Looking Up: Our moon - so close yet so far

Peter Becker

It’s just as well most of us don’t spend much time thinking how we look - at least from outer space. There’s plenty of attention giving down here on the ground about appearances, without wondering what can be seen from “out there.”

Astronauts in orbit, just by looking out their window or when space walking, can see plenty of evidence of human habitation, though of course they can’t see people. Vast croplands are a tapestry of fields. Urban areas stand out from the green of forests. The night side is dotted with the concentrated accumulation of thousands of lights marking our cities and towns.

Yet individual manmade features are elusive. Astronomy Magazine writer Michael E. Bakich noted that contrary to a common myth, even the Great Wall of China is too small and far away to stand out without magnification. While the Great Wall is 5,500 miles long, it spans only 30 feet wide at most. Although some scientists say it could be detected if the atmosphere were clear enough and the sun at the right angle, Bakich notes that to discern something 30 feet across would take a resolving power five times that of the human eye, from low Earth orbit.

Apollo astronauts became an elite group, among whom 12 of them actually walked on the lunar surface. Nine missions, totaling 27 astronauts, went around the moon. From that distance, these men between the years 1968 and 1972, had the awesome privilege of looking back at our home planet. The Earth was described like a Christmas ornament, a fragile glass ball colored green blue and white.

The Earth appears roughly four times as wide in the lunar sky as the moon does from Earth. It would require a telescope to detect anything on Earth smaller than a width of 70 miles, given the average distance of 238,000 miles.

Likewise, the moon that you can see this week in the evening sky - first-quarter moon occurs May 10 - shows only its lovely white face, dotted with gray spots, we have nicknamed the face of the “man in the moon.” None of the millions of craters are visible to our unaided eye, save the white splotch that marks the location of the huge crater Copernicus, within a great area of gray.

Even a small pair of binoculars, magnifying five or seven times, will show a wealth of craters and jumbled mountains along the rough terminator - the line between the sunlit portion and darkened area of the moon. If you were standing on the lunar terminator, the sun would appear on the horizon, either setting or rising. It is here that shadows are the longest, bringing out features with the most relief. As seen in a small telescope, craters stand out along this ragged line, their bowl-shapes partly or completely filled with ink-black shadow. Along the very terminator, only the crater rims might be seen, or possibly the tip of a central mountain peak, just catching the sun’s rays.

Distances are so great that we couldn’t see anything as small as the U.S. flags left on the moon, no matter how large our telescope. Resolving detail in a telescope is complicated by the constantly moving, blurry blanket of air between our feet and the gradual edge of space. Earth’s turbulent atmosphere occasionally settles down enough to allow us to use our telescopes to the best of their capability, but it is usually a rare moment.

To increase our chances for the most stable view, examine the moon, the planets or close spacing of stars when they are high up - and not low near the horizon. When  low, we are looking through the most atmosphere, and the light path is distorted the most. Also beware of using too high a magnification for your telescope. A rule of thumb is that 50x per inch of aperture (the width of your reflector’s main mirror or refractor’s front lens) is the most you can use to advantage. Thus, a 4-inch aperture telescope can handle up to 200x if the atmospheric steadiness (”seeing”), your telescope’s optical quality and alignment are all good.

That will still show you a lot. To figure how “close” you can get to the moon, divide the magnification into the distance. At 200x, you can see the moon as if you were 1,190 miles above the surface!

To get as close as one mile from the moon you would need a telescope magnifying 238,000X - needing an aperture of nearly 40 FEET!

Send your comments and questions to

Keep looking up!