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Solar Eclipses Elsewhere? On the Earth, a total solar eclipse is an event that inspires awe in just about anyone. Unfortunately, they are rare. A solar eclipse occurs in a swaf an average of 100 kilometer wide. Furthermore, the Moon orbits about 5 degrees; above and below the plane of the Earth's equator. So most of the time the Moon misses the Sun by several degrees whenever the Moon passes the Sun. Could you imagine if the Moon orbited about 0.5 degrees; above and below the plane of the Earth's equator? Every time the Moon passed the Sun an eclipse would occur. Total eclipses would be ten times more common and a total eclipse would occur anywhere on the planet an average of once every 40 years. But I digress. Total eclipses occurring on Earth are special. We have more than an eclipse, we also have atmospheric phenomena to go with it. And a lunar eclipse, the inverse of a solar eclipse, is red due to the refraction of the atmosphere. Yet, solar eclipses do occur in other places in the solar system. The corona, however, usually appears on a jet black sky. Still there are some surprises as you will soon learn. Where can these eclipses occur? Could a planet as seen from another planet cause an eclipse? No. In this situation, the eclipsing planet is too small to cover the Sun, due to the great distances between the two planets. What does happen is a transit. Transits can be observed from Earth with a good solar filter. Mercury or Venus occasionally run across the Sun disc as small dots. The next transits of Mercury occur on November 9, 2006, May 7, 2003 and May 7, 2016. The next transits of Venus occur on June 7, 2004 and June 7, 2012. Transits of Venus occur in pairs separated by eight years every hundred years or so. Mercury transits have a more complicated pattern but are more common. A transit of Jupiter from Saturn (viewed from a moon) is a treat. It is like a hole is being punched in the Sun. It is somewhere between a transit and an annular eclipse. The diameter of Jupiter from Saturn is a quarter the diameter of the Sun. Could a planet as seen from one its moon produce a total eclipse? Yes, but the planet tends to cover too much of the eclipse. Even in the best case the planet's apparent angular diameter is 3.4 times the Sun's apparent angular diameter, as seen in the Table 1. (Apparent angular diameter is the amount of angle an object takes in the sky. It is measured in degrees. 180 degrees, or 180° circle the entire sky, a half-circle. There are 60 arcminutes (60') in a degree and 60 arcseconds (60") in an arcminutes. The moon is approximately 30', and Jupiter is usually about 50".) From the Earth faint traces of a corona can be detected about 10° from the eclipse, about 20 times the apparent angular diameter of the Sun and Moon. 20 times bigger is practically the cutoff at seeing anything at mid-eclipse. Table 1: Comparison of Angular Diameter of Planet and Sun from a Moon.AD = Angular Diameter
Could a moon seen from a planet produce a total eclipse? The only one known to produce an eclipse right on is our Moon. Phoebe of Mars is too small. Charon of Pluto is too big. Venus and Mercury do not have moons. Moons from gas giants do produce closer results, but none are right on and a gas giant is not a planet you could really land on. Could a moon as seen from another moon around a gas giant produce a close eclipse? The answer is a resounding yes. Because your viewing location moves around the gas giant, it is a variable distance from the other moon. Or put another way, the angular diameter of the eclipsing moon changes size compared to the Sun (the Sun changes angular diameter but only very slightly). Table 2 shows which moons eclipse the Sun from various moons. The 'Moon' column shows which moon which will do the eclipsing. The 'Matching' column indicates how far away a moon needs to be to form a perfect match with the Sun for the 'Moon' specified in the 'Moon' column. For small bodies in the 'Diameter' column, the smallest dimension is given, to ensure the Sun is completely covered up. The 'Range' column shows the range of distances a moon's mean distance needs to be to perform an ideal eclipse. The 'Inside' column shows the viewing locations from which an eclipse can be seen for the moon in the 'Moon' column. |
All numbers in kilometers
| Planet | Moon | Diameter | Distance | Matching | Range | Inside |
|---|---|---|---|---|---|---|
| Jupiter | Metis | 40 | 127,969 | ~24,000 | 100,000-150,000 | Adrastea (?) |
| Adrastea | 15 | 128,971 | ~9,000 | 120,000-140,000 | Metis (?) | |
| Amalthea | 150 | 181,300 | ~90,000 | 91,000-270,000 | Metis, Adrastea, Thebe | |
| Thebe | 90 | 221,895 | ~54,000 | 68,000-270,000 | Amalthea | |
| Io | 3,630 | 421,600 | 2,178,000 | 1,756,000-2,600,000 | Callisto | |
| Europa | 3,138 | 670,9000 | 1,882,800 | 1,221,400-2,554,000 | Callisto, almost Ganymede | |
| Ganymede | 5,262 | 1,070,000 | 3,157,200 | 2,087,000-4,227,000 | Almost Callisto | |
| Callisto | 4800 | 1,883,000 | 2,880,000 | 997,000-4,760,000 | Ganymede | |
| Saturn | Pandora | 62 | 141,700 | ~64,000 | 77,000-205,000 | Pan to Mimas |
| Epimetheus | 98 | 151,422 | ~100,000 | 51,000-250,000 | Pan to Enceladus | |
| Janus | 150 | 151,472 | ~155,000 | 4000-306,000 | Pan to Tethys & its trojans | |
| Mimas | 392 | 185,520 | 402,000 | 216,000-588,000 | Enceladus to Rhea | |
| Enceladus | 500 | 238,020 | 514,000 | 276,000-752,000 | Tethys to Rhea | |
| Tethys | 1,060 | 294,660 | 1,090,000 | 795,300-1,385,000 | Titan | |
| Dione | 1,020 | 377,400 | 1,049,000 | 671,600-1,426,000 | Titan | |
| Rhea | 1,530 | 527,040 | 1,717,000 | 1,190,000-2,244,000 | Titan and Hyperion | |
| Titan | 5150 | 1,221,850 | 5,294,000 | 4,072,000-6,516,000 | None | |
| Hyperion | 220 | 1,481,000 | ~230,000 | 1,250,000-1,700,000 | Titan | |
| Iapetus | 1460 | 3,561,000 | 1,500,000 | 2,061,000-5,061,000 | None | |
| Uranus | Cordelia | 26 | 49,750 | ~54,000 | 5,000-100,000 | Ophelia to Puck |
| Belinda | 68 | 75,260 | ~140,000 | 65,000-210,000 | Juliet to Ariel | |
| Puck | 154 | 86,010 | ~320,000 | 230,000-410,000 | Ariel | |
| Miranda | 471.6 | 129,780 | 973,854 | 844,100-1,104,000 | None | |
| Ariel | 1157.8 | 191,240 | 2,391,000 | 2,200,000-2,582,000 | None | |
| Umbriel | 1169.4 | 265,970 | 2,415,000 | 2,149,000-2,681,000 | None |
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As can be seen from the complex Table 2, plenty of eclipses occur. Those with question marks are close to matches (with 20%). The smaller moons will give less satisfying eclipses because their shapes are highly irregular. Much of the corona will be blocked out. Below is a summary of the solar eclipses that can be viewed. The left side is the viewing location, the right side is the eclipsing body. Where I have question marks I am unsure if the eclipsing body actually eclipses the Sun. I am unsure about these are small bodies with nearly identical orbits -- there is no guarantee they actually get really close to each other. There could be orbital resonances or other factors. Please note that in Table 2 above and the list below do not take into account several factors. On factor is the parent planet. Where a possible eclipse could occur, the planet could block out the Sun or cast its shadow on the eclipsing body. In addition, for a match to occur, the Sun, the viewing location and the eclipsing body must be on the same plane. Or put another way -- thing must line up. Even a slight difference in the orbital inclinations can make total eclipses rarer. On Earth, if the Moon was in the same plane, we would have a eclipse -- solar or lunar, every month rather than once a year. Orbital eccentricities can alter the matching ranges for solar eclipses slightly as well. Looking at the table 1 below, several interesting things can be noted. For Jupiter moons, the ones for Ganymede and Callisto are the best solar eclipses in the solar system, with the exception of the Earth and Moon. This is because the Sun is still has a significant sized disk -- about 6 arcminutes across. At Saturn the Sun is about 3.3 arcminutes across. Here Titan would be the best location if for one small detail -- you can't see very well through its atmosphere. At Uranus, the Sun is about 1.7 arcminutes across. Just barely enough to see a disk. Lots of eclipses involving small bodies occur here. No eclipses occur from Neptune's moons. With the Sun at 1 arcminutes across from Neptune, solar eclipses from Neptune's moons would be a joke anyway. Table 3: Summary of Eclipsed Moons
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