Sun and Moon - chance or design?

When we look at the natural satellites of the planets of the solar system, it soon becomes obvious that the Earth/Moon system is a freak. The size of the Moon in relation to the Earth is 4:1 in diameters (we shall leave the question of mass ratios out of the present argument).

Mercury and Venus have no known satellites and the two known satellites of Mars are minuscule compared to Mars itself. Likewise the satellites of the giant planets, although larger than our Moon in a few cases (the four Gallilean satellites of Jupiter and Saturn's largest satellite, Titan), all these planets have natural satellites that are much smaller than their primaries.

It has sometimes been suggested to me by those who believe in a divine force in nature, that the deity contrived the Earth/Moon system in such a way that mankind should be able to experience the spectacle of a total eclipse of the Sun. Therefore, having blessed us with a larger than average Moon (note I use the word as a proper noun for the unique object that is our satellite) the deity contrives also to have this body in such an orbit that it appears almost exactly the same size in the sky as the Sun.

This, it has been argued, seems more than a case of mere chance. But is it? Our mathematics, which deals with the phenomenon of probability and chance, shows that the larger the number of specimens in a system the more likely there is to be resemblance if not replication. One then has to note simply that from observations it would appear there are to all intents and purposes an infinite number of stars out there. If that is the case then the chance of finding such a systems as ours is inevitably positive.

Of course all this takes us into the realms of metaphysics. A small minority of us even argue that in a system in which time is integral to the process there can be no such concept as a finite universe. But that is rather more than I intend to discuss here.

Diagram 1

At a purely practical level it is obvious that both the Sun and the Moon vary in apparent diameter depending upon their distances from us. Dia 1 shows a graph of the Sun's diameter during the course of a year; Dia 2, that for the Moon in 2006. It will be seen that the Moon fluctuates in apparent size over a wider range than does the Sun. This is fortunate in one sense since to look at the Sun with the unprotected eye is hazardous whereas the Moon's fluctuating apparent size may be enjoyed and appreciated with perfect safety.

Diagram 2

Telescopic observers will notice that at times the Moon's disc fits comfortably into the field of view using a certain eyepiece, whereas at others it does not.

Diagram 3

During 2006, the Moon's maximum apparent (2008.78") diameter occurred on February 28. Minimum apparent diameter (1763.78") will occur on September 22. The comparative sizes are illustrated in Dia 3.

Some may have noticed the apparent smallness of the near full-moon on March 15/16, which was due to the Moon being near to its greatest distance from Earth (apogee).

The Sun is at its closest to Earth around about January 1 and at its maximum distance at the beginning of July each year. In 2006 the Sun's apparent diameter ranges from 1887.84" to 1951.84", almost too small to notice on a comparative diagram similar to that for the Moon shown in Dia 3.

These variations in the apparent diameters of the two bodies accounts for the difference in the appearance of solar eclipses from annular to total.

JV 16/03/06