Image: Half-full moon
R.M. Sandy / NASA
Mountains and shadowed craters provide plenty of visual interest when the moon is half full.
By Skywatching columnist
updated 1/6/2006 8:50:35 PM ET 2006-01-07T01:50:35

If you have just received a telescope as a holiday gift or resolved to pull the old one out of the closet in 2006, perhaps your first target will be our nearest neighbor in space: the moon. It's a great target that offers endless opportunities even for experienced skywatchers.

But when is the best time to observe the moon with a telescope? 

Many people might assume it is when the moon 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 20x to 40x), or even with good binoculars, we can then see a wealth of detail on its surface.  Around those times when the moon is half-lit or at gibbous phase, those features lying close to the terminator stand out in sharp, clear relief.

The moon arrived at first-quarter phase on Friday at 1:56 p.m. ET.  That marked the moment when its disk was exactly 50 percent illuminated.

Lunar mountains are visible as the sun lights them from the right.  How does it 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 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.

As you watch the waxing moon this week, note that it will pass north of Mars on Sunday evening.  And on Monday night it will appear to cross in front of parts of the Pleiades star cluster for viewers in North America and northern South America.

Finally, 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 lunar phase, occurring for the half-month between first and last quarter (although to 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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