What is the Difference Between Sidereal Day and Solar Day?

The difference between a sidereal day and a solar day lies in the reference points used to measure the time it takes for a planet to rotate.

• Sidereal Day: A sidereal day is the time it takes for a planet to rotate once relative to the stars. It represents the planet's true rotation period relative to the rest of the universe. For example, Earth takes about 23 hours and 56 minutes to complete a sidereal day.
• Solar Day: A solar day is the time it takes for a planet to rotate once relative to the Sun. It is the time between successive sunrises. Due to the planet's orbital motion, a solar day is slightly longer than a sidereal day. For Earth, a solar day is about 4 minutes longer than a sidereal day.

The difference between sidereal and solar days is more noticeable for planets with longer days and more eccentric orbits, such as Mercury. Mercury's sidereal day is 59 Earth days long, while its solar day is much longer. For Earth and Mars, the difference between sidereal and solar days is smaller, with Martian sidereal and solar days being about 24 hours and 37 minutes and 24 hours and 40 minutes, respectively.

Comparative Table: Sidereal Day vs Solar Day

Here is a table comparing the differences between a sidereal day and a solar day:

In summary, a sidereal day is the time it takes for Earth to rotate once around its axis, while a solar day is the time it takes for the Sun to return to the same position in the sky. A solar day is longer than a sidereal day because it accounts for both the Earth's axial rotation and its orbit around the Sun.