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The solar day is the length of time for a position on a rotating body to face the Sun again, on Earth, noon to noon at some given location, i.e., what we normally call a day. The term is used for other bodies such as solar system planets and moons. Examples:
| Body | approx. solar day |
| Mercury | 176 Earth days |
| Venus | 117 Earth days |
| Earth | 24 hours |
| Mars | 24 hours 40 minutes |
| Jupiter | 10 hours |
| Saturn | 10.5 hours |
| Uranus | 17 hours |
| Neptune | 16 hours |
| Pluto | 6.4 Earth days |
| Ceres | 9 hours |
| Moon | 29.5 Earth days |
| Titan | 16 Earth days |
Mars researchers often use the term Sol for the solar day of Mars.
In contrast, a body's sidereal day is the length of time it takes some given location on the body to face the same point within the celestial sphere, basically when it faces the same stars again. Given that a the direction that faces the Sun from a solar system body constantly changes as the body orbits, its solar day generally differs from its sidereal day. The percentage difference is small if the body has numerous solar days in its year: the difference for Earth is about 4 minutes. The two solar system planets with significantly differing solar and sidereal days are Mercury (176 versus 59 Earth days) and Venus (117 versus 243 Earth days).