A Most Peculiar Coincidence

A total solar eclipse happens when the Moon transits the Sun at exactly the right distance to block the whole Sun, but not the Sun’s corona.

370年11月2日 (2339 AD November 2 Old Style) Earth — Tourists are gathering from all the Twenty Planets today to see a natural phenomenon unique to the Earth: a total solar eclipse. The Earth and its major natural satellite, Luna, align periodically so that the system’s primary Sol is exactly blocked from view in a small region of the Earth’s surface, producing an effect known as a “diamond ring” for its similarity to a traditional ornament among the indigenous sophonts, before precisely blocking the star from view and revealing the extended stellar corona to the naked eye.

While satellite transits of a stellar primary are common to all inhabited worlds, and most known planets, the coincidence of size and orbital distance that produces the spectacular visual effect is so unusual that some religious groups on Earth point to it as evidence that the God or Gods have taken a special interest in the Earth.

A solar eclipse happens when the Moon passes between the Earth and Sun, blocking the Sun from view. Normally, this would simply be called a “transit”, and having one body transit the Sun from another is not really all that uncommon. But solar eclipses as we see them on Earth depend on a coincidence so unlikely that it’s entirely possible the Earth and Moon are the only pair of a planet and satellite in the entire Milky Way galaxy for which it happens.

It happens that the Moon’s diameter and distance are such that at least in certain parts of the Moon’s orbit around the Earth, the Moon as seen from Earth is exactly the same size as the Sun. For the part of the Earth’s surface that is on the direct line from the Sun through the Moon, the Sun is perfectly obscured, so that it’s only visible through mountain valleys around the edge of the Moon, producing an effect known as “Bailey’s Beads”.

“Bailey’s beads” the surface of the Sun is visible only through mountain valleys around the edge of the Moon.

Then the Moon completely blocks the Sun from view, and for a short time observers can see the corona, the much more faintly glowing outer atmosphere of the Sun.

The corona — parts of the Sun’s outer atmosphere only visible from the Earth during a total eclipse.

From space, the eclipse is much more obviously what it really is: the shadow of the Moon racing across the surface of the Earth, as this picture from the Mir space station, taken in 1999, shows.

This will be a black mark on your record.

I got thinking about this because a spectacular picture of an event much more rare was the Astronomy Picture of the Day recently. This event was a transit of Venus — effectively a “solar eclipse” by the planet Venus instead of the Moon.

Venus transiting the Sun — as seen in the ultraviolet.

This transit, while happening much less often than a solar eclipse by the Moon, is much less rare in another sense, as it’s what inhabitants of other planets have to be satisfied with — a dot moving across the surface of the star, visible only with instruments.

At least until passenger service to the Earth is established…