrodynamics. ...

## Values

Simon Newcomb's calculation at the end of the nineteenth century for general precession (known as p) in longitude gave a value of 5,025.64 arcseconds per tropical century, and was the generally accepted value until artificial satellites delivered more accurate observations and electronic computers allowed more elaborate models to be calculated. Lieske developed an updated theory in 1976, where p equals 5,029.0966 arcseconds per Julian century. Modern techniques such as VLBI and LLR allowed further refinements, and the International Astronomical Union adopted a new constant value in 2000, and new computation methods and polynomial expressions in 2003 and 2006; the accumulated precession is: Simon Newcomb. ... This article needs cleanup. ... LLR can stand for Lunar Laser Ranging Log Likelihood Ratio ... Logo of the IAU The International Astronomical Union (French: Union astronomique internationale) unites national astronomical societies from around the world. ...

pA = 5,028.796195×T + 1.1054348×T2 + higher order terms,

in arcseconds per Julian century, with T, the time in Julian centuries (that is, 36,525 days) since the epoch of 2000. The J2000. ...

The rate of precession is the derivative of that:

p = 5,028.796195 + 2.2108696×T + higher order terms

The constant term of this speed corresponds to one full precession circle in 25,772 years.

The precession rate is not a constant, but (at the moment) slowly increasing over time, as indicated by the linear (and higher order) terms in T. In any case it must be stressed that this formula is only valid over a limited time period. It is clear that if T gets large enough (far in the future or far in the past), the T² term will dominate and p will go to very large values. In reality, more elaborate calculations on the numerical model of solar system show that the precessional constants have a period of about 41,000 years, the same as the obliquity of the ecliptic. Note that the constants mentioned here are the linear and all higher terms of the formula above, not the precession itself. That is, p = A + BT + CT² + … is an approximation of p = A + Bsin (2πT/P), where P is the 410-century period. A Solar System Numerical Model A Law of Motion for Entities in a Solar System Numerical Model The solar system may be modeled numerically using both Newtons Law of Gravitation and Newtons Second Law of Motion. ...

Theoretical models may calculate the proper constants (coefficients) corresponding to the higher powers of T, but since it is impossible for a polynomial to match a periodic function over all numbers, the error in all such approximations will grow without bound as T increases. In that respect, the International Astronomical Union chose the best developed available theory. For up to a few centuries in the past and the future, all formulas do not diverge very much. For up to a few thousand years in the past and the future, most agree to some accuracy. For eras farther out, discrepancies become too large - the exact rate and period of precession may not be computed, even for a single whole precession period.

The precession of Earth's axis is a very slow effect, but at the level of accuracy at which astronomers work, it does need to be taken into account on a daily basis. Note that although the precession and the tilt of Earth's axis (the obliquity of the ecliptic) are calculated from the same theory and thus, are related to each other, the two movements act independently of each other, moving in mutually perpendicular directions.

Over longer time periods, that is, millions of years, it appears that precession is quasiperiodic at around 25,700 years, however, it will not remain so. According to Ward, when the distance of the Moon, which is continuously increasing from tidal effects, will have gone from the current 60.3 to approximately 66.5 Earth radii in about 1,500 million years, resonances from planetary effects will push precession to 49,000 years at first, and then, when the Moon reaches 68 Earth radii in about 2,000 million years, to 69,000 years. This will be associated with wild swings in the obliquity of the ecliptic as well. Ward, however, used the abnormally large modern value for tidal dissipation. Using the 620-million year average provided by tidal rhythmites of about half the modern value, these resonances will not be reached until about 3,000 and 4,000 million years, respectively. Long before that time (about 2,100 million years from now), due to the increasing luminosity of the Sun, however, the oceans of the Earth will have boiled away, which will alter tidal effects significantly. It has been suggested that Tidal friction be merged into this article or section. ...

## References

• Explanatory supplement to the Astronomical ephemeris and the American ephemeris and nautical almanac
• Precession and the Obliquity of the Ecliptic has a comparison of values predicted by different theories
• A.L. Berger (1976), "Obliquity & precession for the last 5 million years", Astronomy & astrophysics 51, 127
• J.H. Lieske et al. (1977), "Expressions for the Precession Quantities Based upon the IAU (1976) System of Astronomical Constants". Astronomy & Astrophysics 58, 1..16
• W.R. Ward (1982), "Comments on the long-term stability of the earth's obliquity", Icarus 50, 444
• J.L. Simon et al. (1994), "Numerical expressions for precession formulae and mean elements for the Moon and the planets", Astronomy & Astrophysics 282, 663..683
• N. Capitaine et al. (2003), "Expressions for IAU 2000 precession quantities", Astronomy & Astrophysics 412, 567..586
• J.L. Hilton et al. (2006), "Report of the International Astronomical Union Division I Working Group on Precession and the Ecliptic" (pdf, 174KB). Celestial Mechanics and Dynamical Astronomy (2006) 94: 351..367
• Rice, Michael (1997), Egypt's Legacy: The archetypes of Western civilization, 3000-30 BC, London and New York.
• Dreyer, J. L. E.. A History of Astronomy from Thales to Kepler. 2nd ed. New York: Dover, 1953.
• Evans, James. The History and Practice of Ancient Astronomy. New York: Oxford University Press, 1998.
• Pannekoek, A. A History of Astronomy. New York: Dover, 1961.
• Parker, Richard A. "Egyptian Astronomy, Astrology, and Calendrical Reckoning." Dictionary of Scientific Biography 15:706-727.
• Tomkins, Peter. Secrets of the Great Pyramid. With an appendix by Livio Catullo Stecchini. New York: Harper Colophon Books, 1971.
• Toomer, G. J. "Hipparchus." Dictionary of Scientific Biography. Vol. 15:207-224. New York: Charles Scribner's Sons, 1978.
• Toomer, G. J. Ptolemy's Almagest. London: Duckworth, 1984.
• Ulansey, David. The Origins of the Mithraic Mysteries: Cosmology and Salvation in the Ancient World. New York: Oxford University Press, 1989.

John Louis Emil Dreyer (February 13, 1852 &#8211; September 14, 1926) was a Danish-Irish astronomer. ... The Secret Life of Plants Cover Published in 1973, The Secret Life of Plants was written by Peter Tompkins and Christopher Bird. ...

Share your thoughts, questions and commentary here