Get ready for a really cool event Tuesday, February 18. That morning the waning crescent moon will pass directly in front of Mars. For about an hour, depending on where you live, the planet will disappear from sight, hidden by our modest moon. The event is called an occultation, which is similar to an eclipse but involves a large object hiding a much smaller one.
The moon routinely passes in front of countless faint stars as its clocks its way around the sky each month. On occasion it will occult a bright naked-eye star like Aldebaran or one of the planets. Those are very special. Not only do we get to see two bright celestial objects side by side, but one of them disappears right before your eyes. It’s been a few years since a significant portion of the U.S. has witnessed a planetary occultation. I’m happy to report the wait will soon be over.
The December 24, 2007, occultation of Mars. Action starts at the 1:22 mark. (I would have chosen a different musical selection 😉
On Tuesday morning, a half-hour before the occultation begins, Mars will shine just to the left of the waning crescent moon’s bright edge, also called the limb. As the minutes tick by, the moon will close in, and the two will slowly draw together. The moon’s movement may look unhurried to the eye, but that’s because it’s a quarter million miles away. It’s actually whisking along at 2,287 miles an hour (3,680 km/hour) to the east on a beeline to the Red Planet.
The occultation will be visible from the continental U.S., Canada and Central America. If you live on the East Coast the moon’s bright limb will start to cover Mars just after sunrise. Don’t be stopped by daylight — a small telescope will still show the Mars at that hour. Just point it at the moon and look for a spark of orange nearby. From the Midwest, the planet will disappear during early morning twilight and reappear along the moon’s night-side limb around sunrise. Because the reappearance happens in daylight you won’t see the dark limb, so Mars will appear to slowly materialize from a blue sky!
In the mountain states Mars will be covered in a dark sky and reemerge in twilight. Observers in the far west won’t see the disappearance because the moon will still be below the horizon, but they will get a fantastic view of the reappearance when the Red Planet will slowly “rise” from the western edge of the shadowy, earth-lit moon. THAT I’d love to see.
A stellar occultation takes only a split second. The star hovers near the lunar limb as the moon edges ever closer and then poof — it’s gone! Even though stars are truly enormous they’re so far away that from our perspective that they’re essentially points of light. The moon covers them in a flash at disappearance and releases then just as quickly when the occultation is over. Neither does a star fade at the moon’s approach because the moon has no significant atmosphere which would otherwise dim the object.
Not so Mars. Can you guess why? Mars has a measurable disk. Yes, it’s a tiny object compared to a star, but it’s soooo much closer. Even a small telescope will show it as an orange dot. Mars’s disk will be 5.2 arc seconds across. One arc second is equal to 1/1,800 of a full moon diameter. Pretty small but big enough that it will take the moon about 14 seconds to completely cover the planet. Likewise, it will take 14 seconds for Mars to “rise” from the moon’s western limb when it reappears roughly an hour later.
2020 turns out to be a great year for Mars occultations with four additional events in March, August, September, and October, mostly visible from the Southern Hemisphere, though September’s occultation will also be seen from northern Africa and southernmost Europe.
How to Watch
The moon will be a thick crescent Tuesday morning and shine low in the southeastern sky, so make sure you have a clear view in that direction. Mars will shine at magnitude 1.2 and be visible with the naked eye until it’s very close to the bright limb. At that time, switch over to either binoculars or a telescope. A small telescope is best because it will provide a still and steady view unlike trying to hold a pair of unbraced binoculars, but both will do the job.
Plan to start watching about 15 minutes before the occultation begins so you can have the pleasure of seeing the moon approach the planet. They two are a study in contrasts — the gigantic, crater-pocked moon gray alongside red Mars, tiny as a wild strawberry. Remember that you don’t need perfect skies to enjoy the event. Partly cloudy will do.
Click here for a list of hundreds of cities in the occultation zone and viewing times. Times are given in Universal Time or UT. To convert UT to your time zone so you know exactly when to expect the planet to disappear and reappear for your location, subtract 5 hours for EST; 6 hours for CST; 7 hours for MST; and 8 hours for PST. Here’s a sample of U.S. cities and disappearance / reappearance times:
New York City, NY — 7:36:37 a.m. / 9:05:49 a.m. EST
Atlanta, GA — 7:07:29 a.m. / 8:45:13 a.m. EST
Columbus, OH — 7:14:47 a.m. / 8:45:48 a.m. EST
Nashville, TN — 6:04:12 a.m. / 7:38:49 a.m. CST
Duluth, MN — 6:04:56 a.m. / 7:25:47 a.m. CST
Chicago, IL — 6:07:10 a.m. / 7:35:02 a.m. CST
Kansas City, MO — 5:52:48 a.m. / 7:20:43 a.m. CST
Phoenix, AZ — 4:37:27 a.m. / 5:40:07 a.m. MST
Boise, ID — Reappearance only at 5:49:02 a.m. MST
San Francisco, CA — Reappearance only at 4:30:57 a.m. PST
Seattle, WA — Reappearance only at 4:47:41 PST