The Cycles Of The Moon

Imagine looking down upon the Moon's orbit (as in Figure A-2) from the depths of space, out among the stars. As with the sidereal year, one can define a sidereal month as the time the Moon takes to return to the same position relative to the stars; that is, to complete a 360-degree circuit around the Earth. This sidereal month lasts 27.32166 days, taking a long-term average value to smooth out short-term erratic variations.

The sidereal month is significant in that it is also the time the Moon takes to spin on its axis, so that it perennially points the same face towards us. Nevertheless we were able to map more than half of the lunar surface before satellites were launched to return images of the far side, because we can peek just beyond the eastern and western limbs of the Moon at different times, the lunar orbit not being circular. We can also see over the poles slightly, and overall 59 percent of the Moon can be mapped from Earth. Figure 1-2 shows these effects in action.

Is the sidereal month the one we need for eclipse computations? Well, not really. The reason is demonstrated in Figure A-6. It is the synodic month that is relevant for eclipses, as discussed in Chapter 2. This type of month describes the lunar brightness cycle, the length of time from one full moon to the next. Any particular synodic month may last for between 29.2 and 29.8 days, but the average taken over many years is 29.53059 days. That is a very significant figure in eclipse calculations.

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