When is
the best time to observe the moon with a telescope? Most astronomy neophytes
might say it is when it's at full phase — but that's probably the worst time to
look at it! When the moon is full it tends to be dazzlingly bright, as well as
flat and one-dimensional in appearance.
In
contrast, the interval when the moon is at or just past first quarter phase, or
at or just before last quarter phase, is when we get the best
views of the lunar landscape right along the sunrise-sunset line or
terminator.
The
terminator can also be defined as that variable line between the illuminated
portion and the part of the moon in shadow. Along with the fact that a half
moon offers more viewing comfort to the eye as opposed to a full moon,
using a telescope with just small optical power (magnifications of 20 to 40 power),
or even with binoculars, we can then see a wealth of detail
on its surface.
Around
those times when the moon is half-lit or gibbous phase, those features lying
close to the terminator stand out in sharp, clear relief.
The
moon arrives at first quarter (half) phase on Friday, Dec. 5 at 21:25 GMT/4:35 p.m.
EST. That will be the moment when its disk is exactly 50 percent illuminated.
Lunar mountains will be readily visible as the sun lights them from the right.
How
does its brightness compare now with full? Most would probably think it's half as
bright, but in reality astronomers tell us that the first quarter moon is only
1/11 as bright as full. This is due to the fact that, a half moon is heavily
shadowed, even on its illuminated half. And believe or not, it isn't until just
2.4 days before full that the moon actually becomes half as bright as full!
In
contrast to a half moon, a full moon is almost completely illuminated,
especially right around its center; the sun shines straight down even into all
the microscopic crevices and except for perhaps around its immediate edges, you
will find no visible shadows at all. And a moon that is half illuminated is
easier on your eye as compared to the blindingly bright full phase when viewing
it through a telescope.
Moon
hides the Seven Sisters
As you watch the waxing moon this
week, take note that on the evening of Dec. 10 it will lie above and to the
right of the Pleiades star cluster for viewers in North America.
As the night progresses, the moon
will slowly approach the Pleiades, moving at roughly its own apparent diameter
per hour. During the latter hours of the night, the moon will finally encroach
upon the star cluster, in the process hiding some of its members.
Along the East Coast, this happens
at around 2:30 a.m. Eastern Time on Dec. 11, while along the West Coast it'll
be during the late evening hours of Dec. 10 at around 10:30 p.m. Pacific Time.
Keep in mind that the moon will be
almost full; stars will vanish along the sliver of the moon's dark limb and
will reappear along the bright limb. But remember that the moon's light will be
so brilliant that in order to see the stars you'll most definitely will need a
telescope! For more detailed information, including maps of the occultation
zone, as well as times for the six brightest Pleiads for dozens of North
American cities, go here.
The "most-seen" lunar phase
Lastly, have you ever noticed that
when artists portray the moon, they invariably seem to show it as either a
slender crescent or full? Half moons are shown far less frequently, while
gibbous moons are rarely depicted at all. The word gibbous is derived from the
Latin word "gibbus" meaning "hump." An unusual word to be sure, but in
describing the moon between half and full, it's the correct term.
Yet
interestingly, the gibbous moon is the most-seen phase, occurring for the half
month between first and last quarter (although for many it looks full for two
or even three nights around the time of full moon). Because it is in the sky
for more than half the night we're more apt to see the gibbous moon. In fact,
it is even visible during the daytime hours, as will be the case during this
upcoming week in mid or late afternoon. In contrast, the oft-pictured crescent
moon is visible only during the early evening or early morning hours, and
sometimes only briefly.
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.
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