When you think of a day, you normally think of one cycle of daytime to nighttime. That is called a solar day. On Earth, a solar day is around 24 hours. However, Earthâs orbit is elliptical, meaning itâs not a perfect circle. That means some solar days on Earth are a few minutes longer than 24 hours and some are a few minutes shorter. Another way to measure a day is to count the amount of time it takes for a planet to completely spin around and make one full rotation. This is called a sidereal day. On Earth, a sidereal day is almost exactly 23 hours and 56 minutes. We know how long an Earth day is, but how about the other planets in our solar system?