Astronomical Constants

Updated August 27, 2019 | Factmonster Staff
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 lightc. 186,282.4 mi/sec
Astronomical unit (A.U.), or mean distance Earth to Sunca. 93,000,000 mi1
Mean distance, Earth to Moon238,860 mi
General precession50',.26
Obliquity of the ecliptic23° 27'8'.26-0'.4684(t-1900)2
Equatorial radius of Earth3963.34 statute mi
Polar radius of Earth3949.99 statute mi
Earth's mean radius3958.89 statute mi
Oblateness of Earth1/297
Equatorial horizontal parallax of the moon57'2'.70
Earth's mean velocity in orbit18.5 mi/sec
Sidereal year365d.2564
Tropical year365d.2422
Sidereal month27d.3217
Synodic month29d.5306
Mean sidereal day23h56m4s.091 of mean solar time
Mean solar day24h3m56s.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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