Mars, the fourth planet in the solar system, is about half the size of Earth, it's half again as far from the sun and its year is almost twice as long. The length of its day, however, is not much different. It varies by less than an hour.

### Length of the Martian Day

As viewed from the stars, Mars takes 24 hours and 37 minutes to complete a rotation. This is called a sidereal day, which is a little shorter than a solar day, which is the time it takes for the sun to return to the same position in the sky, as viewed by an observer on the surface. A solar day in Mars is 24 hours and 39 minutes long.

### Comparison With Earth

Because Earth is nearly twice as large as Mars, there's a four-minute difference between its sidereal and solar days. A solar day is 24 hours, but a sidereal day is 23 hours and 56 minutes. In terms of solar days, the Martian day is 39 minutes longer than Earth's, but in terms of sidereal days, the Martian day is 41 minutes longer.