How Do Moon Phases Work?

When visible, the Moon is the brightest object in the nighttime sky. What makes the Moon so interesting is that it seems to slowly fade from day to day until it disappears completely. Then, just as it vanishes, it slowly reappears in the sky. In this article, we take a look at how the Moon travels through these different phases.

How Do Moon Phases Work

The Sun on the Moon

It may come as a surprise that the Moon actually casts no light of its own. All of the radiant white glow that we see of our heavenly neighbor is actually a reflection of the Sun’s rays.

the Sun is lighting up our Moon just like it lights up our days here on Earth
A visual representation of how the Sun lights up the Moon.

You heard that correctly – the Sun is lighting up our Moon just like it lights up our days here on Earth. The Earth spins like a top in a circle once every 24 hours, giving us one day and one night during that time. What makes the Moon unusual is that we always get to see the Moon’s same face and the Sun’s light upon only that surface.

The Moon’s Rotation and Revolution

Moon's rotation and revolution around Earth
An illustration of how the Moon rotates and revolves around Earth. (Image credit: “Moon’s rotation and revolution around Earth” by  Siyavula Education  on Flickr  CC BY 2.0)

Like every other object in the Solar System, our Moon travels around another object. In the Moon’s case, it revolves around the Earth approximately once every month.

What makes the Moon somewhat unique is that it spins on its axis at exactly the same time. This event occurs because the Earth’s strong gravitational pull prevents the Moon from rotating on its own.

As a result, we always get to see the same side of the Moon, no matter where in the world we are. Such a phenomenon is known as tidal locking, and a few other objects in the Solar System act the same way.

How Do Moon Phases Change?

The relation of the phases of the Moon with its revolution around Earth
The relation of the phases of the Moon with its revolution around Earth. (Image credit: Orion 8 on Wikimedia Commons CC BY-SA 3.0)

As the Moon revolves around the Earth, its position in relation to the Sun changes with each passing day. Traveling at speeds of 2290 miles per hour (3680 kilometers per hour), the Moon covers a lot of ground in a short amount of time.

Imagine the Moon traveling in a circle around our planet. There are times where the Moon will be closer to the Sun, farther from the Sun, and many cases where it is somewhere in between.

The Sun’s light is never changing, but the Moon’s position constantly is. It is this exact reason that the phases of the Moon change so frequently. Depending on where the Sun’s rays reach the Moon, we can see all of it, some of it, or none of it at all.

The Phases of the Moon

When we look up at the sky on a clear night (or day), there’s a chance we can catch the Moon in one of its eight phases. Interestingly enough, these phases always follow the same pattern. If you know the current phase of the Moon, you’ll be able to determine the time of day that’s best to see it.

Without further ado, let’s take a brief look at 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)

A New Moon occurs when the Moon is positioned between the Earth and the Sun. All of the Sun’s rays reach the side of the Moon that we on Earth never see.

The Waxing Moon

It takes the Moon approximately two weeks to travel from its location during a new Moon half way around the Earth. During this time, more and more of the Earth-facing side of the Moon is exposed to sunlight. When the Moon is getting more prominent in the sky, we refer to it as a waxing Moon.

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

Transitioning from a New Moon, the Moon will start out as a thin crescent. Over the course of a week, the Moon will grow until it appears half-visible in the sky. We call this phase a quarter Moon because we are, in actuality, only seeing one quarter of the Moon in the sky.

Once it passes the quarter Moon stage, the Moon continues to get larger in the sky as it draws closer to being fully lit. Since the Moon appears to have a hump, it’s dubbed gibbous after the Latin word for the same thing.

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

During the phases of a waxing Moon, you’ll see the Moon rise during daylight hours and set some time after dark.

Full Moon

Full Moon
Full Moon. (Image credit: NASA)

Once the Earth-facing side of the Moon is completely exposed to the Sun, we have what we all know as a full Moon. At this point, the Moon is in the opposite direction of the Sun. It rises at sunset and shines spectacularly throughout the night.

The Waning Moon

After the Earth-facing side of the Moon Moon reaches its peak exposure, it begins to travel back around the Earth. As it does so, it gets slightly smaller in the sky each night. Whenever the Moon gets smaller in this way, we refer to it as a waning Moon.

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

In the exact opposite fashion of the waxing Moon, a waning Moon moves from Full to New over a two-week time frame. It spends its first week in varying sizes of the gibbous phase before returning to a quarter. This quarter Moon, however, shows the opposite side of the waxing quarter!

As the Moon continues to disappear from the sky, it will settle into a week in its crescent phase before finally returning to a New Moon.

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

A waning Moon will rise at some point during the night and set at some point during the day.

Half the Moon Is Always Lit

The Moon is always exactly 50% lit by the Sun’s rays
The same amount of sunlight reaches the Moon no matter what phase we currently see in our sky.

No matter how much of the Moon is visible to us on Earth, the same amount of sunlight always reaches the lunar surface. Even when the Moon is entirely dark to us, the far-facing side of the Moon is completely lit. If we see half of the Moon, rest assured that half of the far side of the Moon is also lit.

Contrary to popular belief, there is no “dark side of the Moon.” Each side of the Moon receives the same amount of light throughout a lunar month. Because of the Moon’s relationship with the Earth, a location on the Moon is exposed to the Sun for roughly 14 days straight!

This video takes us through a demonstration of the phases of the Moon.


It is the Earth’s steady hold that keeps the Moon from spinning freely in the sky. As a result, we are always left looking up at the same side of our celestial neighbor. As the Moon travels around us, we get to see the Sun paint this lunar surface month in and month out for as long as time allows.

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.