Moon Phases (CosWatch Blog 4)

Welcome to the second of Fun Science‘s CosWatch blog posts, which you can read through with your young scientist and learn the phases of the Moon.

What is CosWatch?

There’s so much to see in the night sky! You may have seen Brian Cox on TV describing the “wonders of the universe”, or Carl Sagan talking about the “awesome machinery of nature”, and they’re absolutely right. But while huge rockets and observatories can help, space isn’t just for people with expensive equipment. You can see amazing things from millions of miles away from your very own back garden. Each week, I’m going to talk about one of these incredible objects, and how you can find them.

This time, we’re going to be talking about the Moon, and why it changes shape!

This amazing shot from NASA shows the Moon creeping up behind some trees at the US National Arboretum in 2018. A large number of the Earth’s beings would have looked upon sights like this all through their evolution.

Tell me about the Moon

The Moon is the Earth’s only natural satellite. Satellites in nature are objects that orbit planets and minor planets. The Earth is a planet, which makes the Moon, which goes around the Earth, a natural satellite! It takes just under a month to orbit around the Earth, and it takes about 1.28 seconds for light reflected off of it to reach our planet.

It’s a solid, rocky mass, like the first four planets (Mercury, Venus, Earth and Mars). It does technically have an extremely thin atmosphere, but nothing substantial. Scientists believe the Moon may exist because an early version of Earth collided with a planet the size of Mars, and the remaining rocks formed into the Earth and the Moon. This is called the “Giant-impact hypothesis”, and has a fair bit of evidence. Scientists really developed results that suggests that the Moon may be much older than this, but this doesn’t necessarily contradict the “giant impact” idea.

This graph shows us roughly what scientists imagine might have happened.

Because The Moon is visible from Earth, it has been an object of fascination for human cultures since the dawn of civilization. The Moon has been described by various peoples and races as a female God, a male God, a rabbit, a Buffalo, and countless other unusual and interesting interpretations. This is probably because of the different colours of the Moon; the darker parts, or “Maria”, look like shapes compared to the brighter “Highland” parts. These areas aren’t actually any particularly different; “Maria” simply reflect a few percent less light, which makes them appear darker.

This lovely art from Matthew Meyer demonstrates how people – or rabbits! – may have seen likenesses of themselves or stories they’d heard in the Moon.

It is the only celestial body other than Earth that humans have  visited; between 1969 and 1972, twelve astronauts have walked on the moon, collecting samples and enjoying the reduced gravity; anything on the Moon weighs about six times less than on Earth. This is because the Moon is roughly six times smaller than the Earth.

Why does the Moon change shape?

The Moon is tidally locked with the Earth, which also means it is in “synchronous rotation”. What that means, simply, is it moves around the Earth just as fast as it turns on its axis. Or, even more simply, the same side is always facing the Earth.

However, the Moon also doesn’t produce its own light, and only reflects that of the Sun. That means that as it moves around the planet, the half of it that is lit by the Sun moves around but the actual Moon doesn’t. This is difficult to visualise, but this image shows it quite well.

There are eight “phases” of the Moon. They are either “waxing” phases, when it appears to be getting bigger, or “waning” ones, where it appears to be getting smaller.

Waxing or waning? Shape Appearance
New Moon This phase is barely visible, because no light can reach it!
Waxing Crescent

More about this image…

Quarter

(or “first quarter”)

More about this image…

Gibbous

More about this image…

Full Moon

More about this image…

Waning Gibbous

More about this image…

Quarter

(or “third quarter”)

More about this image…

Crescent

More about this image…

The Moon’s visibility, age, and regular changes make it one of the earliest clocks in the world. Early human beings would use it to keep track of time, just like the seasons of the Earth.

How can I see the Moon?

As you might imagine, it isn’t too difficult to spot the Moon when it isn’t in its “New Moon” phase. It is the brightest object in the night sky; in fact, it reflects so much of the Sun’s light it can be seen quite often in the day.

The speed of the Earth’s rotation relative to the Moon’s orbit means that it rises about fifty minutes later every evening.  Like the Sun, it rises in the East. As with Jupiter, your best course of action for working out when it will appear is using the National School Observatory‘s website; unlike Jupiter, you’re unlikely to mistake it for anything else!

In conclusion:

  • The Moon is Earth’s only large, long-term natural satellite.
  • It has been observed by humans since time immemorial, and the different-coloured areas (“Maria” and “Highlands”) look like shapes.
  • The Moon moves around the Earth as fast as it turns around, meaning the same “face” is always looking at us, and light moves around it, causing the different phases.
  • It can be seen easily with the naked eye, except during “New Moon” phases.

What’s next on CosWatch?

Next time, I’ll be talking about Uranus, the third gas giant, and the seventh planet in our Solar System. Somerset residents will be particularly interested in this one!

Notes:

Fun Science recently created a “Planets and Space” home kit, pre-orderable now for only £5.00. Check it out here!