One of the most remarkable celestial coincidences is that the Moon and Sun — as viewed from the Earth — have almost the same apparent size. Depending on its position in orbit, the Moon can appear either larger or smaller than the Sun, resulting in solar eclipses on Earth occurring in two varieties: total, when the Moon is close enough to appear larger than Sun and completely cover it, and annular, where a more distant Moon fails to completely cover the Sun’s photosphere, resulting in a “ring of fire”.

This size coincidence is striking, especially since it hasn’t always been the case, nor will it be the case forever. Billions of years ago, the Moon was much closer to the Earth and total eclipses were far more common, yet less spectacular because the Sun’s corona and prominences wouldn’t have been visible all around the Sun. Eventually, tidal-driven recession of the Moon from the Earth will put an end to total solar eclipses visible from Earth and all subsequent eclipses will be annular. Some have actually argued that the closely comparable apparent sizes of the Sun and Moon have contributed in some way to the evolution of human intelligence, providing an “anthropic” explanation of why we happen to be observing such a marvel at the epoch in geological time when it happens to occur.

In the first week of July, 2004 three completely unrelated celestial phenomena occurred within days of one another: the full Moon, the passage of the Moon through perigee (the moment when it most closely approaches the Earth), and the passage of the Earth through aphelion—its greatest annual distance from the Sun. Thus, the full Moon occurred less than 12 hours after lunar perigee, and only three days later the Earth arrived at aphelion. The coincidence of these events permitted taking photographs of the perigean full Moon and the Sun near aphelion, all with the same camera and optics, to illustrate the difference in the apparent sizes of the Sun and Moon in these circumstances.