Astro Bob: Peekaboo! Full Moon hides Mars
On Wednesday evening, the moon will cover up the planet Mars. Don't miss this must-see event.
DULUTH — If you've never witnessed an occultation before you're in for a real treat. On Wednesday evening across much of North America (except the Southeast and parts of the East Coast), skywatchers will see the full moon slowly encroach upon and then gobble up the planet Mars. From some locations, Mars will disappear for more than an hour behind the Moon.
An occultation is sort of like a conjunction, when the moon and planet (or two planets) line up together. But in an occultation the alignment is so nearly perfect that the nearer object covers the other — like hitting a bull's eye.
Lunar occultations of stars occur routinely. Stars wink out suddenly because despite their enormous size, they're so far away they appear as mere points of lights even at high magnification. The hard edge of the moon approaches and swiftly covers the stellar pip. At 4,212 miles across (6,780 km) Mars is about twice as big as the moon, but since it's some 50.6 million miles (814 million km) away it appears tiny in comparison — 107 times smaller!
But it's disk is still much larger than a star, so it will take the moon a number of seconds to cover it. As seen from the Duluth, Minnesota, region, the moon takes its first bite at 9:06 p.m. CST and finishes the planet off 43 seconds later. Watching Mars go bye-bye will be lot of fun to see up close in a telescope or even a pair of binoculars.
What I'm really curious about is how close we'll see Mars get to the moon with the naked eye before it disappears in the lunar glare. When the two rise together Wednesday evening, they'll be close but easy to separate, about 1-2 degrees apart. That alone should look amazing — I mean you've got a full moon plus Mars at its brightest, and they're side by side. Have your phone ready to take a picture of the pair with a pretty foreground, maybe something holiday-related like a lit tree.
Then keep watch. The moon moves eastward in its orbit (to the left in the northern hemisphere) and will slowly sneak up on the planet, closing the gap between them. At some point you'll see them nearly touching. That should occur around 10-10:30 p.m. EST; 9-9:30 p.m. CST; 7-7:30 p.m. MST and 6-6:30 p.m. PST.
The full moon is super bright but so is Mars, now at magnitude -1.8 and more radiant than Sirius, so I suspect we'll be able to see the planet right up to the moon's edge without optical aid. But honestly, I don't know, so I'd love to hear what you see. Please share your observations on my Astro Bob's Astronomy for Everyone Facebook page.
The moon's path across Mars varies depending on your location. From some locales, it's covered for just a few minutes, in others, more than an hour. After the planet disappears it will reappear minutes up to an hour-plus later on the opposite side of the moon as if rising from its edge. You don't want to miss that either!
To find out disappearance and reappearance times for your city, check out the International Occultation Timing Association's (IOTA) page devoted to the event. Times shown are in Universal Time (UT) hours, minutes and seconds. To convert UT to EST, subtract 5 hours from the times shown; 6 hours for CST; 7 hours for MST and 8 hours for PST. For example, if you live in Chicago, Mars will disappear at 3:10:58 a.m. UT. Subtract 6 hours and you get 9:10:58 p.m. Dec. 7.
For a better world map than the one included here, please see my recent Sky & Telescope post on the topic.
Even if you don't get to see the occultation, you'll be in for one awesome conjunction. The moon passes extremely close to Mars from many U.S. cities. From Philadelphia, they'll be just 1/30 of a moon-diameter apart at 10:51 p.m. EST and just as close viewed from Boston at 11:01 p.m. If clouds block the view, Italian astronomer Gianluca Masi will live stream the event on his Virtual Telescope site starting at 10 p.m. CST (4 U.T.) on Dec. 7.
According to broadcast meteorologist and astronomy aficionado Joe Rao ,favorable occultations of Mars occur for a specific location on average once every 14 years. The next one for North America will take place on January 14, 2025.
Remember that you can enjoy the occultation in every which way — with the naked eye, binoculars or a telescope. For close-up photography, use a telephoto lens with your camera mounted on a tripod. Start with ISO 400, a lens setting of f/8 and exposure around 1/500 to 1/2000 of a second. Inspect the images on the camera's back screen, and if it's not quite what you want, adjust the exposure accordingly. Good luck and enjoy!