It's almost August, and you know what that means. The August Mars Hoax is starting to go around again.

I just got a call from an astronomy educator in Louisiana. A TV station in Shreveport called him to check the news they'd heard that Mars will soon come so close to Earth that it will appear as big and bright as the Moon.

Actually, "hoax" is the wrong word for this thing, unless some joker is now spreading it knowingly (quite possible). It's an e-mail chain letter claiming that Mars will come closer than ever in history on August 23rd and will look as big as the full Moon. If your well-meaning great-aunt or your cousin's brother-in-law's dog hasn't sent it to you, it's probably just a matter of time.

What's going on is this. Back on August 23, 2003 (that's 2003 with a 3, folks), Mars had an especially favorable opposition, coming close enough to appear 25 arcseconds wide. That's still pretty tiny even in a telescope — smaller, for instance, than Jupiter always appears.

Back then, someone somewhere pointed out that at a magnification of 75× in a telescope, Mars would appear as big in the eyepiece (½° wide) as the Moon does unmagnified. True enough. But two things happened, as often do with chain letters. First, it got rewritten bit by bit to improve the story as people passed it around, so that the "75× in a telescope" was downplayed or, in some versions, left out. Second, the chain letter kept going and going, with the same breathless excitement, long after August 2003 receded into the mists of history.

In 2010, for instance, Mars's opposition happened on January 29th, and even that was a poor one, with Mars appearing just 14 arcseconds wide. In August this year Mars appears about as small as it gets, as it sinks low in twilight on the far side of the Sun from Earth.

Every year I give members of the news media, when they phone Sky & Telescope, the following quote:

"The Mars chain letter is not a bad thing, it's a good thing. It is basically harmless, so it is an immunization. That is, if you make a fool of yourself to your family and friends by sending it to them, you may be embarrassed enough that you won't send them the next e-mail chain letter you get, which could be a lot less benign."

P. S.: The first place to check for facts about any internet rumor, hoax, or urban legend is www.snopes.com. Bookmark it.