What is a Gibbous Moon?

When we look up at the Moon in the sky, we always see the same “Man on the Moon” staring back at us. However, at different times of the month, the Moon appears to take on different shapes. Let’s take a look at these “phases” of the Moon, which will help us better understand just what a gibbous Moon is.

Gibbous Moon
A gibbous moon is visible above Earth’s atmosphere, photographed by an Expedition 17 crew member on the International Space Station. (Image credit: NASA)

What Does the Word Gibbous Mean?

As we study the Moon and how it appears to us in the sky, taking a look at the word “gibbous” will offer us a clue as to how it applies to the Moon. With origins in Latin, gibbous refers to a convex, humped, or bulging shape.

Why Does the Moon Appear to Change Shape?

As the Moon revolves around our Earth over the course of roughly 28 days, it goes through a total of eight different phases. Of course, the Moon isn’t actually disappearing and reappearing.

8 phases of the moon
8 Phases of the Moon. (Image credit: “moon-in-phases” by Spirit-Fire on Flickr CC BY 2.0)

The part of the Moon we see is from sunlight that hits the Moon’s surface and is reflected to us on Earth. As the sunlight hits the Moon in different areas over those 28 days, it appears to take on different shapes.

Waxing vs. Waning

Waxing vs Waning Moon
Waxing vs Waning Moon. (Image credit: NASA)

When we describe the Moon’s phases, we talk about them either waxing or waning. These fancy terms simply refer to whether the Moon appears to be getting larger or smaller in the sky.

If the Moon is getting larger, we call it a waxing Moon.

If the Moon is getting smaller, we call it a waning Moon.

How can you tell if the Moon is waxing or waning?

As the Moon orbits the Earth in the Northern Hemisphere, sunlight gradually moves across the lunar surface from right to left. This means a Moon lit on the right side is waxing (or getting bigger), and a Moon lit on the left side is waning (getting smaller).

It’s important to note that the exact opposite happens if you are in the Southern Hemisphere. There, the Sun will first appear on the left side of the lunar surface and get bigger from there. Once at its fullest, the Moon will shrink from left to right until it disappears completely.

If you want to know whether it is currently a waxing or waning moon, check out this great resource from moongiant.com: Today’s Moon Phase.

What Are the Phases of the Moon?

New Moon

New Moon
By the modern definition, New Moon occurs when the Moon and Sun are at the same geocentric ecliptic longitude. (Image credit: NASA)

During a new Moon, we on Earth are unable to see the Moon at all. Although not quite on the same plane, the Moon is situated between the Earth and the Sun. This means only the part of the Moon that faces away from Earth receives any light.

Waxing Crescent Moon

Waxing Crescent Moon
Waxing Crescent Moon. (Image credit: NASA)

Other than a full Moon, a Crescent Moon is perhaps what most people picture when they think about the Moon. During this phase, the Moon is starting to get larger, but still only a sliver of the Moon is visible as it moves out from being between the Earth and Sun.

Most of the Sun’s light is still shining on the away-facing side of the Moon that we can never see.

A waxing crescent Moon is seen following closely behind the Sun as it moves across the sky. It will rise shortly after the Sun and set shortly after the Sun does.

This phase spans the time it takes for a new Moon to become a quarter Moon.

First Quarter Moon

First Quarter Moon
First Quarter Moon. (Image credit: NASA)

As the Moon continues to get more prominent in the sky, it eventually reaches a point where we can see exactly half of its Earth-facing side. This phase is coined a quarter Moon because, in actuality, we on Earth are seeing precisely one-quarter of its surface.

A first-quarter Moon will rise around noon and will follow the Sun across the sky. It will be highest in the sky at sunset and will finally reach the horizon around midnight.

Waxing Gibbous Moon

Waxing Gibbous Moon
Waxing Gibbous Moon. (Image credit: NASA)

Once past the quarter moon phase, it develops a bulge as the Sun moves further across its surface over the next seven days. The hump that the Moon now features as it approaches its fullest is the reason it’s referred to as a gibbous Moon.

At the gibbous phase, a majority of the Earth-facing side of the Moon is now visible. The away-facing side of the Moon is now almost entirely dark.

A gibbous Moon will continue to appear larger in the sky over the course of a week and will rise in the afternoon, later with each passing day.

Full Moon

Full Moon
Full Moon. (Image credit: NASA)

As you can probably guess, the Moon appears at its full size to us on Earth. Now, the Earth is located on a slightly different plane between the Earth and the Sun. This allows the Sun to cast all of its light on the side of the Moon we all know.

A full Moon does the exact opposite that the Sun does, rising at sunset and shines brightly throughout the night.

Waning Gibbous Moon

Waning Gibbous Moon
Waning Gibbous Moon. (Image credit: NASA)

After the Moon reaches its peak size, it moves into a waning gibbous phase. Here, the Moon’s hump is now located on the opposite side of what it was before. It rises some time in the evening and can be seen until just after sunrise.

Third Quarter Moon

Third Quarter Moon
Third Quarter Moon. (Image credit: NASA)

As the Moon continues its orbit around the Earth, it once again reaches a point where it appears half-visible in the sky. For the same reasons as the first quarter Moon, we are once again able to view one quarter of the entire surface of the Moon.

A third-quarter Moon rises at midnight and is visible throughout the morning before setting around the middle of the day.

Waning Crescent Moon

Waning Crescent Moon
Waning Crescent Moon. (Image credit: NASA)

Before the Moon once again briefly disappears from our view as a new Moon, it first travels through the waning crescent phase. Here, the Moon appears as a smaller sliver each morning as it begins to position itself between the Earth and the Sun once again.

A waning crescent will rise a little before the Sun each morning, and the Sun will chase it across the sky. It will set before the Sun at some point in the afternoon.


The Moon is our constant celestial neighbor, orbiting our planet faithfully and producing these magnificent phases. Of naming conventions, gibbous is undoubtedly the most obscure but represents almost half the time the Moon is visible. Admittedly, the Moon may not be at its most glamorous with a hump!

About Noah Zelvis

Noah is a content writer who has had a love of all things astronomy for as long as he can remember.
When not reaching for the stars, you’ll likely find Noah traveling or running.