(Listen to this article on the Sky Alpha Breakfast Show)
Humans have been looking at and learning about the night sky for a very long time. We have been looking at the night sky since the first days of ancient civilisation. Understanding the night sky allowed early humans to navigate across land and sea, to invent calendars and think beyond the experiences of the everyday.
Our view of the night sky is constantly changing. Alongside the regular movement of the stars across the sky, beautiful events occur throughout the year. Many of these can be seen with the naked eye. Of these, the most obvious and beautiful involve the movement of our Moon.
UNDERSTANDING OUR PERSPECTIVE OF THE NIGHT SKY: EARTH
In order to understand what we see in the night sky, we need to understand the position that we are looking at it from.
Humans live on Earth. Earth is a large sphere made mostly of rock. It is constantly travelling around its nearest star, the Sun, and the almost circular path it follows is called its orbit. This means the Earth is what we call a planet.
Other planets also travel around the Sun. The list of these planets is:
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
Mercury is the closest planet to the Sun. Neptune is the furthest away.
The Earth is also spinning. This means the side that faces the Sun is constantly changing. During the day, you are the side that faces the Sun. As the Earth spins around the Sun seems to move across the sky, until we are looking the other way and it becomes night. This is when the stars become visible.
WHAT IS THE MOON?
The planets are not the only bodies of rock near to the Sun. A moon is a smaller body of rock that travels around a planet instead of a star.
Earth has just one moon and it is the largest object in our night sky, despite being relatively small. The reason it appears so big is because it is much closer than any of the other objects in space. However, it is still around 384,000km away!
WHY DOES THE MOON’S SHAPE CHANGE?
If the Moon is just made of rock and doesn’t make any light of its own, you might wonder how we can see it at all and where the moonlight comes from. The light from the moon is actually light from the Sun that is reflected off the Moon’s surface.
This means that we can only see the side of the Moon which is currently facing the Sun.
The Moon moves around the Earth every 29.5 days. This means the part of the Moon facing the Sun relative to the Earth is always changing. When the Earth is between the Moon and the Sun, the entire face of the Moon reflects back to Earth and is fully visible. This is a full Moon and is when the Moon looks like a full circle. As the Moon moves around us, less and less of it becomes visible. This continues until the new moon, when the Moon cannot be seen.
Many of the calendars humans have developed are inspired by this cycle of the Moon, including the months we use today, which are even named after the moon!