Sky Talk June 2011: Some Frequently-Asked Questions About the Moon

Although it’s the most readily visible (aside from the Sun itself) and nearest to us of all celestial bodies, the Moon poses puzzling questions to many people — especially those new to the hobby of skywatching. Here are some of the most often asked ones.

Q: Why is the First Quarter Moon as shown on calendars actually half full?
A: The answer here is quite simple. When the Moon appears half-illuminated or half-full, it’s one quarter of the way around its monthly orbit. Likewise, at Last Quarter it’s three quarters of the way around. Yes, simple — but still very confounding terminology indeed!

Q: Why is the Full Moon so much brighter than the First Quarter one?
A: You would expect it to be twice as bright since there’s twice the area reflecting the Sun’s light back down to us. But the Moon when full is actually 12 times brighter! The reason here is not so simple: when at its quarters phases, the Sun is shining almost horizontally at a glancing angle along the surface (especially near the terminator, or dividing line between light and dark), casting very long shadows from surface features ranging from mountains and craters to boulders and even small rocks and pebbles. All those shadowed areas reflect little or no light. But when full, the entire surface of the Moon is bathed in direct sunlight and reflected our way — there are no shadows. This explains why the best times to observe our lovely satellite with binoculars and telescopes are around the quarters when surface relief is strikingly exaggerated by shadows. The Moon when full looks flat and bland — sadly disappointing many new to observing who expect to see jagged mountain ranges and deep craters!

Q: Is there really a dark side to the Moon?
A: Yes – and no! As the Moon orbits the Earth, its entire surface is illuminated by sunlight at one time or another, back as well as front. Known as “complimentary phases,” this means that when the Moon is full to us, its back side is totally dark. When the Moon is new and dark to us, the back is bathed in direct sunlight. In between, the back side experiences the compliment of whatever phase we are seeing. When a crescent to us, it appears gibbous there. At the quarters, both front and back are equally half illuminated.

Q: If the Moon rotates, why do we never see the back side?
A: That’s because the Moon rotates in the same period as it revolves around the Earth. To see this, stand in the middle of a room and have someone face you several feet away. Now, ask them to slowly move around you while always facing in the same direction (which means they are not rotating on their “axis” but only revolving in their “orbit” around you). As you follow their movement, you’ll first see the side of their face and then the back of their head! If the Moon didn’t rotate, we would see the back side just as in this example.

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