The Physical Size Of The Moon

In all of our discussions of eclipses so far we have tacitly assumed that the intrinsic physical sizes of the Sun and the Moon are not changing appreciably.

The Moon is a rocky body. When it was young and hot it was slightly larger because by heating things you generally make them expand. That is why a glass jar may crack if you pour in a boiling liquid: the rapidly heating interior tries to expand against the cool exterior, breaking it asunder. Imagine you are making jam and have gotten to the point where you pour the steaming liquor of fruit, sugar, and pectin into the jars. You should be sure that you immerse those jars first in boiling water not only to sterilize but also to heat them, thus avoiding breakage due to temperature differentials.

Although the Moon may have been a little larger when young, it has long since completed its cooling and reached its equilibrium dimensions. (The puckering of its surface during this cooling and contracting from an initially molten state is thought to explain some of its peculiar surface features like cracks and rills—similar to the wrinkling of a prune as it dries out.) From the perspective of eclipse calculations, the physical size of the Moon may be taken to be unchanging. Over the time scale of human history, even the gradual increase in its mean distance from the Earth is not a significant effect compared to the monthly in-and-out movement varying its angular size.

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