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--- plantary_ephemerides [2010/03/25 00:14] – created ong
+++ plantary_ephemerides [2010/03/28 21:40] (current) – external edit 127.0.0.1
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 Because the planets all approximately lie on the ecliptic plane, latitude was generally not important in measurements. However because they orbit around the sun, rather than earth, at varying speeds, their motion is much more complicated than the moon's. Most notable are the turning points at which the planets change their direction for a period of time (called the regression period) from eastward to westward, after which they resume their eastward movement.
-The Babylonians observed that the motion of the planets was periodic, and hence so were the planetary phenomena they wished to record. They therefore were interested in determining the length of these periods, which could be used to predict future occurrences.
+The Babylonians observed that the motion of the planets was periodic, and hence so were the planetary phenomena they wished to record. They therefore were interested in determining the length of these periods, which could be used to predict future occurrences. Some of these periods could be ascertained empirically by watching the night sky over a long stretch of time.
+Often, however, the period for the event concerned was too long, or too subtle for direct observation. In this case, the Babylonians calculated the period length, and then based on the recorded date for the event on a particular occasion, could predict the dates of future occurrences.
+As with the moon, the Babylonians conducted observations of the planets with reference to the ecliptic(link). The planets moved along the ecliptic through the years in the same direction of the sun, and eventually completed one trip around it. After this they would exhibit the same phenomena at the same times as the previous cycle. However for a stretch of their orbit the planets would appear to change directions against the sun, in a phase called retrogression. Thus the Babylonians determined the beginning and ending points of each planet's regression. Also, as with the new moon the planets could be in a position relative to the earth such that sunlight obscured their presence in the sky. For the 'outer' planets (that is, Mars, Jupiter, and Saturn), there was only one such phase of invisibility in the (fundamental?) period, and the Babylonians determined the beginning and ending points of this phenomena as well. The inner planets (Mercury and Venus) had two such phases of disappearance, one occurring during the time the planet was seen as an 'evening star', and the other when it was seen as a 'morning star'. For all planets the Babylonians determined the times of opposition as well.