Orbital Period

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The orbital period of a satellite is the time taken to make one orbit. For the Moon the orbital period relative to the Sun is 29.53 days, but relative to the stars is only 27.26 days. That's because while the Moon goes round the Earth, the Earth also goes round the Sun, and that makes the Moon go round a full star-relative orbit, then have to go a bit further.

The Earth moves about 1 degree per day (actually 365.25/360), so in 29.53 days the Moon has to go around the 360 degrees, plus an extra 29.53*(365.25/360), making 390 degrees in total. That takes 29.53 days, so the 360 degrees must take 29.53/390*360 days, giving 27.26 (and a bit) days.

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