Are there eclipses on other planets?
The Sunday Nation
Nairobi,
20 October 2013
The apparent size of an object is the ratio of its actual size to its distance from the observer. For this reason, the farther away an object is, the smaller it appears to be. One of the best illustrations of this principle is the sun and the moon.
The sun is about 1.4 million kilometres in diameter while the moon is only 3,400km. Yet, when viewed from Earth, these two heavenly bodies appear to be the same size. If don’t believe that, just wait for the coming solar eclipse and watch as the moon almost blocks the sun from sight.
The reason the two appear equal is that the sun is very far away (150 million km) while the moon is quite close (384,000km). Dividing the size by the distance, we find that the apparent size of the sun is 0.0093 while that of the moon is 0.0089. The difference is very small thus the two do indeed seem to be equal in size.
An interesting question then arises: do eclipses occur on other planets?
If we lived on Jupiter, we’d have over 60 moons in the sky: Four of them are very large while the rest are pretty small, nay, tiny. The largest is named Ganymede and it is about 5,600km in diameter – larger than the planet Mercury.
Ganymede revolves around Jupiter at a distance of 1.07 million km. Thus its apparent size (actual diameter divided by distance from planet) is 0.0052. Now Jupiter lies some 780 million kilometres from the sun. Therefore, the apparent size of the sun when viewed from this planet is 0.0018.
Clearly, this moon (Ganymede) appears to be about three times the size of the sun. Therefore, during an eclipse, it completely blocks out the sun from view.
But even though it is the largest in actual size, Ganymede looks as if it is the second largest when viewed by a Jupiterian. The moon with the largest apparent size on Jupiter is Io; it appears to be about five times the diameter of the sun. The reason for this is that, at 421,000km away, Io is much nearer to the host planet (Jupiter) than Ganymede (1.07 million km).
Life on Jupiter must be quite interesting insofar as eclipses are concerned: the four largest moons of this planet all appear larger than the sun. Thus each of them is capable of blocking the sun completely.
Still, one more question remains: since the apparent size of gets smaller as we go farther away, how does the sun compare with the other stars?
The other stars are so far away that, when viewed from earth, they look like dots in the sky. However, by studying their distances, brightness and character of their light, it is possible to evaluate their sizes. It turns out that the sun is an average star – not too large, not too small; not too bright, not too dim. Rather unexciting, when you think about it!
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