How sidereal and solar days differ
A sidereal day measures one full rotation of Earth relative to the distant stars. A solar day measures the time it takes the Sun to return to the same point in the sky, from noon to noon. Because Earth moves along its orbit while spinning, it has to turn a bit further each day to face the Sun again, which makes the solar day longer.
1 sidereal day = 23 h 56 m 4.0905 s = 86,164.0905 s
1 solar day = 24 h = 86,400 s
1 sidereal day = 0.99726957 solar days
Worked example
To convert 3 sidereal days to solar days, multiply by 0.99726957: 3 × 0.99726957 = 2.99181 solar days, about 11.8 minutes short of three full 24-hour days.
Where sidereal time is used
- Telescope tracking: equatorial mounts turn at the sidereal rate, one revolution every 23 h 56 m 4 s, to keep stars centered.
- Finding objects: local sidereal time equals the right ascension currently crossing your meridian, so astronomers use it to know what is overhead.
- Geostationary satellites: their orbital period equals one sidereal day, which keeps them fixed over one spot on the equator.
- Observing plans: stars rise about 3 minutes 56 seconds earlier each night, roughly 2 hours earlier each month, which is why the night sky changes with the seasons.
Sidereal and solar days on other worlds
Venus is the odd one out: it spins backwards so slowly that its solar day is shorter than its sidereal day.
Mean solar day vs apparent solar day
The 24-hour day is a mean solar day. The actual time from one solar noon to the next changes slightly through the year because Earth's orbit is elliptical and its axis is tilted. The accumulated difference, called the equation of time, makes sundial noon run up to about 16 minutes ahead of or behind clock noon.
Frequently asked questions
What is the difference between a sidereal day and a solar day?
A sidereal day (one rotation relative to the stars) is about 23 h 56 m 4 s, while a solar day (noon to noon relative to the Sun) is 24 h. The solar day is roughly 4 minutes longer.
How long is a sidereal day in solar time?
About 0.99727 mean solar days, or 23 hours, 56 minutes and 4.0905 seconds.
Why is a sidereal day shorter than a solar day?
Earth orbits the Sun as it spins, so it must rotate about 1 extra degree (roughly 4 minutes) to bring the Sun back to the same position, making the solar day longer.
How many solar days are in a sidereal day?
Exactly 0.99726957 solar days, slightly less than one.
How many sidereal days are in a year?
About 366.25 sidereal days versus 365.25 solar days, because Earth gains one extra rotation relative to the stars per orbit.
How much earlier do stars rise each night?
About 3 minutes 56 seconds, the difference between a solar and a sidereal day. Over a month that adds up to roughly 2 hours.
What is local sidereal time?
The right ascension of the stars currently on your meridian. When local sidereal time is 5h 35m, the Orion Nebula (RA 5h 35m) is due south from mid-northern latitudes.