Sky Talk May 2010: Moon-Gazing

Every month skywatchers are given the opportunity of exploring an alien world so obvious that it’s the second brightest object in the sky after the Sun. We’re referring, of course, to the Moon! And while much can be seen and enjoyed with the unaided eye alone (ranging from its ever- changing phases to lunar eclipses), it’s a virtual wonderland for those viewing it with binoculars and telescopes.

Many entire books have been devoted to the wonders to be seen on our lovely satellite by binocular and telescopic observers. Among these are: the lunar “seas” and “oceans” and bays”; mountain ranges, craters and valleys; domes and rilles and rays; eerie “earthshine” on the crescent Moon (light from the Pacific reflected onto the lunar surface in the evening and from the Atlantic in the morning); occultation’s of planets and bright stars and glittering star clusters like the Pleiades; strange events known as Transient Lunar Phenomena (obscurations, flashing lights and colorations); and eclipses of both the Moon itself and of the Sun caused by the Moon.

The most pleasurable way to become familiar with the Moon’s features is to undertake a “sightseeing” tour using a basic lunar map like that accompanying this article. While this can be done with binoculars (which should be tripod-mounted for stability), the fun really begins when using a telescope. Low-power, wide-field views of our satellite like that given by the Edmund Astroscan with its six Full-Moon diameters of sky at 16x show it dramatically suspended and seemingly floating in space (which it actually is!) Going to ever higher powers, while reducing how much of the Moon is actually seen, shows it in amazing detail — so much so, that at magnifications of several hundred power, observers have likened it to being in orbit around it! (Many of the telescopes offered in the Scientifics catalog can achieve such magnifications on nights of steady atmospheric conditions, or good “seeing.”) To find out how close your telescope is bringing the Moon to you, simply divide its average distance (roughly 240,000 miles) by the magnification you’re using. Thus, 24x brings you to within 10,000 miles of its surface, while 240x places you just 1000 miles above it!

There are a couple of important facts to keep in mind about Moon-gazing. The best place to see lunar detail is along the “terminator” — the dividing line between night and day on the Moon and the point at which the Sun casts the longest shadows, thereby exaggerating surface relief. This is at its most dramatic around the time of First-Quarter (Moon half-full), during which surface features stand out in utterly amazing and bewildering detail. (The extent of the effect of shadowing on the Moon is revealed by this surprising fact: you would think that the Full Moon would be twice as bright as the Half Moon — but in fact it’s 12 times as bright due to the near-absence of shadows on the surface at Full!) Another important point. You might assume that if you’ve examined a certain lunar feature in detail, it will look exactly the same at the next lunation (or phase). But in fact, due to the Moon’s complex orbital dynamics and libration (wobbling on its axis), it takes seven years before the very same lighting conditions repeat themselves!

–James Mullaney
Former assistant editor at Sky & Telescope magazine and author of seven books on stargazing.


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