386. Who was the “Newton of Antiquity”?
This title has been given to Hipparchus, who flourished in the second century B. C. He was the most celebrated of the Greek astronomers. He calculated the length of the year to within six minutes, discovered the precession of the equinoxes, and made the first catalogue of the stars, 1081 in number.
387. What are the eight motions of the earth?
1. Diurnal rotation on its axis.
2. Annual revolution in its orbit.
3. Precession of the equinoxes, which requires 25,816 years for the equinoctial points to make a complete revolution of the ecliptic.
4. Change of perihelion. In the year 4089 B. C. the earth was in perihelion at the autumnal equinox. It is now in perihelion on the 1st of January. In the year 17267 A. D., the long cycle of 21,356 years will be completed, and for the first time since the creation of man the autumnal equinox will coincide with the earth’s perihelion.
5. Change in the obliquity of the ecliptic. The orbit of the earth vibrates backward and forward, each oscillation requiring a period of about 10,000 years.
6. Nutation caused by the moon. This movement requires eighteen and three fourths years for completing a revolution.
7. Planetary perturbations.