Harvest Time. These words have an almost magical ring to them for many of us. The Summer heat is giving way to cooler temperatures and with them clearer daytime and nighttime skies with Fall soon upon us. (The Autumnal Equinox actually happens on September 22nd.) And farmer’s markets are overflowing with an amazing variety of fruits and vegetables. Combine this with a big beautiful Full Moon, and the feeling that life is good is inescapable.
Full Moon occurs on September 20th this month, and for several evenings on either side of this date it still looks big and full. It seems so much brighter then than it does when half full—and it is. You would expect that it would be twice as bright since twice as much of it is showing. But in fact, it’s actually 12 times as bright! The reason for this is both subtle but fascinating. When the Moon is at its half phase, it is 90 degrees from the Sun and sunlight in falling on it at a glancing angle. This causes features such as craters and mountains to stand out in vivid relief and cast long dark shadows across the lunar surface, greatly reducing the amount of sunlight reflected back to us here on Earth. When the Moon is full it is directly opposite the Sun and sunlight hits it face-on, resulting in virtually no surface relief—everything looks featureless (except for the bright rays radiating from some of its craters)!
This results in something very important every telescope owner—especially those just starting out in their celestial exploration—needs to know. When the Moon is around its half phase, surface features jump out in majestic relief. But when at or near its full phase, there is virtually no surface relief and the Moon looks flat! (The same goes for viewing our satellite with binoculars.) So if you are looking for spectacular views of craters and mountains, avoid the Full Moon. Sadly, many have deliberately waited until then to try out their new telescope—and are frustrated and disappointed, unable to see anything like they expected. I personally know of those who were so disappointed and upset that they returned their telescopes for a refund as being defective!
— James Mullaney
Former assistant editor at Sky & Telescope magazine & author of nine books on stargazing. His latest, Celebrating the Universe!, is available from HayHouse.com.