December 31st, 2009

Doc Tectonic (sepia)

Orbital Mechanics

If you have ever considered putting an Earth-like planet in orbit around a gas giant:

* It will become tidally-locked in a few tens of thousands of years, which is somewhat inconvenient if you wanted a particular rotation speed.

* But you can put it in an orbit close enough to its primary to have a 24-hour orbital period (and hence a 24-hour day) and come nowhere even close to the Roche limit. (Because the limit is probably near or below the top of the gas giant's atmosphere.)

* Being tidally-locked, your ocean tides aren't going to be affected by the primary (which is fixed in the sky), but will be due to the sun and any other moons the primary may have. (But that said, the inner and outer poles will probably be where the oceans are located.)

* At a distance of 300,000 km (roughly a 24-hour orbit), Jupiter has an angular width of around 13 degrees -- which is 25 times as big as the sun/moon from Earth. It will be REALLY BIG in the sky.

* And assuming a 24-hour day, the daily solar eclipse (as the moon passes through the primary's shadow) will last for a little under an hour.

* At the inner pole, lunchtime is the only time you'll get serious darkness. At the outer pole, the only obvious evidence of the gas giant's existence will be its effects on any other moons.