Rotation Period and Planetary Day: The Other Planets
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.