Mars will not be as big as the moon
Posted August 25, 2014 6:40 a.m. EDT
Updated August 25, 2014 8:32 a.m. EDT
The decade-old “Mars Spectacular” rumor is making its way back into in-boxes and Facebook feeds.
Claims that Mars will “as big as the full moon” and a “once in a lifetime experience” along with a striking image of twin moons over a Russian monastery are not true.
Though Mars is about twice the diameter of the moon, the red planet is still too far to appear that large in our skies, even at the closest points in its orbit.
This rumor is rooted in the Aug. 27, 2003, event when the orbits of Earth and Mars brought them within 34.6 million miles. It was a great time to view Mars with a moderately sized telescope. Early versions of the email rightly stated that Mars would look “as big as the full moon, when viewed through a telescope.” As the email was forwarded that last part was dropped or even replaced it with “to the naked eye.”
The rumor resurfaced each August for the next several years and then mercifully went away for a few years. It’s back this year, driven by sharing on Facebook and other social media.
Earth and Mars aren’t even very close in their orbits right this week.
Astronomers use degrees and arc minutes to compare the apparent size of objects in the sky. There are 90 degrees from the horizon to the zenith (directly overhead) and each degree has 60 arc minutes. To get an idea of this scale, stretch your arm out against the sky. The width of your pinky is about 1 degree, your fist is about 10 degrees wide and NC State fans making the wolf ears with their index and pinkies are actually measuring 15 degrees of sky.
On Wednesday, Aug. 27, the moon will subtend 29.58 arc minutes and Mars 0.12 arc minutes or about 250 times smaller. Only a thin waxing crescent moon will be visible Wednesday, not the full moon shown in the Facebook image.
Looking ahead, the orbits of Earth and Mars will next bring them close in four years. On July 31, 2018, Mars will be about 35.8 million miles away. Even then, Mars will appear only 0.4 arc minutes wide or 74 times smaller than the moon.
Most people mean well when they pass on astronomical rumors like these, but they are missing out on the real sights of the night sky. Mars is visible this week for about 2 hours after sunset. Look for it low in the southwestern sky about 3 degrees below and to the left of cream-colored Saturn. Mars will continue to slide beneath Saturn each evening this week before forming a triangle with the nearly first quarter moon Sunday night.
Tony Rice is a volunteer in the NASA/JPL Solar System Ambassador program and software engineer at Cisco Systems. You can follow him on twitter @rtphokie.