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The Planets

The Moon

The Moon is often the biggest and brightest object in the night sky, and many of its surface features can be seen with the naked eye. It’s one of the most interesting celestial bodies to observe, whether through binoculars or a telescope.

The Far vs. Near Side of the Moon

Because the Moon is locked with the Earth’s orbit, the same side of the Moon—the near side—faces the Earth at all times. Only a few people (the astronauts who orbited the Moon for NASA’s Apollo Program, for example) have seen the far side of the Moon. The rest of us always view its near side. The Moon actually wobbles a bit in its orbit (an effect astronomers call libration), so over the course of a month, it’s possible to view up to 59% of the Moon’s surface.

Lunar Phases and When to Moongaze

Half of the Moon (the side facing the Sun) is always lit, while the other half usually looks black in the night sky. Sometimes the dark side of the Moon becomes dimly visible from earthshine (sunlight that bounces off the Earth and hits the Moon). The sunlit face sweeps across the near side of the Moon in a 29.5-day cycle, so at different times of the month, we see different lunar phases. There are a few key terms astronomers use to talk about phases:

Waxing: The Moon is getting fuller
Waning: The Moon is getting less full
Quarter: The Moon is half full
Crescent: The Moon is less than half full
Gibbous: The Moon is more than half full
New Moon: The Moon is not visible, as its orbit places it between the Earth and the Sun
Full Moon: The entire near side of the Moon is lit

Before you plan to moongaze on a given night, make sure you know the current lunar phase. A Full Moon is actually a bad time for observing features on the Moon itself. Craters and mountain chains (see “Features of the Moon,” below) pop out in highest definition near the terminator (the line where the lit side of the Moon meets the dark side), so Quarter and Gibbous Moons are the best times to moongaze. If you know when the last New Moon occurred, you can use the table below to predict the phase:

Days Into Lunar Cycle	Moon Phase	When the Moon Is Visible	What the Moon Looks Like
1 day	New	Not visible
4 days	Waxing Crescent	8 a.m. to 10 p.m.
7 days	First Quarter	11 a.m. to 1 a.m.
10 days	Waxing Gibbous	2 p.m. to 4 a.m.
14 days	Full	5 p.m. to 7 a.m.
18 days	Waning Gibbous	8 p.m. to 10 a.m.
22 days	Last Quarter	11 p.m. to 1 p.m.
26 days	Waning Crescent	2 a.m. to 4 p.m.

Features of the Moon

Even when you look at the Moon with your naked eye, you’ll discern that there are two types of surfaces: bright ones (highlands, which are covered in craters) and darker ones (lowlands, which are filled with smooth, dark, volcanic rock). The “Man in the Moon” that some people see is a collection of lowlands that make up the eyes and mouth of a jolly cartoon face. With strong binoculars or a small telescope, you can see more features in good detail:

Craters: Holes of all sizes left from asteroids, meteoroids, and sometimes comets that impacted the Moon’s surface. These often have raised edges and central peaks (mountainous regions in the middle).
Maria: Large, dark, flat “seas” where lava has covered older, cratered surfaces. Maria is pronounced “MA-ree-uh”; the singular is mare, pronounced “MA-ray.”
Rays: Bright lines radiating from craters. These rays are made of pieces of the lunar surface that were ejected when the crater was formed. The most prominent rays are visible around the Tycho crater.
Mountain ranges: Chains of mountain peaks that stretch across the surface of the Moon. The largest, the Apennines, can be seen with the naked eye near the terminator during the First Quarter Moon.
Rilles: Small valleys of lava flow that wind hundreds of miles across the Moon’s surface.