Planet Rotation Speeds

[dmm]

Spoo said:
Typically larger planets rotate faster (at least in the Solar System).

Body ; Sidereal Period ; Synodic Period="Day"
Mercury ; 58.6467 days ; 175.940 days
Venus ; - 243.02 days ; - 116.75 days
Earth ; 23 hr 56 min 4.1 sec ; 24 hr 0 min 0 sec
Moon ; 27.322 days ; 29.53 days
Mars ; 24 hr 37 min 22.66 sec ; 24 hr 39 min 35.24 sec
Jupiter ; 9 hr 55 min 30 sec ; 9 hr 55 min 33 sec
Saturn ; 10 hr 29 min 32 sec ; 10 hr 29 min 33 sec
Uranus ; - 17 hr 14.4 min ; - 17 hr 14.4 min
Neptune ; 16 hr 6.6 min ; 16 hr 6.6 min
Pluto ; - 6 days 9 hr 17.6 min ; - 6 days 9 hr 17.0 min

[MasterChiToes]

The Terrestrial planets can be expected to turn slower due to 'tidal forces' dissipating their rotational angular momentum. If you check out the length of a day on ancient earth, it was much shorter than the Jovian planets.

In general, for a stable climate, I would agree that smaller planets better turn slower.

[Spoo]

MasterChiToes said:
The Terrestrial planets can be expected to turn slower due to 'tidal forces' dissipating their rotational angular momentum. If you check out the length of a day on ancient earth, it was much shorter than the Jovian planets.

In general, for a stable climate, I would agree that smaller planets better turn slower.

I think you would have to make a number of assumptions about the event that formed the Moon to know what Earth's original rotation rate was. Although, I agree that it must have slowed since then.

Also, tidal forces would have little effect on Mars (two very small moons), and none on Venus (no moons) - although something must have happened to Venus to make it rotate "backwards". It's generally thought that terrestrial planets with a satellite as large as the Moon are very rare. However, Mercury's rotation is very strongly determined by tidal forces from the Sun.

[Baron Munchausen]

Spoo said:

I think you would have to make a number of assumptions about the event that formed the Moon to know what Earth's original rotation rate was. Although, I agree that it must have slowed since then.

Also, tidal forces would have little effect on Mars (two very small moons), and none on Venus (no moons) - although something must have happened to Venus to make it rotate "backwards". It's generally thought that terrestrial planets with a satellite as large as the Moon are very rare. However, Mercury's rotation is very strongly determined by tidal forces from the Sun.

Venus's odd rotation (very slow retrograde) combined with the entire surface being only ~500 million years old (evenly random distribution of craters over entire planet = same age for entire surface, the number of craters gives the rough age estimate) adds up to one known cause: a huge farking impact event.

[dmm]

As a longtime reader/viewer of SF, I can tell you that your analysis is messed up. Up until the 1950s, Venus was inhabited by a technologically-advanced humanoid race consisting entirely of extremely hot babes. Then something happened, and now the place is a wasteland. Perhaps their cycles all synched up.

[Renegade 13]

Spoo said:
I think you would have to make a number of assumptions about the event that formed the Moon to know what Earth's original rotation rate was.

From everything I've read over the past several years, the general consensus is that a roughly Mars-sized planetoid ran head-on into the proto-Earth, hitting with sufficient force to liquify, well, pretty much everything. The impact blasted up sufficient material into orbit that eventually coalesced into the Moon. This theory is supported by the fact that all the Moon material they've ever studied is the same as the material the Earth's crust is composed of, but nothing deeper than the crust, suggesting a massive impact origin.

[Spoo]

Renegade 13 said:
From everything I've read over the past several years, the general consensus is that a roughly Mars-sized planetoid ran head-on into the proto-Earth, hitting with sufficient force to liquify, well, pretty much everything. The impact blasted up sufficient material into orbit that eventually coalesced into the Moon. This theory is supported by the fact that all the Moon material they've ever studied is the same as the material the Earth's crust is composed of, but nothing deeper than the crust, suggesting a massive impact origin.

Of course. What I meant is, we don't know things like the relative velocities or the angle of impact. Thus, we don't know what Earth's original rotation rate was.

[Renegade 13]

Ahhh I misinterpreted. But yeah, they can estimate those quantities, but have no way of pinning them down with any degree of scientific accuracy.

[MasterChiToes]

Fun Fact:
Though the sun has 99.9% of the solar system's mass, it has less than .5% of its angular momentum. Jupiter, with only a fraction of a percent of the mass in the solar system has about 99% of the angular momentum of the solar system.

http://urantiabook.org/archive/science/jupiter.htm