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Chapter 15: Precession

Difficulty Level: At Grade Created by: CK-12

The priests of ancient Babylonia and Egypt were also pioneer astronomers. They studied the heavens, mapped their constellations, identified the path of the Sun and estimated the periods of the Moon and Sun as they moved across the sky.

But it was a Greek astronomer, Hipparchus of Nicea, who made the first major new discovery in astronomy. Comparing observations more than a century apart, Hipparchus proposed that the axis around which the heavens seemed to rotate shifted gradually, though very slowly.

Viewed from Earth, the Sun moves around the ecliptic, one full circuit each year. Twice a year, at equinox, day and night are equal and the Sun rises exactly in the east and sets exactly in the west. Ancient astronomers had no good clocks and could not tell when the day and night had the same length, but they could identify the equinox by the Sun rising exactly in the east and setting exactly in the west. At those times the Sun's position is at one of the intersections between the ecliptic and the celestial equator.

Around the year 130 BC, Hipparchus compared ancient observations to his own and concluded that in the preceding 169 years those intersections had moved by 2 degrees. How could Hipparchus know the position of the Sun among the stars so exactly, when stars are not visible in the daytime? By using not the Sun but the shadow cast by the Earth on the moon, during an eclipse of the Moon! During an eclipse, Sun, Earth and Moon form a straight line, and therefore the center of the Earth's shadow is at the point on the celestial sphere which is exactly opposite that of the Sun.

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Difficulty Level:

At Grade



Date Created:

Feb 27, 2012

Last Modified:

May 08, 2015
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