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 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 Terms | Astronomical Measurement | Conversion of Universal Time (UT) to Civil Time |
