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.
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.