Editing Sidereal time (section)

==Sidereal days compared to solar days on other planets==
Six of the eight solar [[planet]]s have [[retrograde and prograde motion|prograde]] rotation—that is, they rotate more than once per year in the same direction as they orbit the Sun, so the Sun rises in the east.{{Sfn|Bakich|2000}} [[Venus]] and [[Uranus]], however, have [[retrograde motion|retrograde]] rotation. For prograde rotation, the formula relating the lengths of the sidereal and solar days is:
{{block indent|1=number of sidereal days per orbital period = 1 + number of solar days per orbital period}}

or, equivalently:
{{block indent|1=length of solar day = {{sfrac|length of sidereal day|1 − {{sfrac|length of sidereal day|orbital period}}}}.}}

When calculating the formula for a retrograde rotation, the operator of the denominator will be a plus sign (put another way, in the original formula the length of the sidereal day must be treated as negative). This is due to the solar day being shorter than the sidereal day for retrograde rotation, as the rotation of the planet would be against the direction of orbital motion.

If a planet rotates prograde, and the sidereal day exactly equals the orbital period, then the formula above gives an infinitely long solar day ([[division by zero]]). This is the case for a planet in [[tidal locking|synchronous rotation]]; in the case of zero eccentricity, one hemisphere experiences eternal day, the other eternal night, with a "twilight belt" separating them.

All the solar planets more distant from the Sun than Earth are similar to Earth in that, since they experience many rotations per revolution around the Sun, there is only a small difference between the length of the sidereal day and that of the solar day – the ratio of the former to the latter never being less than Earth's ratio of 0.997. But the situation is quite different for [[Mercury (planet)|Mercury]] and Venus. Mercury's sidereal day is about two-thirds of its orbital period, so by the prograde formula its solar day lasts for two revolutions around the Sun – three times as long as its sidereal day. Venus rotates retrograde with a sidereal day lasting about 243.0 Earth days, or about 1.08 times its orbital period of 224.7 Earth days; hence by the retrograde formula its solar day is about 116.8 Earth days, and it has about 1.9 solar days per orbital period.

By convention, rotation periods of planets are given in sidereal terms unless otherwise specified.