Don’t Get Excited: There Won’t Be Two Moons In The Sky Tonight


The Sunday Nation


28 August 2005


There is an email going round claiming that, tonight, the planet Mars will appear as large as the moon. The letter also claims this spectacular event will occur because the red planet will be at its nearest distance from Earth in recorded history - “only 55 million kilometres away”. The email further says that this rare event will next occur in the year 2287.

Well, don’t get too excited – this email is a hoax. First of all, Mars will not reach its closest today. That will happen on October 31 and the distance to the red planet on that day will be 69 million km. Secondly, The closest approach in recorded history occurred on August 27, 2003 when Mars came to about 55 million km from Earth. To see a simulation of the motion of the two planets, visit and click on the “articles” link.

The truth is that tonight, Mars will be 102 million km away and its size as viewed from Earth will not be anything close to that of the moon! Let’s see what the figures say…

The moon is 3,470km in diameter and lies about 380,000km from Earth. Imagine drawing two straight lines, one from each side of the moon and stretching them all the way to your eye thus forming a triangle with two sides 380,000km and the third side 3,470km. What would be the angle between the two long lines?

The answer is: about half a degree. This is the apparent size of the moon as viewed from Earth. Now let’s do the same with Mars: Its diameter is 6,900km and tonight it will be 102 million km away. Therefore its apparent size will be only 0.008 degree. In other words it will be only one sixtieth of the size of the moon.

Even on October 31, the apparent size of Mars will be only 0.01 degree – that is one 50th of the size of the moon. So, tonight the red planet will appear just as small as usual and most people will not be able to distinguish it from the rest of the stars in the sky.

One question still remains: how close would Mars need to be for it to appear as large as the moon? The answer is 790,000km away. Although that would really a spectacular even to watch, there is no telling how it would affect life on Earth.

Since the mass of Mars is ten times that of the moon, then at 790,000km away the gravitational pull of the red planet would be about two and a half times that of the moon. The greater gravity would cause big changes in tides on our seas. It could also affect the weather by causing “tidal “waves in the atmosphere.

The moral of this story? Two things: first, don’t believe everything you read on the Internet and second, you don’t want another moon in the sky, it might bring the end of life on Earth.

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