The moon reaches full phase Tuesday. But as far as full moons go, it's not the most impressive. In fact, it's the smallest full moon of the year.
About 13 hours after it's officially full, the moon will arrive at the point in its orbit farthest from Earth (called apogee), a distance of 252,518 miles (406,389 kilometers). The moon's apparent angular size that night will be at a minimum in 2010.
Though the casual viewer may not notice the difference, the Aug. 24 moon will appear 12.3 percent smaller than the full moon of Jan. 30, which nearly coincided with perigee — the moon's closest point in its orbit relative to Earth. [Moon Mechanics]
Observing the moon
When is the best time to observe the moon with a telescope? Most astronomy neophytes might say during full phase, but that's probably the worst time. The moon tends to be dazzlingly bright as well as appearing one-dimensional.
The intervals when the moon is at or just past first-quarter and last-quarter phases are when we get the best views of the landscape right along the moon's sunrise-sunset line, or terminator. [Skywatcher's Guide to the Moon] (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 than a full moon does, we can see a wealth of detail on its surface using a telescope with relatively small optical power (magnifications of 20 to 40 power), or even good binoculars. Around those times when the moon is half-lit or in the gibbous phase (as it will be next weekend), those features lying close to the terminator stand out in sharp, clear relief.
The moon was at first-quarter phase on Aug. 16 at 2:14 p.m. ET. That was the moment when its disk was exactly 50 percent illuminated. Lunar mountains became visible as the sun lit them from the right.
How bright it is
How does brightness of a half moon compare with that of full? Most would probably think it's half as bright, but astronomers tell us the half moon of the first quarter is only one-eleventh as bright as full. This is due to the fact that it is heavily shadowed, even on its illuminated half. Believe or not, it isn't until just 2.4 days before full that the moon actually becomes half as bright as when it's full.
A full moon is almost completely illuminated, especially right around its center; the sun shines straight into all the microscopic crevices, and perhaps except for around the immediate edges, you will find no visible shadows at all.
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, and gibbous moons are rarely depicted at all. Ansel Adams' famous photograph "Autumn moon" is an outstanding exception, involving a waxing gibbous moon that Adams imaged from Yosemite National Park's Glacier Point in California back in September 1948.
What 'gibbous' means
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.
The gibbous moon also is the most-seen phase, occurring for the half month between first and last quarter. 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. That will be the case just after sunrise on the weekend following the full moon. 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.