December 21, 2007

Full Moon Meets Mars

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

A similar encounter in 2003 created a great photo

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

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.

Pacific States:

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.