Mars will look as big as the Full Moon

This astronomy myth crops up every time Mars and Earth reach their closest point in space.

By Michael E. Bakich

Myth: Mars will look as big as the Full Moon

Mars makes a great telescopic sight when it’s closest to Earth. However, it will never appear as large as the Full Moon to the naked eye.

Photo by NASA

Not only is this statement untrue, but, like the Energizer bunny, it keeps going and going. The roots of this myth started with a widely circulated e-mail that appeared before the August 2003 close approach of Mars. On August 27 of that year, Mars' orbit around the Sun carried it to a point called opposition, which means that the planet appears on the opposite side of the sky from the Sun as we view it from Earth. At opposition, Mars rises at sunset, is highest in the sky at midnight, and sets at sunrise. An opposition marks the closest point between Earth and Mars, so Mars appears bigger and brighter than any other time that year.

But not all oppositions are equal. On August 27, 2003, the Red Planet appeared bigger and brighter than anyone alive will see again — well, unless you live to the year 2287.

After that? Astronomers' best guess is that Mars will reach its maximum size once again about 60,000 years in the future. But don't worry. It will be nearly as big and bright as it appeared in 2003 many times before then. In other words, the August 27, 2003, opposition was not unique.

Mars reaches opposition every 26 months. So, on November 7, 2005, and December 24, 2007, the astronomical community heard the same old song: "Mars will appear as big as the Full Moon to the naked eye." This statement is wrong. For Mars to look that large, it somehow would have to jump out of its orbit and move some 34 million miles (55 million kilometers) closer to the Sun.

Unlike most myths, I can explain exactly how this one began. Consider the original statement: "Through a telescope at 75x, Mars will appear as big as the Full Moon appears to the naked eye." In this sentence, "at 75x," means "at a magnification of 75." So, on August 27, 2003, if you looked at Mars through a telescope equipped with an eyepiece that provided a magnification of 75, Mars would have appeared as large (in other words, it would have had the same angular size) as the Full Moon does to your naked eye (that is, without any magnification).

Here's how the math works:
The Full Moon of August 2003 occurred August 12 in the Eastern time zone of the United States. Elsewhere in the country, the date was August 11. When full, the Moon had a diameter of 31.4 arcminutes (1 arcminute equals 1/60 of 1°). To compare this number to Mars' size, we have to change it into arcseconds: 1 arcsecond equals 1/60 of 1 arcminute, so the Full Moon's width is 31.4 arcminutes x 60, which equals 1,884 arcseconds. On August 27, 2003, Mars had a diameter of 25.11 arcseconds. To get the comparison, just divide Mars' size into the Full Moon's size: 1,884/25.11 = 75.

Aha! So the Full Moon would have appeared 75 times bigger than Mars in late August 2003. Therefore, if you magnified Mars 75 times — say, through a telescope — it would have appeared as large as the Full Moon does to the naked eye.

The next time Mars is closest to Earth will be January 27, 2010. At that time, the planet will lie 61.72 million miles (99.33 million km) from Earth. Compare that to August 27, 2003, when Mars was only 34.65 million miles (55.76 million km) from Earth.

OK, I can't resist: On January 27, 2010, if you look at Mars through a telescope that magnifies 132x, it will appear as large as the Full Moon does to the naked eye.

Numbers can be fun if you get the facts right.

Learn more about astronomy with these resources

Know the facts — not the fiction — about astronomy and sign up for Astronomy magazine's free weekly e-mail newsletter.
See stunning images of comets, galaxies, nebulae, and more in our Online Reader Gallery.
Read the latest astronomy news.
Go behind the scenes of the hobby, science, and magazine in our editors blog.

	Mercury	Mercury	Venus	Venus
RISE	—		—
SET	—		—
		—		—