Friday, August 24, 2007

Mars as big as the moon? Rediculous.

My mom just sent me an email which claimed that on August 27th, mars will be in opposition to the Earth and as a result will appear the same size as the full moon.

The truth is, around this time is when Mars is closest to Earth and will appear like the undisputed champion of the sky in terms of brightness.

The truth also is that if one eye was in a consumer telescope and the other open toward the moon the two orbs would appear to have a similar diameter.

Mars will pass more than 55 million miles from Earth, which the email also referred to (with different but similar numbers), but could it appear the same in size? Let's do the math.

The moon's diameter is 3,474 miles.

The moon's distance from Earth is 238,857 miles.

Mars' diameter is 6,749 million miles.

In order for Mars to "appear" the same size as the moon, it's distance would have to be calculated like this: MarsDistance = MarsDiameter * ( 1 + (MoonDistance / MoonDiameter))

The result would require Mars' distance from Earth to be 473,919 miles from earth.

In short, since Mars is roughly double the size of the moon, its distance would have to be roughly double from the earth to appear the same size as the moon.









actual distance

55,000,000 (est)



needed distance





The email was right that the Moon would pass closest around August 27th, and that it would be more than 55 million miles from Earth. Where it is wrong is that it would appear the same. It would need to come 54,500,000 miles closer than that to even come close.

I am no specialist, but I would imagine we would have all kinds of gravimetric issues on Earth if anything the size of Mars ever passed that close to us. But that is just my speculation.

When you Google this it's clear that this email started innocently (that the telescope and naked eye would make the Moon and Mars similar in size) but was lost as it was passed along. The email can be traced back to 2003.

Nonetheless, don't miss out on looking for Mars this month – it's the big red star (not to be mistaken for the moon)!

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