A solar day is defined as the time it takes for the Sun to appear in the same position in the sky. This is slightly longer than the time it takes for the Earth to complete one full rotation on its axis, which is called a sidereal day.

The difference between a solar day and a sidereal day is due to the Earth's orbit around the Sun. As the Earth rotates, it also moves a tiny bit in its orbit, so it needs to rotate slightly more than 360 degrees to bring the Sun back to the same position in the sky.