What are the Phases of the Moon?
The Moon photographed from the International Space Station just above the Earth. Credit: NASA

What are the Phases of the Moon?

12/05/2022Written by Michael Darch

Exploring our Moon and its various monthly phases, along with how the Moon affects us here down on Earth.

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The Moon is Earths silent companion always there, but have you ever wondered why it changes the way it looks throughout the month? Well, the Moon is always watching us and now it’s time to learn how to properly watch back! Find out when and where the Moon appears and how to impress your nearest and dearest by correctly identifying the various phases it goes through. 

Why does the Moon appear to change shape?

Why does the Moon appear to change shape?
The Moon takes slightly different amounts of time to complete an orbit compared to how long it takes to go through its complete cycle of phases. Credit: Surprizi
Why does the Moon appear to change shape?
This shows the orbit of the moon in relation to the Earth and the Sun. No matter where it is, it is always half lit by the sun Credit: Orion 8

Have you ever noticed that the Moon is constantly moving across the Earth’s sky, but it looks different depending on when in the month you are looking at it? 

What’s going on? The Moon takes 27.3 days to orbit the Moon and come back to where it began circling (sidereal month). However, because the Earth also orbits the Sun at the same time, the Moon needs a few extra days to travel a bit further and catch up to end up back in line with the Earth and Sun and complete a lunar phase cycle (synodic month) which takes 29.5 days.  

It’s important to note that half of the Moon is always being lit-up by the Sun in this cycle. So, the Moon doesn’t change shape itself, what changes is how much of the lit-up part of the Moon we can see looking up from the surface of Earth. This diagram shows what’s really going on. 

One side of the Moon is always lit up – the side facing towards the Sun. But as the Moon orbits the Earth, the amount of the Moon that we on Earth see lit-up changes throughout the month. We call these the phases of the Moon. 

How to identify where in the Moons phases we are.

How to identify where in the Moons phases we are.
Starting from the top left and going to the bottom right we can see all the moons phases. Credit: Hamed Rajabpour and Nariman Ghorbani
How to identify where in the Moons phases we are.
Each phase has its own name with the new moon marking the beginning / end of the cycle. Credit: Mark Mathosian
How to identify where in the Moons phases we are.
The temperature fluctuates tremendously on the Moon. The bright side can be 120 degrees Celsius while the dark side goes as low as -253 degrees Celsius. Credit: NASA (digitally enhanced by Rawpixel Ltd)
How to identify where in the Moons phases we are.
An iconic full moon, the grey parts are volcanic plains called maria. In the past they were mistaken for oceans. Credit: NASA

You can tell which phase the Moon is in just by looking. The lunar cycle can be broken down into eight separate phases: 

  • new moon 
  • waxing crescent 
  • first quarter 
  • waxing gibbous 
  • full moon 
  • waning gibbous 
  • last quarter 
  • waning crescent
  • Then back to the beginning with the new moon

The cycle begins with a new moon. This is when the Moon and Sun are seen in the same part of the Earth’s sky. The Sun and Moon are roughly in the same line of sight, with the Sun lighting up the far side of the Moon that we don’t see from the Earth. At new moon we’re looking at the shadowed side of the Moon and subsequently it appears completely dark to us – we don’t see it. 

The terminology used for the other phases can be a bit confusing so let’s break it down. Firstly, when we say the Moon is waxing, we mean that the lit-up side of the Moon appears to be growing. The word waxing is a descendant from an old word “weaxan”, which means ‘to increase’. 

Waning is the opposite term and means to decrease. This stems from the word “wǫnian”, which means ‘to lessen’. So, when we say waning regarding the Moon, we are saying that the lit-up side is starting to shrink. 

If you want an easy way to remember whether the Moon is waxing or waning and you’re in the UK, the trick is to see which side of the Moon is lit-up. If it’s the right side it’s waxing, if it’s the left it’s waning. 

We use “crescent” because at these phases the Moon looks like a crescent shape! Simple. “Gibbous,” however, is a specific term for the way the Moon looks – like it’s ballooning when over half of it is lit-up. 

Confusingly, when the Moon is half-full, it is called a quarter moon. Named as such because the first quarter moon occurs a quarter of the way through the cycle, with the last quarter being three quarters of the way through. 

So, we’ve talked about the position of the Moon at new moon. Well, the full moon is the opposite of this. Now the Moon is on the opposite side of the Earth compared to the Sun, so we can see the entire side lit-up. And here’s a fun fact: you only ever see full moons at night (from around dusk till dawn)! 

If you would like to find out when the upcoming full moons are, check out our blog: Full Moons and Full Facts 

So why is any of this important?

So why is any of this important?
This beetle struggles with the magnitude of the task ahead. Fortunately, the Moon can help in this task! Credit: Belinda Cave
So why is any of this important?
This simple diagram shows the Moon's gravitational effect on our oceans making the water ”bulge” around our planet. Credit: Chiswick Chap
So why is any of this important?
Iranian Islamic calendar. Credit: Public Domain

The Moon has a huge effect on living organisms on Earth. It’s been around since before life began on our planet and so has shaped a lot of subtle things. The Moon can affect animal behaviours with some creatures like dung beetles and sand hoppers having evolved to use the Moon to navigate around. 

Of course, the big way the Moon effects the Earth is in the form of our tides, because without the Moon our tides would be miniscule. The fact we have the tides is good for life on Earth because the tides churn the ocean which affects the global weather and spreads nutrients about the globe, which in turn increases the health of our oceans’ ecosystem. 

Humans have been using the phases of the Moon to track the passage of time and create lunar calendars. In 2013, in Scotland, the earliest lunar calendar was discovered. It was a large monument made up of pits that allowed people 10,000 years ago to track the movements of the Moon. 

 The Sumerians used a lunar calendar back in 3200 BCE. They used the lunar cycle to create 12 months with alternating days of 29/30. The Egyptians followed, as did the Assyrian and Elamite calendars.  

Creating a calendar represented a huge leap in scientific understanding because it allowed us to make accurate predictions about the seasons. This allowed humans to better plan according to what was happening around them, which is very useful for things like agriculture and hunting. 

But it’s not just ancient cultures, modern ones still do use the Moon in their calendars. The Islamic society use the Lunar Hijri. It is a calendar based on twelve lunar months and it’s used to track the proper days of Islamic ritual and holidays. 

China uses a combination of both (a calendar influenced by the cyclic motion of the Moon and Sun) – this system is called a lunisolar calendar. Leap years in this calendar add an extra 13th month (which are slotted into the lunisolar calendar every few years).  

At this point in the Moon's crescent phase, it is only visible as a sliver. Credit: NASA

It’s amazing just how much our silent neighbour affects our lives here on Earth. Here’s an idea to help you remember the phases, why not try keeping a Moon diary? Every day for a month sketch the way the Moon looks and by the end of the month you will have your own Moon diary! 

About the author: Michael Darch is a Space Communications Presenter at the National Space Centre.