Explore Moon Mountains from Your Backyard

When isthe best time to observe the moon with a telescope? Most astronomy neophytesmight say it is when it's at full phase — but that's probably the worst time tolook at it! When the moon is full it tends to be dazzlingly bright, as well asflat and one-dimensional in appearance.

Incontrast, the interval when the moon is at or just past first quarter phase, orat or just before last quarter phase, is when we get the bestviews of the lunar landscape right along the sunrise-sunset line orterminator.

Theterminator can also be defined as that variable line between the illuminatedportion and the part of the moon in shadow. Along with the fact that a halfmoon 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 detailon its surface.

Aroundthose times when the moon is half-lit or gibbous phase, those features lyingclose to the terminator stand out in sharp, clear relief.

  • TheDisappearing Moon: Why and Where it Hides

Themoon 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.

Howdoes its brightness compare now with full? Most would probably think it's half asbright, but in reality astronomers tell us that the first quarter moon is only1/11 as bright as full. This is due to the fact that, a half moon is heavilyshadowed, even on its illuminated half. And believe or not, it isn't until just2.4 days before full that the moon actually becomes half as bright as full!

Incontrast to a half moon, a full moon is almost completely illuminated,especially right around its center; the sun shines straight down even into allthe microscopic crevices and except for perhaps around its immediate edges, youwill find no visible shadows at all. And a moon that is half illuminated iseasier on your eye as compared to the blindingly bright full phase when viewingit through a telescope.

Moonhides the Seven Sisters

As you watch the waxing moon thisweek, take note that on the evening of Dec. 10 it will lie above and to theright of the Pleiades star cluster for viewers in North America.

As the night progresses, the moonwill slowly approach the Pleiades, moving at roughly its own apparent diameterper hour. During the latter hours of the night, the moon will finally encroachupon the star cluster, in the process hiding some of its members.

Along the East Coast, this happensat around 2:30 a.m. Eastern Time on Dec. 11, while along the West Coast it'llbe during the late evening hours of Dec. 10 at around 10:30 p.m. Pacific Time.

Keep in mind that the moon will bealmost full; stars will vanish along the sliver of the moon's dark limb andwill reappear along the bright limb. But remember that the moon's light will beso brilliant that in order to see the stars you'll most definitely will need atelescope! For more detailed information, including maps of the occultationzone, as well as times for the six brightest Pleiads for dozens of NorthAmerican cities, go here.

The "most-seen" lunar phase

Lastly, have you ever noticed thatwhen artists portray the moon, they invariably seem to show it as either aslender crescent or full? Half moons are shown far less frequently, whilegibbous moons are rarely depicted at all. The word gibbous is derived from theLatin word "gibbus" meaning "hump." An unusual word to be sure, but indescribing the moon between half and full, it's the correct term.

Yetinterestingly, the gibbous moon is the most-seen phase, occurring for the halfmonth between first and last quarter (although for many it looks full for twoor even three nights around the time of full moon). Because it is in the skyfor 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 thisupcoming week in mid or late afternoon. In contrast, the oft-pictured crescentmoon is visible only during the early evening or early morning hours, andsometimes only briefly.

Joe Rao serves as an instructorand guest lecturer at New York's Hayden Planetarium. He writes about astronomyfor The New York Times and other publications, and he is also an on-camerameteorologist for News 12 Westchester, New York.

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Joe Rao
Skywatching Columnist

Joe Rao is Space.com's skywatching columnist, as well as a veteran meteorologist and eclipse chaser who also serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications. Joe is an 8-time Emmy-nominated meteorologist who served the Putnam Valley region of New York for over 21 years. You can find him on Twitter and YouTube tracking lunar and solar eclipses, meteor showers and more. To find out Joe's latest project, visit him on Twitter.