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A Most Peculiar Coincidence

Transits of moons across the Sun aren't rare at all. Solar eclipses are something else.

by
Charlie Martin

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August 22, 2013 - 11:00 am
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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”.

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All Comments   (9)
All Comments   (9)
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I think at just the right points of their orbits, Io or Europa as seen from Callisto or Callisto as seen from Ganymede might cover the Sun exactly. There may be similar arrangements possible for Saturn's moons.
48 weeks ago
48 weeks ago Link To Comment
Obviously you used some pretty old photos to illustrate your essay.

For example, I saw no sign of the landing lights at Armstrong Field, Luna City.

Or did you Photoshop them out?
48 weeks ago
48 weeks ago Link To Comment
Trying for the old-time feel.
48 weeks ago
48 weeks ago Link To Comment
Get your fill of them now, because the moon is retreating from the Earth slowly and at some point in the future will never be the same apparent size as Sol as viewed from Earth. No more eclipses.
48 weeks ago
48 weeks ago Link To Comment
The distance from the earth to the moon isn't a constant, and the plane of the moon's orbit around the earth isn't parallel to the plane of the earth's orbit around the sun. Add that to the earth's axis being tilted, and eclipses are all over the damn place.
48 weeks ago
48 weeks ago Link To Comment
Transits, yes. The closeness of the match between the Moon's angular diameter and the Sun's is remarkable.
48 weeks ago
48 weeks ago Link To Comment
Maybe it's not a coincidence that their angular diameters' are close.
48 weeks ago
48 weeks ago Link To Comment
Maybe not, but explain a mechanism.
48 weeks ago
48 weeks ago Link To Comment
What's even more remarkable is that the moon used to be much closer, and in the future will be further away. The (relatively) scant time for eclipses to be just right is the time when a species came about to marvel at them. Almost as if it were planned that way.
48 weeks ago
48 weeks ago Link To Comment
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