The Best Eclipses in the Solar System
Slowly the world darkens, but without the warming of color that accompanies sunset. The flecks of light in the shadows of trees take on a crescent shape. Suddenly the sun is gone, and in its place hangs a magnificent crown of white fire, accompanied by the blazing gems of Mercury and Venus. The spectacle lasts for only a minute or two. There is a blinding flash at one side of the crown, and then the light of the sun returns, as slowly as it faded. A total solar eclipse is indeed a spectacular sight on this Earth of ours.
The total eclipse that will cross parts of Southeast Asia and the North Pacific to end in the Gulf of Alaska on March 17 this year is an example of something rare in the solar system. In solar eclipses here on Earth, the sun's radiant atmosphere, the corona, is almost entirely visible during totality. For this to be possible, the moon must appear to be almost exactly the size of the sun. (More accurately, the ratio of the moon's diameter to its distance must be very slightly greater than the ratio of the sun's diameter to its distance.) When this is the case, the sun's disk is hidden by the moon, but its luminous atmosphere remains visible. Of course this matching of sizes also means that the total eclipse lasts only very briefly in any one place.
Since the distances of the earth from the sun and the moon from the earth vary slightly, the moon sometimes appears smaller than the sun. If an eclipse occurs while the moon is farther away from the earth than normal and the earth is nearer the sun than usual, the eclipse will be annular -- a thin ring of direct sunlight will show all around the moon.
Eclipses are certainly visible on other planets, but not like ours. For one thing, many moons are not even round. On Mars, the closer satellite, Phobos, is potato shaped, and even though it is very close to its planet, its longest diameter would reach only two thirds of the way across the sun's disk. Jupiter's Amalthea, another small and close moon, would more than span the diameter of the sun lengthways, but its longest diameter is always pointed towards Jupiter. Its eclipses of the sun would look very much like those by Phobos, except that the whole display would be smaller.
Jupiter's Io is the only moon that would look larger than our Moon, even though all of the round moons of other planets have larger apparent diameters than the sun. The sun appears smaller because the planets around which these moons revolve are more distant from our central star. The round moons of Jupiter, Saturn, Neptune and Uranus are two to ten times the apparent diameter of the sun. These moons can indeed produce total eclipses, and ones lasting longer than our ephemeral affairs. But most of the sun's corona would be hidden, and much of the event's beauty would be lost.
Our March eclipse will be visible for most Alaskans as a partial eclipse (if skies are clear) between about 5:30 and 7 p.m. AST. Aleutian weather makes it unlikely that totality will be visible from a boat south of the Aleutians (a high-flying plane would be a better bet), but we can supply exact coordinates of the path of totality to anyone planning to be in the area.
