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If Earth's Moon Were Ganymede-Like

On my alternate Earth, its moon has a diameter of 3,274 miles and orbits the planet from a distance of about 665,000 miles. A five-mile-deep mantle separates its rocky crust from its core made entirely of iron. Questions follow:

  1. Will Earth still rotate at 24 hours and revolve around the sun at 365 days with a moon of this size at this distance?
  2. How quickly will this moon leave Earth's orbit?
  3. How big and how bright would it look in the night sky?
  4. How long would one lunar cycle--new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, last quarter, waning crescent, new--take?
  5. How would tides and eclipses be affected?


1. Sure. If you want. The moon doesn't really affect either of these.
2. What are you talking about? Barring tidal forces from the earth's rotation causing it to speed up, if anything the moon would be moving very, very slowly towards the earth. However, with your moon so far out, its orbit may be unstable. It risks getting dragged out of Earth orbit and into solar orbit. I'm not sure about that, though.
3. It would appear to be roughly half the radius of the moon, since it's so far out.
4. The lunar cycle would last 127.59 days. The various phases are evenly spaced throughout the cycle, so that's easy enough for you to figure out.
5. Nobody would probably ever notice an eclipse, since the moon would cover such a small portion of the sun. Tides would be far less intense, but would follow your moon in the same way they follow our moon.

You don't seem to know too much about space. Are details about the thickness of the moon's crust and composition of its core relevant to the story?


Actually, you know what? The orbit would last 127.59 days. The lunar cycle would last 196.14 days. It's because the earth is moving around the sun, causing the "target" for one cycle to slowly shift further and further along the orbit path throughout the year.


Yes, that's because tidal forces from the earth's rotation are gradually transferring momentum from the earth's rotation into the moon. Eventually, the earth will become tidally locked with the moon, and the process will veeeeeeeeery slowly reverse. Calculating the rate at which this happens is... difficult. It has to do with the flexibility of the earth's crust, and a bunch of other crazy factors.

On your world it would likely happen slower because of the reduced tidal forces, but I can't tell you much more than that.


Just to clarify, when I say that "the process will reverse" I mean that the moon and earth will slowly drift back together, not that the earth's rotation will speed back up.