It's b-a-a-a-ck – the claim that Mars will appear to be so large in the sky that it will look as if Earth has two moons.
It is fair to say that Mars itself has two moons – Phobos and Deimos. But Earth? A second, temporary lunar look-alike won't happen. Ever.
On Aug. 27, when this bogus spectacular is supposed to take place, the planet will be a mere pinprick of light in the western sky at dusk, sitting close to Venus. It will be some 195 million miles from Earth.
In relative terms, the moon Friday night will be roughly 400 times larger than Mars will appear.
In astro-speak, Mars will have an apparent diameter of about 4.4 arcseconds. The moon, by contrast, will have an apparent diameter of about 29.5 arcminutes. While those units aren't something most people use every day, you can get an intuitive grasp by thinking in terms of time. Twenty-nine minutes is substantially longer than four seconds.
The e-mail extolling the prospect that Mars will temporarily make it appear as though Earth has two moons has been making the rounds since 2003. In August of that year, the orbits of Earth and Mars brought the two planets to their closest encounter in some 60,000 years. But close is a relative term. The two planets were still some 34.6 million miles apart.
Although two moons is not on Earth's astronomical agenda Friday night, by all means, check out the evening sky anyway. Venus, Mars, and Saturn will make a nice grouping low on the western horizon as darkness falls.