Discovery of precession
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Precession of the equinoxes is caused by a change in the direction of the Earth's rotation axis in space. As a result of this motion, the stars as seen from Earth appear to change position slowly. The equinoctial points, where the ecliptic crosses the celestial equator, appear to "precess" through the zodiac. Also, the celestial poles appear to trace circles in the sky. The Earth goes through one complete precession cycle in a period of approximately 25,800 years, so the discovery of precession requires astronomical observations over many centuries.
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[edit] Priority Questions and the Discovery of Precession
The discovery of precession is usually attributed to Hipparchus of Rhodes or Nicaea, a Greek astronomer who was active in the 2nd century BCE. 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), and it was explained in Isaac Newton's Philosophiae Naturalis Principia Mathematica (1687) as a consequence of gravitation (Evans 1998, p. 246).
Various claims have been made that other cultures discovered precession independent of Hipparchus. At one point it was suggested that the Babylonians may have known about precession. According to al-Battani, Chaldean astronomers had distinguished the tropical and sidereal year. He stated that they had, around 330 BC, an estimation for the length of the sidereal year to be SK = 365 days 6 hours 11 min (= 365.258 days) with an error of (about) 2 min. It was claimed by P. Schnabel in 1923 that Kidinnu theorized about precession in 315 BC (Neugebauer, O. "The Alleged Babylonian Discovery of the Precession of the Equinoxes," Journal of the American Oriental Society, Vol. 70, No. 1. (Jan. - Mar., 1950), pp. 1-8.) Neugebauer's work on this issue in the 1950s superseded Schnabel's (and earlier, Kugler's) theory of a Babylonian discoverer of precession.
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 or even cared what was going on. For instance, from the Middle Kingdom on they used a table with entries for each month to tell the time of night from the passing of constellations: these went in error after a few centuries because of their calendar and precession, but were copied (with scribal errors) for long after they lost their practical usefulness. 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, which suggest they were not assiduous observers of the sky (Parker).
Identifying alignments of monuments with solar, lunar, and stellar phenomena is a major part of archaeoastronomy. Stonehenge is the most famous of many 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 Egyptian claim is theoretically possible (it is true that monuments are sometimes oriented towards stars) but it is not supported by textual evidence.
Yu Xi (fourth century CE) was the first Chinese astronomer to mention precession. He identified the rate of precession as 1° in 50 years (Pannekoek 1961, p. 92).
[edit] 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 BCE), 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 tell us about 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.
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 BCE, and March 21, 135 BCE (Toomer 1984, p. 135 n. 14).
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 365.2467 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, may have been allowing for errors in observation.
A third work On Intercalary Months and Days resulted from Hipparchus' work on precession. It described a lunisolar calendar that gave an average value for the length of the year close to his measurement of the tropical year. The Hipparchic cycle calendar was not influential.
[edit] The 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 BCE 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.
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 BCE.
[edit] Later Studies of Precession
The first astronomer known to have continued Hipparchus' work on precession is Ptolemy in the 2nd century CE. 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.
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 CE, who accepted Ptolemy's explanation. Theon also reports an alternate theory:
- 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).
[edit] References
- 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.
