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Astronomical Constants

Light-year (distance traveled by light in one year) 5,880,000,000,000 mi
Parsec (parallax of one second, or stellar distances) 3.259 light-years
Velocity of light c. 186,282.4 mi/sec
Astronomical unit (A.U.), or mean distance Earth to Sun ca. 93,000,000 mi1
Mean distance, Earth to Moon 238,860 mi
General precession 50',.26
Obliquity of the ecliptic 23° 27'8'.26-0'.4684(t-1900)2
Equatorial radius of Earth 3963.34 statute mi
Polar radius of Earth 3949.99 statute mi
Earth's mean radius 3958.89 statute mi
Oblateness of Earth 1/297
Equatorial horizontal parallax of the moon 57'2'.70
Earth's mean velocity in orbit 18.5 mi/sec
Sidereal year 365d.2564
Tropical year 365d.2422
Sidereal month 27d.3217
Synodic month 29d.5306
Mean sidereal day 23h56m4s.091 of mean solar time
Mean solar day 24h3m56s.555 of sidereal time
1. Actual mean distance derived from radar bounces: 92,935,700 mi. The value of 92,897,400 mi (based on parallax of 8".80) is used in calculations.
2. t refers to the year in question, for example, 2003.

Astronomical TermsAstronomical MeasurementConversion of Universal Time (UT) to Civil Time

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