Image: Planet Venus appears close to Crescent Moon
Mike Hewitt  /  Getty Images
The planet Venus appears as a bright spot next to the crescent moon during Monday's rare alignment, as seen from Brighton, England.
By Skywatching columnist
updated 12/5/2008 4:48:54 PM ET 2008-12-05T21:48:54

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 at 4:35 p.m. ET. 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 astronomers tell us that the first-quarter moon is actually only one-eleventh 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. A moon that is half illuminated is easier on your eye, compared with 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. ET 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. PT.

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 Pleiades 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 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.

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