Sunday, October 27, 2013

Motion of the Moon (or Moons) around the Earth

The next details to work out have to do with the Moon or Moons of your world. The Moon and Sun are responsible for the tides on Earth, and the cycle of the Lunar orbit has a large effect on life on Earth.

In addition to the influence of the Moon in an ordinary orbit, eclipses of the Sun or Moon are very impressive experiences and have played a role in history many times. Eclipses are tricky to model and I am not sure yet how to handle them so I am going to leave them out of the Quick Start for now. I hope to figure out a good model for them and will update this post after I do.

The period of your Moon's orbit around your world is the first thing to decide. You may or may not have a unit of time between a day and a year based on this period, but that is something to think about. This period will be the mean time from a phase -- full Moon for instance -- to the same phase in the next orbit. Depending on the orbit, this length can vary quite a bit as it does for the real Moon.

Until I have figured out how to handle eclipses there is not much point in determining other features of the Moon's motion. The orbit of the Moon is slightly inclined to the orbit of the Earth so that the Moon crosses the Earth's orbital plane twice per orbit. If the Moon is full (in other words, the Earth is directly between the Moon and Sun) at the very moment that the Moon is crossing the Earth's orbital plane, there will be a lunar eclipse. If the Moon is new (in other words, the Moon is directly between the Earth and Sun) at the moment the Moon is crossing the Earth's orbital plane, locations on the Earth directly behind the Moon from the Sun will have a solar eclipse.

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