What are the Phases of the Moon?

Why does the Moon appear to change shape?

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 lunar orbit). 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 lunar orbit) 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.

So why is any of this important?

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).