Why are There Moon Phases?


If you are a constant observer, one of the things you may be wondering about is why are there moon phases? When you observe the moon from the Earth, you will notice that it goes through a cycle of light and dark appearances. The stages that the moon goes through in this cycle is called moon phases. To understand moon phases, we need to examine the moon’s orbital position as it relates to the Earth and the sun.

The moon’s revolution around the Earth makes it appear as though it is changing its shape in the sky.

What are Moon Phases?

Before we discuss what causes moon phases, it’s good to first understand what they really are. A moon phase describes how much and which parts of the moon are being seen as light and shadow. As the moon orbits around the Earth, we can observe the phases as they change. It’s the amount of moon you can see from the Earth depending on how much light it receives from the sun. This changes each day.

The moon is illuminated when the sun reflects light on it. The part of the moon that faces the sun is lit up and the part that is facing away shows shadows.

  • Full Moon

    During a full moon phase, the entire moon is lit up. This is when you can see it so bright in the sky. It is especially nice to see a full moon when camping. You can take your sleeping bag out on the grass or sit comfortably on a nice camping chair to watch the moon in its full glory.

  • New Moon

    On a new moon, on the other hand, all of it is seen as a shadow. This is when you can barely see any moon at all.

  • Quarter Moons

    At first quarter and third quarter phases of the moon, only half of the moon is seen as light and the other half as a shadow. It is also commonly called a half moon.

  • Crescent

    In between these quarter phases, we have the crescent and the gibbous moons. This is when the moon’s lighted area or shadowed area takes on a crescent shape. Most astronomers prefer watching the moon on their telescope during the crescent moon phases because it is when it is truly beautiful and stunning.

Why are There Moon Phases?

Now that you know better what the moon phases are, you can more easily understand why there are moon phases.

  • Travels Around the Earth

    Just like the Earth, half of the moon receives light from the sun while the other half is in shadow at any given time. This is why the Earth has time zones. One part of the world is in daylight while on the other side, it is night time.

  • Different Angles

    As the moon orbits or travels around the Earth, people can see the moon from different angles, depending on where they are. As such, people see different percentages of the moon’s light and shadow.

When it’s a full moon, the moon is on the opposite side of the Earth from the sun. Because of this, we can see the whole side of the moon that is lit. During a new moon, however, the exact opposite alignment happens. The moon is in between the Earth and the sun. As a result, we can only see the shadowed side of the moon. And so on.

The moon phases depend on the position of the moon relating to the sun and the Earth. As it makes its way around the Earth, we can see the bright parts of its surface at different angles. This is what people have decided to call phases of the moon.

In Between Moon Phases

There are 4 ways that we can describe the in-between moon phases. They are waxing, waning, crescent, and gibbous.

When the moon is waxing, its lit area appears to be increasing. And when it is waning, the lit area of the moon appears to be decreasing. Think of it as the AM and PM in time in a 24-hour day cycle.

During a crescent, the moon appears less than half illuminated. On gibbous, the moon appears more than half illuminated.

What is a Lunar Eclipse?

During a lunar eclipse, the Earth casts a shadow on the moon when it is on a full moon phase. It temporarily makes it go dark either fully or partially. We experience partial eclipses several times per year but a total lunar eclipse occurs very rarely. Eclipses are quite short events and you can observe the moon going from full to totally dark and back to full again in just a few hours.

How Can You Keep Track of the Moon Phases?

Today, you don’t have to be an expert to know what moon phases are taking place at any given time of the month. There are apps that you can download that are available on both Google Play and Apple App Store so you can easily check the moon phases on your mobile device.

Did you know that you can also ask Siri or Google to tell you the current moon phases? Simply access Siri or Google from your phone and ask the question. They will immediately search the internet for answers and tell you right away what the moon phase is. Sometimes, you will even get a link to get more information about it.

Sometimes, moon phases are built-in in your calendar applications whether they are on your phone, tablet or computer. Even the traditional physical calendars have moon phases indicated on particular days of the month. You can also see moon phases on weather apps. These apps tell you the current weather in your location and will also let you know the moon phase. There are also several websites that let you track the phases of the moon.

Learning about moon phases is fun and you don’t have to be a professional scientist to understand what causes them, and how you can know the current moon phase.