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What Is the Sidereal Period of Planets?

In the deep vacuum of space, there is no sound, yet the planets move around their orbits as if to a tempo. There is no music, yet they spin around their axes as if to a dance. The combined choreography of these movements causes a discrepancy in the time required for a planet to complete one rotation with respect to the sun and the time required to complete a rotation with respect to the fixed stars. Scientists call the former a synodic period, or a solar day, and the later is called a sidereal period. The sidereal period corresponds to the amount of time it takes for the planet to rotate exactly 360 degrees and is unaffected by the planet's orbit around the sun.

Mercury
- Mercury's sidereal period is 58.6467 days. Yet at the end of this period, Mercury is still in the dark. It takes another 117.2933 days for the sun to rise. This makes Mercury's nights and days twice as long as its year. Part of the reason for this unusual phenomenon is that Mercury is the closest planet to the sun, which makes its orbit much smaller than that of any other planet. Another factor is its elliptical orbit, which brings it 20 percent closer to the sun than usual at certain times in the year. This causes such an increase in orbital speed, that it surpasses its rotational speed and makes the sun appear to move backwards for several days.
Venus
- Venus rotates backwards in respect to most planets. Its sidereal period is 243.02 days, but instead of the constellations rising in the east and setting in the west, the opposite occurs. Unlike Mercury, it's solar day is shorter than its sidereal period, lasting only 116.75 earth days. This causes the sun to rise twice during some sidereal periods.
Earth
- Earth's sidereal and synodic periods are very similar. One earth day is 24 hours. This means that if a location on earth is facing towards the sun at noon on Monday it will also be facing the sun at noon on Tuesday. However, the exact time it takes for the Earth to rotate on its axis is 23 hours 56 minutes and 4.1 seconds. So while many people describe a day as one rotation around the Earth's axis, it is actually slightly longer than the rotation period.
The Inner Planets
- The sidereal and synodic periods of most inner planets differ only by minutes or seconds. The sidereal period of Mars is 24 hours, 37 minutes and 22.66 seconds. Jupiter's is the shortest of all the planets at 9 hours, 55 minutes and 30 seconds, with Saturn following closely behind at 10 hours, 32 minutes and 35 seconds. Uranus, like Venus, has a backward rotation that lasts 17 hours, 14 minutes and 24 seconds. Neptune's sidereal period is almost identical to its synodic period, which both last 16 hours and 6.6 minutes. Pluto, although no longer considered a planet, also spins on its axis, with a sidereal period of 6 days, 9 hours and 17.6 minutes.

Astronomy