If skies are clear in your area on Sunday night, Dec. 23, you'll be able to partake in a rather unusual sight as the full moon appears to glide very closely above the planet Mars.
Mars, which made its closest approach to the Earth on Dec. 18, will be only hours from a Christmas Eve opposition with the sun and is now shining prominently with a bright yellow-orange glow.
And if you're favorably positioned in certain parts of the Pacific Northwest, western Canada, or Alaska, you'll actually see the moon occult (hide) Mars for a short time as the pair sits low above the east-northeast horizon.
A similar encounter in 2003 created a great photo opportunity.
The farther north and west you are located, the closer together the moon and Mars will appear; the time of closest approach will come earlier in the evening as you head west. Those along the U.S. East Coast will get their best views during mid-evening between about 8:40 and 9:10 p.m. EST. For skywatchers along the West Coast, closest approach comes as evening twilight fades between about 5:30 and 5:45 p.m. PST.
Two full moons?
It is rather ironic that Mars and this upcoming full moon will appear side-by-side, considering that several months ago, an Internet hoax regarding Mars and the moon unfortunately received wide circulation. This infamous e-mail message duped countless people into believing that there would be "two full moons" shining side by side in the sky, implying that Mars would seem to loom as large as the moon itself. As preposterous as this hyperbole sounded, many nonetheless made elaborate plans to be outside on the appointed night, fully expecting to see Mars swollen to an incredibly large size. Of course, it was not to be.
Now keep this in mind: With this upcoming pre-Christmas tableau, Mars will be only 1/127 as large as the disk of the moon. So to the naked eye it will appear not as a disk, but as a non-twinkling, albeit brilliant "star."
No doubt many last-minute holiday shoppers who are out on Sunday evening might do a double-take should they cast their gaze up toward the winter's first full moon and wonder, "What is that star that happens to be hovering below it?" But unless they're looking through a high-power eyepiece of a telescope nobody should expect to see Mars even remotely resembling a moon-sized object! Something for all of you to keep in mind next summer, if that insipid Martian hoax gets recycled yet again.
Below are details of how this event will appear for several different regions of North America. Note that 10 degrees in the sky is about equal to the width of your fist on an outstretched arm.
Eastern States/Maritime Provinces/Southern portions of Quebec/Central and Eastern Ontario:
For places in the Eastern time zone, the moon and Mars will appear separated by roughly 2 to 3-degrees as they rise from east-northeast at dusk.
Mars will be situated below and to the left of the moon. Notice that moon will appear to approach Mars as the evening progresses by its own diameter each hour. By about 7:00 p.m., the moon will seem to be sitting directly above Mars and an hour later, it will have shifted to the upper left of Mars. They will appear closest together at around 8:55 p.m. — give or take about 15-minutes depending on your exact location. Mars will appear to get as close as about 21 arc-minutes relative to the moon's lower right limb ... that's about two-thirds of the apparent width of the moon.
For the rest of the night, the moon will slowly pull away to the left (east) of Mars. For the Canadian Maritimes, Mars and the moon are closest at 10:22 p.m. for Halifax, NS and 11:12 p.m. for St. John's, NL.
Central States/Western Ontario/Southern Manitoba:
For places in the Central time zone, Mars and the moon will be separated by only about a couple of degrees (or even slightly less) as they emerge from the east-northeast horizon at dusk.
Soon after 6:00 p.m., the moon will be sitting directly above Mars. At about 7:40 p.m. — give or take about 10 minutes — they'll appear closest, with Mars situated about 16 arc minutes (about half the apparent width of the moon) below and to the right of the moon's edge.
The moon then pulls away from Mars at its own diameter each hour for the rest of the night.
Mountain/Plains States/Canadian Prairies:
From the Mountain time zone, the moon will appear seem to be sitting less than its own apparent width, directly above Mars at 5:45 p.m. Less than an hour later at around 6:35 p.m. — give or take five minutes — you'll see Mars sitting only about 9-arc minutes — or less than one-third of the moon's apparent width — from its lower right edge.
From California, the moon will hover just above Mars at 5:00 p.m. PST, but the pair will be very low to the east-northeast horizon.
From Los Angeles, closest approach comes at 5:32 p.m. — only 8 arc minutes separate Mars from the moon's lower right limb. They'll be only half as close as this (about one-eighth of the moon's apparent diameter) as seen from San Francisco at 5:37 p.m.
Traveling farther north (toward Oregon) places you closer to the southern limit of where Mars will actually disappear behind the moon's right-hand edge.
In the occultation zone
If you live anywhere north of a line that will run southwest to northeast from near Newport, Oregon to Eastport, Idaho, and continuing on up toward western Hudson Bay, you will see the moon cross in front of Mars and temporarily eclipse it.
This includes the northwestern part of Oregon, much of Washington state (except the southeast) and a small sliver of northernmost Idaho. Also within the viewing zone is a large part of western Canada, as well as the entire state of Alaska. Some notable cities are within the zone, including Portland, Seattle, Vancouver, Edmonton and Anchorage. From Portland, for instance, Mars will be hidden from 5:46 to 5:50 p.m. PST.
The glare from the brilliant moon will make the disappearance and reappearance all but impossible to see with the naked-eye alone; binoculars or better yet, a telescope should be used as Mars gradually closes in on the moon and later moves away from it.
A map of the visibility zone along with a schedule for dozens of cities in North America, as well as Europe and Asia (where Mars will be occulted during the predawn hours of Dec. 24) can be accessed here.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.