Mars: As big as the full moon? A: No.

You might have been the recent recipient of an email that breathlessly states that this August (2006) "...Mars will look as large as the full moon to the naked eye." I had several friends and family members send me this one. It's actually a) not true, and b) recycled falsehoods from 2003, when Mars did have a close approach to each (here close means 54 million km).

This email kills me for a number of reasons. What really hurts is that the people who have sent it to me are excited about space - they convey a bit of that child-like wonderment we all felt the first time we saw stars in the night sky unblemished by city light pollution. I love it when people are excited about space science and I hate to dim any of that enthusiasm, but I think it is important to bring people's expectations in line with reality.

Perhaps the best source to quickly check on emails such as these is Phil Plait's Bad Astronomy blog. Phil has attempted to slay this particular dragon several times, but like the hydra, new heads keep popping up.

There is some speculation that this email was written by a professional as a joke. The 2003 version of this email contains the following phrase:

"it [Mars] will attain a magnitude of -2.9 and will appear 25.11 arc seconds wide. At a modest 75-power magnification Mars will look as large as the full moon to the naked eye.

I think part of the reason this email seems so convincing is that is has some accurate-sounding facts included. It gives the apparent diameter of Mars in the sky to two decimal places! The 2003 email had a paragraph break between "At a modest 75-power magnification" and "Mars will look as large as the full moon to the naked eye." Thus, if you were quickly skimming the text, you may miss the first clause about looking through a telescope.

The emails I have received in 2006 had the wording altered:

"At a modest 75-power magnification.
To the naked eye Mars will look as large as the full moon!"

So someone has taken an misperception in a 2003 email and turned it into an outright lie in 2006. In 2003, Mars did appear pretty darn close, but close is a relative term. You'd still need binoculars to verify that is was a disc instead of a star. As the email(s) correctly state, Mars at closest approach is about 25 arcseconds across. The full Moon, however, is about 1800 arcseconds across. But what's a factor of 72 between friends and family members? By the way, 2006 is not going to be a good year for Mars-watching. Mars will be on the other side of the Sun. A good rule of thumb is that (relatively) good Mars observing times come around once every two years: 2003, 2005, 2007, etc. This two-year alignment cycle is the same reason why NASA has a launch window to Mars every two years.