Spaceweather.com is an informative web site on the latest goings on above earth’s surface, including eclipses, aurora sightings, sunspots, and unusual atmospheric phenomena. The June 7, 2017, posting featured a stunning time-lapse video of two of Jupiter’s four Galilean moons casting their shadows on the planet’s cloud tops. Although these distant eclipses look impressive from our perspective, from Jupiter’s cloud tops not so much.
We read on the spaceweather.com web site:
Envious Earthlings can take solace: Eclipses on Jupiter aren’t nearly as beautiful as they are on our planet. Here on Earth, the Moon and sun are almost precisely the same size — an incredible coincidence that produces phenomena such as Baily’s Beads and the Diamond ring. When the disk of the sun is so precisely covered, we can see the ghostly tendrils of the sun’s outer atmosphere (the corona) shimmering across the sky.
On Jupiter, however, local moons appear to be much larger than the distant sun. Io, for example, is 6 times wider than the solar disk; it completely swallows the sun’s corona. This mismatch in size mitigates the beauty of the occasion. The best eclipses are homegrown, after all.
We come to realize just how special solar eclipses are on earth’s surface when we compare them to what solar eclipses would look like from other planets in our solar system. While Jupiter’s moons appear too big in relation to the sun, some of the moons around other planets appear too small.
The first pictures of an eclipse from the surface of another planet were taken by NASA’s Opportunity rover on the surface of Mars in March 2004. See the top of this post for a series of pictures of Phobos taken by the Curiosity rover on August 20, 2013.
Notice the irregular appearance of the moon’s silhouette. The moons of Mars are so small that they can’t form themselves into spherical shapes, like our moon. Phobos completes an orbit around Mars in a mere eight hours! As a result, a transit of Phobos across the sun’s disk lasts only thirty seconds. The sun appears 35 percent smaller from Mars, making it harder for any would-be Martian observers to see fine details on the sun. And it only gets worse for the planets beyond Mars.
In contrast, the sun appears only 3 percent bigger than the moon, on average, as viewed from earth’s surface. That’s a close match. The sun appears larger from earth than from any other planet with moons. Plus, eclipses from earth last a maximum of 7.5 minutes. Finally, we would be neglectful if we failed to mention the obvious facts that earth provide a solid surface to observe from and a transparent atmosphere. These are the reasons we get the best solar eclipses in the solar system. Of course it’s all sheer coincidence, don’t you know.
Photo source: JPL/Caltech/NASA.