Miranda (mandydax) wrote,


There's a perennial email that gets forwarded since about August of 2003, when Mars came the closest to the Earth in tens of thousands of years. Evidently, the originators of this thing didn't understand the distances involved and also didn't put the year on it. The claim is that sometime in August, Mars will make a close approach to Earth and will appear as large as the Moon in our skies. Of course this year, as it is about every other year, Mars will actually be on the far side of the Sun, hundreds of millions of kilometers away. I've read the debunkings at Universe Today, and Bad Astronomy, and even at NASA. I decided to do my own independent debunking using a few statistics.

I did some calculations and the nearest that Mars could come to Earth (if the semi-major axes of their orbits lined up) is 54,571,299km. (PerigeeMars=PerihelionMars-AphelionEarth) The farthest that the Moon can get from the Earth currently is 405,696km (ApogeeMoon). This means that Mars would be at its closest 134.5 times farther away than the Moon at its farthest (PerigeeMars/ApogeeMoon). So, for Mars to appear the same size as the moon, its radius would need to be 134.5 times as great as the Moon's. (This is the same reason that the Moon and Sun are about the same size in the sky; the Sun is about 400 times as large as and 400 times farther away than the Moon). The Moon's equatorial radius is 1738.14km, so Mars would need to be 233,802km in radius. This is 36.66 times that of the Earth, and 3.27 times that of Jupiter, the largest planet in the Solar system. For an even more dramatic comparison, Mars would need to have about 1/3 the radius of the Sun. Mars, barring some very dramatic change in the orbits of the planets or of the Moon, will never appear as large as the Moon in the Earth's sky. Math, internet people; L2math.
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