The Gibbous Moon: A Skywatching Guide

Gibbous Moon Skywatching Sights
Some sights to look for on the gibbous moon, which dominates the evening sky this week. (Image credit: Starry Night Software)

Full moons tend to get most of the stargazing attention, but there's plenty to see now on Earth's nearest neighbor in its "waxing gibbous" phase.

The sun's shifting light will throw different parts of the moon into relief over the next few days, yielding interesting views of lunar craters and other landforms. 

Here's a brief guide of what to look for, after a bit of background on moon phases.

A moon phase primer

In its monthly trip around the Earth, the moon passes four very distinct landmarks or phases: new moon, first quarter, full moon and third (or last quarter), before returning to new moon again. These mark 0 degrees, 90 degrees, 180 degrees and 270 degrees, respectively, as measured from the position of the sun. [The Moon's Phases and Cycle (Infographic)]

All four phases are exact instants in a particular month. This cycle, they fall on the following days: new moon, Feb. 10; first quarter, Feb. 17, full moon, Feb. 25; and last quarter, March 4.

In between these four instantaneous phases, the moon is said to be in four transitional phases, which each last slightly more than a week. "Waxing crescent" comes after new moon, "waxing gibbous" follows first quarter, "waning gibbous" comes after full moon and waning crescent follows last quarter.

A "waxing" moon is getting larger while a "waning" moon is shrinking. Most people are familiar with a "crescent" shape, but the word "gibbous" is not normally in most people's vocabulary. It is from a Middle English word for "hump-backed" but can also refer to a pregnant belly.

Of course, the moon doesn't really change shape. The moon's phases are caused by the changing angle from which the sun illuminates it as the moon makes its way around the Earth.

What to look for

The current phase of the moon is waxing gibbous. The sun is shining at an angle from behind the Earth. We are past first quarter so more than half the moon is in sunlight, but the sun's light doesn't quite reach the far left of the moon. As always, the most interesting area to observe is close to the "terminator" — the dividing line between day and night on the moon.

Depending on which night you look at the moon, different areas will be shown in high relief by the light of the rising sun.

On the northern half of the moon, look for the curving arc of the Mare Crisium, along with the smaller arc of the Sinus Iridum. These features were named in the 17th century, before astronomers knew that the moon was a dry and airless world. The names translate from the Latin as the "Sea of Crises" and "Bay of Rainbows," respectively.

Just north of the Mare Crisium is the distinctive oval crater Plato, whose smooth dark floor is broken only by a few tiny craterlets. This crater is 68 miles (109 kilometers) in diameter, and is a perfect example of a crater whose floor filled with lava shortly after impact. Since then, a handful of impacts have created small pits in the smooth floor, visible only in fairly large telescopes under perfectly steady viewing conditions, an excellent test for the optical quality of a telescope. [Telescopes for Beginners: A Buyer's Guide]

Toward the southwest limb of the moon, look for the large crater Gassendi, named for 17th century French astronomer Pierre Gassendi. Sixty miles (101 km) in diameter, Gassendi's floor is full of many interesting features: a slightly off-center mountain, several large craters, a system of rilles (shallow grooves) and a strange cluster of conjoined domes (gentle swellings).

Near the lunar south pole is one of the youngest craters on the moon, named for Tycho Brahe, the famous Danish observer of the 16th century, whose observations were used by Johannes Kepler to determine the laws that govern the motion of the planets.

Tycho is 63 miles (102 km) in diameter. Its young age is betrayed by the bright ground it exposed and then sent out in long circumlunar rays in all directions.

Another bright young crater is Aristarchus in the northwest quadrant. Aristarchus lived in the 3rd century BCE and had ideas way ahead of his time, notably that the Earth revolved around the sun rather than the other way around. Although fairly small, at 25 miles (40 km) in diameter, Aristarchus' brightness makes it an easy crater to spot.

The names we use today for craters on the moon were mainly applied by Giovanni Riccioli in the 17th century.

Riccioli used names from ancient Greece, Rome, and medieval Europe near the center of the moon and placed the modern (for his day) astronomers around the limb of the moon. Nowadays modern scientists, all deceased, are placed anywhere on the moon. The only craters named for living people are a handful that bear the monikers of Apollo astronauts.

This article was provided to by Starry Night Education, the leader in space science curriculum solutions. Follow Starry Night on Twitter @StarryNightEduFollow on Twitter @Spacedotcom. We're also on Facebook & Google+

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Geoff Gaherty
Starry Night Sky Columnist

Geoff Gaherty was's Night Sky columnist and in partnership with Starry Night software and a dedicated amateur astronomer who sought to share the wonders of the night sky with the world. Based in Canada, Geoff studied mathematics and physics at McGill University and earned a Ph.D. in anthropology from the University of Toronto, all while pursuing a passion for the night sky and serving as an astronomy communicator. He credited a partial solar eclipse observed in 1946 (at age 5) and his 1957 sighting of the Comet Arend-Roland as a teenager for sparking his interest in amateur astronomy. In 2008, Geoff won the Chant Medal from the Royal Astronomical Society of Canada, an award given to a Canadian amateur astronomer in recognition of their lifetime achievements. Sadly, Geoff passed away July 7, 2016 due to complications from a kidney transplant, but his legacy continues at Starry Night.