This weekend I received a forwarded email from a relative claiming that Mars is going to be so close to Earth and so brilliant on August 27, 2007, that it will be like Earth has a second moon. Unfortunately, this email is four years old and still flat out wrong.
Mars, like the Earth, orbits the sun in an ellipse (kinda like an oval). The Earth's orbit is very nearly circular. Every January, the Earth is 92 million miles from the sun; in July, the Earth is 94 million miles from the sun. That change is so small, we don't notice it on Earth. Mars, however, has a more elliptical orbit. At its closest to the sun, Mars is about 130 million miles from the sun; at its furthest, it is 155 million miles away from the sun. That difference is big enough to be noticed on the planet; the big dust storms now threatening our Mars rovers only happen when Mars is closest to the sun (which it is now).
But Mars being closest to the sun doesn't mean it is closest to the Earth. The Earth orbits the sun faster than Mars, and so can be elsewhere in its orbit when Mars is closest to the sun. This animation shows how the position of the Earth and Mars change relative to the sun and relative to Each other.
On August 27, 2003, the Earth and Mars were as close as they had ever been in recorded history. This is just because of geometry, and both planets happening to be at the right places in their orbits (you can play with the animation to see how this happened). At its closest in that year, Mars was about 35 million miles (56 million km) away from the Earth (exactly, it was 34,646,418 miles away). Even at that "small" distance, though, Mars did not look like a second moon to the Earth, as the email I received claims. It looked like a really bright star, unless you had a telescope, in which case Mars would look impressive compared to normal, though still smaller than Venus and Jupiter appear through a telescope. This is quite far from being a "second moon" to the Earth!
This year, Earth will again be relatively close to Mars, but not until December 18, when Mars will be 55 million miles away (88.42 million km). And, Mars will only be about half the apparent size it was back in 2003.
This email has turned up every year since 2003. It was mostly wrong then, and it is even more wrong now. So, if you get an email like this, please don't forward it to everyone you know. If you get an email with astronomy news you are uncertain about, feel free to drop me an email, and I'll let you know if it is real. You can also look for yourself at The Urban Legend Reference Pages or at the Bad Astronomy website.