Years to Sidereal Days (yr to d (sid))
1 year equals 366.242501 sidereal days (rounded).
How to convert years to sidereal days
To convert years to sidereal days, multiply the number of years by 366.242501. The factor is rounded to 10 significant digits. To go the other way, multiply sidereal days by 0.002730431333 (rounded).
sidereal days = years × 366.242501
Example: 10 years = 3,662.42501 sidereal 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 day varied by place and period, so this converter uses the most commonly cited value. Treat results as estimates, not exact figures.
About these units
Year (yr)
The average Gregorian calendar year of 365.2425 days (31,556,952 seconds). A common year has 365 days and a leap year 366.
A year is the Earth's trip around the Sun, but there are several versions. The tropical year, which tracks the seasons, is about 365.2422 days. The sidereal year, measured against the stars, is about 365.2564 days, roughly 20 minutes longer, because the Earth's axis precesses. Calendars must squeeze these into whole days, which is the origin of leap years. English year descends from Old English gear, while Latin annus, the source of annual, comes from a different root.
The Gregorian calendar, introduced by Pope Gregory XIII's bull of 24 February 1582, fixed the problem of Julian drift that had moved the March equinox away from 21 March and upset the date of Easter. Ten days were skipped, so Thursday 4 October was followed by Friday 15 October. Its rule of skipping leap days in century years not divisible by 400 gives a mean year of 365.2425 days, the value this site uses for the year. Britain and its empire switched in 1752 and Russia in 1918. Scientists write the year as a, with the Julian year fixed at 31,557,600 seconds.
1 year = 31,556,952 seconds (exactly).
Sources: Wikipedia: Year; Wikipedia: Gregorian calendar
Sidereal Day (d (sid))
About 86,164.09 seconds (23 h 56 min 4 s), one rotation of the Earth relative to the stars.
Sidereal comes from the Latin word for star, and the unit measures how long Earth takes to turn once relative to the distant stars rather than to the Sun. The figure used here is about 86,164.09 seconds, or 23 hours 56 minutes 4.09 seconds. NASA's Earth fact sheet gives the same period as 23.9345 hours.
The sidereal day is about four minutes shorter than the mean solar day because Earth also moves roughly one degree along its orbit during each rotation, so it must turn slightly further before the Sun is back on the meridian. Over a year this adds up to one extra turn, giving about 366.24 sidereal days against 365.24 solar days.
For centuries astronomers measured it by timing stars as they crossed the meridian and checking observatory clocks against star catalogues. From the 1970s, radio methods such as very long baseline interferometry and pulsar timing proved more precise, and since 2003 the Earth Rotation Angle has replaced the older sidereal time conventions in formal use.
1 sidereal day = 86,164.0905 seconds (approximately).
Sources: Wikipedia: Sidereal time; NASA Earth Fact Sheet
Related conversions
- Sidereal days to years (reverse)
- Years to seconds
- Years to minutes
- Years to hours
- Years to days
- Years to weeks
- Years to months
- Years to milliseconds
- Years to microseconds
- Years to nanoseconds
- Years to decades
- Years to centuries
- Years to millennia
- Seconds to sidereal days
- Minutes to sidereal days
- Hours to sidereal days
- Days to sidereal days
- Weeks to sidereal days
- Months to sidereal days
See all time units.