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precession of the equinoxes

(Encyclopedia)CE5 Precession of the equinoxes (the points at which the earth's celestial equator intersects its ecliptic) is due to the slow rotation of the earth's axis around a perpendicular to the ecliptic. ...

nutation

(Encyclopedia)nutation, in astronomy, a slight wobbling motion of the earth's axis. The causes of nutation are similar to those of the precession of the equinoxes, involving the varying attraction of the moon on th...

precession

(Encyclopedia)precession: see gyroscope.

sidereal year

(Encyclopedia)sidereal year, time required for the earth to complete an orbit of the sun relative to the stars. The sidereal year is 365 days, 6 hr, 9 min, 9.5 sec of mean solar time (see solar time). It is 20 min ...

dog days

(Encyclopedia)dog days, name for the most sultry period of summer, from about July 3 to Aug. 11. Named in early times by observers in countries bordering the Mediterranean, the period was reckoned as extending from...

Draco, in astronomy

(Encyclopedia)Draco [Lat.,=the dragon], northern constellation lying SE of Ursa Minor and N of Lyra and Hercules. It is traditionally depicted as a dragon. Draco contains the bright star Eltanin (Gamma Draconis). T...

equinox

(Encyclopedia)equinox ēˈkwĭnŏks [key], either of two points on the celestial sphere where the ecliptic and the celestial equator intersect. The vernal equinox, also known as “the first point of Aries,” is t...

Pisces

(Encyclopedia)Pisces pīˈsēz [key] [Lat.,=the fishes], constellation lying directly S of Andromeda and on the ecliptic (the sun's apparent path through the heavens) between Aries and Aquarius; it is one of the co...

Cancer, in astronomy

(Encyclopedia)Cancer [Lat.,=the crab], in astronomy, constellation lying on the ecliptic (the sun's apparent path through the heavens) between Gemini and Leo; it is a constellation of the zodiac. It contains the st...

Hipparchus, Greek astronomer

(Encyclopedia)Hipparchus, fl. 2d cent. b.c., Greek astronomer, b. Nicaea, Bithynia. He is the first systematic astronomer of whom there are records. He made his observations chiefly on the island of Rhodes. Ptolemy...