What is the Difference Between Rotation Period (Sidereal Day) and Planetary Day?

Each planet in the solar system rotates around an imaginary line through its center called the axis. Any measurement of a planet's movement has to be made relative to another object (or collection of objects), so the time it takes for a planet to complete a rotation depends on what this relative object is.

Astronomers measure every planet's rotation relative to two different things: the sun and the collection of background stars. For all practical purposes, the background stars are so far away they can be considered fixed points. A planet's rotation period, also called its sidereal day, is the amount of time required for it to complete one rotation about its axis relative to the background stars.

As the planet rotates, it also moves along its orbit, so its position relative to the sun changes. To complete a rotation relative to the sun, it requires a little longer because of this change of position. This period is called the planetary day or solar day.

To an observer on Earth, as the year passes, the stars appear to rotate once around the heavens. This is a result of the Earth's motion along its orbit around the sun. The period from noon (the sun's apparent high point above the Earth) on one day to noon on the next day is exactly 24 hours — one solar day. But the period between the apparent high point for any particular star from one night to the next is slightly under 24 hours — about 23 hours 56 minutes, one sidereal day. A sidereal day, in other words, is four minutes shorter than a solar day.

So to an earthbound observer, the stars appear to rotate around the sky slightly faster than the sun does. This difference in apparent speed accounts for the one apparent rotation of the stars in a year - the change of the constellations from season to season.

Click here to view an animation demonstrating the difference between a sidereal day and a solar day.

For each of the outer planets, there is only a slight difference between the rotation period and the planetary day. Their orbits are so large, and their rotation so fast, that the planet travels only a tiny distance along its orbit for each rotation, so the solar day is only a tiny fraction larger than the sidereal day.

For Mercury, the innermost planet, the difference is much greater. Mercury rotates slowly on its axis and has a small orbit. Unique among the planets, it has a neat, 3:2 ratio between its rotation period (58.65 Earth days) and its orbital period, or year (88 Earth days). Its planetary day is as long as two of its years. Therefore, the planetary (solar) day on Mercury is three times as long as the rotation period (sidereal day).

The situation with Venus' rotation is even more peculiar. Venus rotates very slowly, and its motion is retrograde — opposite the direction of the Earth's rotation. Venus is the only planet whose rotation period is actually longer than its planetary day: 5832.5 hours (rotation period) versus 2802 hours (planetary day). To indicate the retrograde rotation, the length of the rotation period is sometimes given as a negative number.

NASA's Planetary Fact Sheet