There is nothing so lovely as seeing a big bright full-Moon rising over the eastern horizon. But when our satellite is around its full phase it is the bane of serious stargazers (who are often called “Moon dodgers”!). Although moonlight doesn’t interfere with the planets and brightest stars, it brightens the sky enough to mask fainter wonders like star clusters, nebulae and galaxies.
The Moon will be full on September 2nd. And despite the above, it offers a number of fascinating features for skywatchers. Most obvious is the famed “Moon Illusion”—the fact that it looks huge upon rising but becomes smaller as it climbs higher in the sky. Many explanations have been given but none solve the mystery. For example, it looks big because it is near objects like trees and houses to compare it with when on the horizon. But here’s the kicker—bend over and look at the Moon through your legs, or alternately through a cardboard tube. When you do, it shrinks to its usual size! Also, photographs taken when low and high in the sky show it to be slightly smaller when on the horizon than when overhead because at that time it is the 4,000 mile radius of the Earth further from us.
Another illusion is that as you watch the Moon “rising,” it is actually the Earth “setting!” It’s the 900 miles-per-hour (depending on your latitude) rotation of our planet eastward that makes the
Moon appear to be moving up and westward in the sky. But your horizon is actually dropping! Once you realize this, so strong is the impression of motion that some observers have reported a feeling of vertigo.
Also fun is identifying various lunar features with the unaided eye, such as the dark maria (or “seas”) and large craters like Copernicus or Tycho—both of which have bright streaks or rays radiating outward from them. This activity becomes even more interesting using binoculars, which reveal hundreds of features all across the face of the full Moon and along its edge. But these are not the dramatic mountains and craters so obvious around the time of first or last quarter. At full-Moon the Sun is shining directly head-on—but at the half phases sunlight is coming in horizontally, causing features to stand out in striking relief due to their shadows.
One other thing to be alert for at full Moon are rapid “flyovers” of geese, planes, and even satellites—including the International Space Station! They are seen dramatically silhouetted against the lunar surface, as they silently glide across the face of our lovely Queen of the Night.
— James Mullaney
Former assistant editor at Sky & Telescope magazine & author of 10 books on stargazing. His latest, Celebrating the Universe!, is available from HayHouse.com.