How to Observe the Moon with a Telescope

I still can remember that very special August night, now more than 50 years ago, when I got my first magnified look at the moon. It was the night of a lunar eclipse and my grandfather brought out the binoculars that he used during hunting season. When I looked at the moon for the very first time through Grandpa's binos, I was hooked

Perhaps what makes the moon such a special object is the fact that it doesn't matter whether you live in a rural area or in the middle of a brightly lit city. The moon can always be readily observed. It always looks spectacular whether you're using binoculars or a telescope. It never looks precisely the same no matter how often you view it, and can be observed even on hazy or partially cloud-covered nights.

You really can't get anywhere here on Earth without consulting a map and similarly, you can't identify much on the moon without a map either. So that's the best place to start. [Best Telescopes for Beginners]

Choose a moon map

With a lunar map and perhaps a photograph of the moon as a guide, you can easily study the moon and identify a number of its most prominent features. 

In most cases, a lunar map will be oriented to show the moon as it would appear to your unaided eye or through binoculars: with its north side up. However, be aware that many astronomical telescopes provide an inverted (upside-down) view, and some even give a reversed (mirror-image) view. Some telescopes even do both. [Related: How to Take Care of Your Telescope]

If your telescope turns the moon upside-down while your map shows the moon right-side up, just turn the map upside down. On the other hand, you'll get a reversed view if you're using a telescope where the eyepiece fits into a right-angle attachment called a star diagonal.  In such a situation you'll have to mentally flip the moon in your eyepiece right-for-left to match the moon on paper.

One of the most useful lunar maps is the "Sky & Telescope Field Map of the Moon," (available in both normal and mirror-reversed versions). The latter is mainly for observers with refractors and Cassegrain telescopes, which reverse the moon's image left to right. This map is on a large-enough scale to show fine detail, but is folded in four to make it easy to use at the telescope. It is laminated with plastic to protect it from dew.

The view through a telescope

The craters and mountains on the moon are easier to see through a small telescope. The general rule of thumb regarding magnification is 50 power for each inch of aperture of your objective lens. So if, for example, you are using a 2.4-inch refracting telescope, the maximum magnification you should use is 120 power.  If you have a 4-inch, reflecting telescope, the maximum magnification is 200 power.  [Target: The Moon - Special Telescope Tricks]

"But wait a minute," you might say, "My telescope came with an eyepiece that promises 350 power. Why can't I use that?"

Well … you could, except you'll probably be disappointed. Besides the fact that the least little movement of your telescope tube will be magnified and cause an acute case of the shakes, if you look at the moon with such an eyepiece, you'll likely be exceeding the limits of your telescope's mirror or objective lens. Just like a photographer can only enlarge a photograph to a certain degree before the image gets grainy and fuzzy, using high magnification usually results in a much dimmer image that will appear to rapidly scintillate and quiver. Remember that you're looking into space through the window of our atmosphere and higher powers will only accentuate its turbulence almost always present at its very high levels.

In fact, because those tempestuous atmospheric conditions only infrequently allow you to use your maximum magnification, I would suggest using magnifications at one-half of maximum. So if you have a 2.4-inch telescope, use 60 power; for a 4-inch telescope, try 100 power. Overall, you'll likely get the most pleasing views. [Video: Which Telescope Type Is Right for You]

Binoculars are a good start

I have said it before in our Space.com forum and I will say it again: before purchasing a telescope, get acquainted with the night sky first with binoculars. Neophytes to astronomy might initially consider this a come-down, but are then pleasantly surprised the first time they look at sky objects with a pair of binoculars; they can reveal many sights that most folks think require a telescope – including the crescent phase of Venus, the four Galilean satellites of Jupiter, comets, asteroids, double stars, star clusters, nebulae, galaxies and, of course, the craters, mountains and plains on the moon. The moon shows at least as much detail in binoculars as Galileo saw with his crude telescopes.  

For astronomical work your binoculars should have objective lenses with a diameter of 50 mm. The usual power of this type of glass is 7, therefore they are labeled as 7 x 50s. 

Through binoculars you can see that the moon's surface has mountains, plains and craters: large hollows with ridges around them. The maria (pronounced MAH-re-ah; the plural of mare), which form the dark patches, were once thought to be oceans and seas.  In 1651, the Italian astronomer Giambattista Riccioli gave them poetic names like Mare Serenitatis ("Sea of Serenity") and Sinus Iridum ("Bay of Rainbows"). These names are still used, although we now know that these dark areas are flat plains of lava and that there is little or no water on the moon. Spend a few nights outdoors identifying various lunar landmarks and as Leslie Peltier soon learned, they will soon become as familiar to you as the geography of the Earth

The best time to look

Many people think that the very best time to observe the moon is when it is full. Actually, that's probably the very worst time to look at it for reasons I'll explain later. 

The very best time to observe the moon is two or three days after first quarter, for several reasons:

1. The moon is in fine position for evening study.
2. Nearly all of the major lunar features can be seen.
3. The moon is not sufficiently bright to cause loss of detail through glare.
4. As the line of darkness – called the terminator – recedes, features near the border stand out in bold relief; the shadows become stronger and details are more easily seen.

Your first glance through binoculars will reveal the maria and the larger lunar craters. Using your lunar map and perhaps an accompanying photograph, spend a few nights identifying various lunar landmarks.

Examine the terminator

Observing the region near and along the terminator will reveal a host of fascinating lunar features. There, the light comes from the side and the mountains cast long shadows, just as they would here on Earth in the early morning or late afternoon. That's why brightly lit lunar peaks stand out and the moon appears more rugged. Some of these magnificent mountain ranges, such as the Leibnitz Mountains, rise to more than 20,000 feet (6,070 meters). But most are about the same height as those here on Earth. The Apennines, Caucasus, Doerfel Mountains, and the Pyrenees all offer splendid viewing.

Be sure to check out the moon in its waxing gibbous phase, two or three days past first quarter when it's about 73 to 86 percent illuminated.  In binoculars or a telescope, two days after first quarter and situated right on, or immediately adjacent to, the terminator, you'll readily see the crater Copernicus, christened the "Monarch of the Moon," by English lunar mapmaker Thomas Gwyn Elger. Three or more days after first quarter you'll get a good view of a ray system surrounding this crater, which resembles a gigantic splash pattern. 

But down near the moon's lower limb is a crater with an even more outstanding system of rays: Tycho, one of the youngest of the major lunar craters. The Rev. Thomas William Webb, a British astronomer, poetically referred to Tycho as "the metropolitan crater of the moon." Its brilliant rays extend outward like a sunflower in all directions for many hundreds of miles. Webb claimed that it's even visible to the naked eye at full moon, something you might like to test for yourself if you have good eyesight. In his book "Exploring the Moon through Binoculars" (McGraw-Hill, 1969), Ernest H. Cherrington, Jr. says that Tycho's rays " … give the full moon the general appearance of a peeled orange, the crater marking the point where the sections meet." 

Avoid the full moon!

As the terminator moves on, the sun climbs higher in the lunar sky and the shadows grow shorter and shorter and the lofty lunar mountains will seem to melt into the landscape and almost disappear. This is why a full moon appears flat and one-dimensional, because the sun is shining on it almost directly; there's not much to see in contrasted detail from dark to light areas. In addition, the full moon is absolutely dazzling, especially during the wintertime when it climbs high into the nighttime sky on clear, crisp, sparkling nights. Did you know that a full moon is 11 times brighter than a first-quarter moon? And when viewed through binoculars or a telescope a full moon can seem almost blindingly bright.

But if you really want to look at the full moon comfortably, then perhaps consider purchasing a moon filter.  Many telescope manufacturers offer such filters to help reduce glare on the moon's surface so you can see more detail and features. These filters are especially useful to owners of large-aperture telescopes and most will thread directly onto the eyepiece barrel. [See the Best Telescopes for Kids]

Share your lunar view!

Finally, here is a suggestion: Take your telescope to a busy street in your hometown, point it toward the moon and crowds will surely gather for a look at our nearest neighbor in space.  If I may paraphrase the mysterious voice from the 1989 movie, "Field of Dreams": "If you set up your telescope, they will come." Perhaps you might even consider developing a schedule during various phases (even perhaps full moon).  You'll get a big kick hearing all the "oohs" and "ahs" or exclamations of "Wow!" just one peek through the eyepiece will get.  

Editor's note: If you capture an amazing photo of the moon that you'd like to share with Space.com and its news partners for a story or gallery, you can send images and comments in to: spacephotos@space.com.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmer's Almanac and other publications, and he is also an on-camera meteorologist for News 12 Westchester, N.Y. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.