Sidereal Years to Days (yr (sid) to d)
1 sidereal year equals 365.256363 days exactly.
How to convert sidereal years to days
To convert sidereal years to days, multiply the number of sidereal years by 365.256363. This factor is exact. To go the other way, multiply days by 0.002737803092 (rounded).
days = sidereal years × 365.256363
Example: 10 sidereal years = 3,652.56363 days.
Months and years are not constant. This site uses the average Gregorian year of 365.2425 days, so one 'year' converts to 365.2425 days, not 365; use 'common year' for exactly 365 days.
Approximate value. Standards for the sidereal year varied by place and period, so this converter uses the most commonly cited value. Treat results as estimates, not exact figures.
About these units
Sidereal Year (yr (sid))
About 365.256363 days, the time for Earth to return to the same position relative to the fixed stars.
Measured against the fixed stars, one orbit of Earth around the Sun takes about 365.256363 days, or roughly 365 days 6 hours 9 minutes 10 seconds. That is the sidereal year, and it is about 20 minutes 25 seconds longer than the mean tropical year used for the seasons.
The gap exists because of the precession of the equinoxes, which Hipparchus identified in 127 BC after comparing the two kinds of year. Ptolemy later reported his sidereal figure as about 365 days 6 hours 10 minutes, already close to the modern value. Earlier peoples had tracked the year by star events instead, such as the Egyptian watch for the rising of Sirius before the Nile flood, and Hesiod's farming advice tied to the Pleiades.
Indian astronomy kept the sidereal year in its calendars. The Surya Siddhanta gave a value near 365 days 6 hours 12 minutes 36.56 seconds, slightly long by modern measurement. NASA's fact sheet lists the sidereal orbital period of Earth as 365.256 days.
1 sidereal year = 31,558,149.76 seconds (approximately).
Sources: Wikipedia: Sidereal year; NASA Earth Fact Sheet
Day (d)
24 hours or 86,400 seconds. The mean solar day; the actual rotation of the Earth is about 4 minutes shorter (see sidereal day).
Old English dæg is the root of our day, with close cousins in German Tag, Dutch dag and Icelandic dagur, all descending from Proto-Germanic dagaz. It is the oldest unit here, because the cycle of light and dark needs no instrument. Yet the day we count is a constructed average: the actual interval from one noon to the next varies through the year by up to about 7.9 seconds, because Earth's orbit is elliptical and its axis is tilted.
Astronomers smooth this out with a fictitious mean Sun, giving the mean solar day, and the day in the SI sense is fixed at exactly 86,400 seconds. Reality has drifted slightly: the mean solar day has recently averaged about 86,400.002 seconds, and tidal drag from the Moon is lengthening it by about 2 milliseconds per century. That is why 27 leap seconds were added to UTC between 1972 and 2022. One rotation measured against the distant stars is shorter, about 23 hours 56 minutes 4.09 seconds, and a year contains one more of these sidereal days than solar days.
1 day = 86,400 seconds (exactly).
Sources: Wikipedia: Day; BIPM: SI Brochure, 9th edition
Related conversions
- Days to sidereal years (reverse)
- Sidereal years to seconds
- Sidereal years to minutes
- Sidereal years to hours
- Sidereal years to weeks
- Sidereal years to months
- Sidereal years to years
- Seconds to days
- Minutes to days
- Hours to days
- Weeks to days
- Months to days
- Years to days
- Milliseconds to days
- Microseconds to days
- Nanoseconds to days
- Decades to days
- Centuries to days
- Millennia to days
See all time units.