How to Observe the Moon: Craters, Seas and Mountains

The Moon is the one target in the night sky that looks spectacular through every instrument, from your naked eye to the largest telescope.

It is bright enough to observe from the middle of a city, it changes every night,

and it holds more detail than you could explore in a lifetime.

The trick is knowing when and where to look.

This guide explains why the best views come away from full Moon, what the major landforms are,

which features to find at each stage of the lunar month and which simple techniques sharpen the view.

Why the Terminator Is Where the Action Is

The terminator is the line that separates the sunlit part of the Moon from the dark part. Along this line,

the Sun is just rising or setting on the lunar surface, so it shines at a very low angle.

Crater walls, mountain peaks and gentle ridges cast long shadows, and the landscape leaps into three dimensions.

Away from the terminator, the Sun is higher in the lunar sky and the shadows shrink, so the surface looks flatter.

That is why full Moon, when the Sun shines straight down on the whole face we see,

is actually one of the least rewarding times for detail.

The terminator shifts every night, so the Moon is a slightly different landscape every time you return to it.

Know the Landscape

Seas

The dark gray patches are called maria, Latin for seas, a name given by early observers who wondered whether they held water.

They are actually vast plains of solidified lava that flooded huge impact basins billions of years ago.

They have relatively few craters, which tells geologists they are younger than the surrounding terrain.

Highlands

The bright regions are the lunar highlands,

older and far more heavily cratered. Much of the Moon's southern half is a jumble of overlapping craters with hardly a smooth patch between them.

Craters, Mountains and Rilles

Craters were blasted out by impacts and range from tiny pits to walled plains more than a hundred miles across.

Many large ones have terraced walls and central peaks.

The great mountain ranges are mostly the uplifted rims of ancient impact basins.

Look also for rilles, long winding or straight channels, and wrinkle ridges snaking across the seas.

Getting Oriented

Which way is up depends on where you are and what you use.

Observers in the Southern Hemisphere see the Moon rotated compared with northern views, and many telescopes flip or mirror the image.

Rather than memorizing left and right, orient yourself by the large seas, which are easy to recognize with the naked eye.

Start with Mare Crisium, the isolated oval sea near the edge of the disc that is lit first after new Moon.

Next to it lies Mare Tranquillitatis, where the first crewed landing touched down in 1969, and beyond it Mare Serenitatis.

The biggest dark areas are Mare Imbrium and the sprawling Oceanus Procellarum on the side lit later in the month.

A printed lunar map, or a Moon app set to match your telescope's view, makes identification much easier.

A Tour Through the Lunar Month

These highlights are roughly in the order the terminator reaches them after new Moon.

The days are approximate and vary a little from month to month.

Young Crescent, Days 3 to 5

Mare Crisium is the first large feature to appear.

Along the terminator, look for the big craters Langrenus and Petavius, the latter with a prominent central peak and a rille running across its floor.

While the crescent is thin, enjoy earthshine faintly lighting the rest of the disc.

Around First Quarter, Days 6 to 8

This is one of the richest periods.

The trio of Theophilus, Cyrillus and Catharina stands on the edge of Mare Nectaris, and Theophilus shows a cluster of central peaks.

Near the middle of the disc, the chain of Ptolemaeus, Alphonsus and Arzachel runs north to south.

Farther north, the curving Apennine Mountains form the rim of Mare Imbrium, and the Alps are cut by a long,

straight gash called the Alpine Valley.

Waxing Gibbous, Days 8 to 12

About a day after first quarter, look in Mare Nubium for the Straight Wall,

a fault that casts a thin dark line when the Sun is low.

Next comes Copernicus, a magnificent crater with terraced walls, often called the Monarch of the Moon.

A few nights later, the curved mountains around Sinus Iridum, the Bay of Rainbows,

catch the sunlight while the bay's floor is still dark, creating a glowing arc.

Dark-floored Plato, near the Alps, is a good test of how well you are seeing.

Full Moon

Shadows vanish at full Moon, but bright ray systems take over.

Tycho, a geologically young crater in the southern highlands, sends brilliant rays across much of the disc,

and Copernicus and Kepler have rays of their own. Aristarchus stands out as the brightest spot on the Moon.

Full Moon is also a good time to learn the shapes of the seas.

Waning Phases

After full Moon, the terminator sweeps across the same features in the same order, but lit from the opposite side,

which makes familiar craters look new.

The waning Moon rises later each night, so these views belong to late-night and early-morning observers.

Gear and Techniques

Every level of equipment has something to offer:

• Naked eye: the pattern of seas, the changing phase, earthshine and the Moon's motion against the stars.
• Binoculars: dozens of craters along the terminator and the outlines of mountain ranges, especially with a 10x50 pair on a tripod.
• Small telescope: crater walls, central peaks and rilles, starting at around 50 to 100 times magnification.
• Larger telescope on a steady night: fine detail at higher powers, including tiny craterlets and delicate rilles.

The Moon can be dazzling in a telescope.

A neutral-density Moon filter or a variable polarizing filter reduces glare and eye fatigue.

Never use a Moon filter to look at the Sun;

only a certified solar filter fitted over the front of the telescope makes solar viewing safe,

and without one you should never point any optics at the Sun.

The steadiness of the air, called seeing, limits how much magnification you can use.

When the view boils, drop the power.

The Moon also looks sharpest when it is high in the sky, where you look through less turbulent air,

and after your telescope has cooled to the outdoor temperature.

Libration: Peeking Around the Edges

The Moon always keeps roughly the same face toward Earth, but its slightly tilted,

slightly oval orbit makes it appear to rock gently over the month.

This effect, called libration, lets us see a little around the edges,

so that over time about 59 percent of the surface is visible from Earth.

Features near the edge, such as Mare Crisium, shift noticeably closer to or farther from the limb from one month to the next.

Sketch, Photograph and Keep a Log

The most effective way to learn the Moon is to record what you see.

A quick sketch of a single crater along the terminator forces you to notice details you would otherwise skim past.

You can also hold a phone camera carefully to the eyepiece to capture a surprisingly good image.

Note the date, time, phase, instrument and seeing conditions with each observation.

Many astronomy clubs run lunar observing programs with lists of features to find,

which give each session a goal and help you learn the whole face of the Moon systematically.

Your Next Clear Night

Check the phase, find the terminator and pick two or three features from the tour above.

Observe at low power first, then increase magnification if the air is steady.

Come back the next night and the same region will look different under a new angle of sunlight.

Few targets reward return visits as generously as our nearest neighbor.