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Astronomy and space

Phases and orbits of the Moon

The biggest clue to why the Moon always looks different when you look up at the sky is that it is constantly moving in relation to Earth and the Sun. It pops up in different places and at different times because it orbits the Earth.

And it appears to have phases because the amount of lunar surface bathed in sunlight we can see from Earth depends on where we and the Sun are. Knowing how this dance between the Moon, Earth and Sun plays out lets us understand the Moon’s constantly changing appearance.

Moon fact: The Moon’s phases repeat every 29.5 days, but it’s orbit around the Earth only takes 27. Why? In that time, as our Moon moves around Earth, the Earth also moves around the Sun. Our Moon must travel a little farther in its path to make up for the added distance and complete its phase cycle.

Why does the Moon always look different?

The way the Moon looks to us is continually changing. It moves across the sky rapidly over the course of a night. And from night to night it rises and falls at different times and in different parts of the sky. Most vividly, its entire appearance changes over the course of two weeks, morphing from a bright circle to a circle sliced in half and finally fading to nothing.

All these changes seem complicated but each one can be explained by thinking about where the Earth, Sun and Moon are in relation to one another.

Changing position on the sky

If we first think about the Moon in relation to the Earth, we can explain how it appears to travel across the sky over the course of a night, and why it rises and falls at different times and in different locations.

The first bit is easy. Just like the Sun and the night time stars, the Moon’s apparent rising in the east and setting in the west each day is not from the Moon’s orbit around the Earth, it’s from the Earth spinning.

The lunar orbit is slower and harder to see – but you can still spot it. By looking at where the Moon is in relation to stars in the background one night, and then comparing to where it is several hours later or on the next night you’ll notice it has moved east. This movement is from the Moon’s orbit, which takes 27 days, 7 hours and 43 minutes to go full circle.

It causes the Moon to move 12–13 degrees east every day. This shift means Earth has to rotate a little longer to bring the Moon into view, which is why moonrise is about 50 minutes later each day. As it rises at a later time, the Moon appears in a different part of the sky.

As the earth and moon orbit the sun, the sides facing the sun at any given point in time are illuminated.

Moon phases

To grasp why the Moon’s appearance changes, we need to bring in the Sun. Like the planets, we only see the Moon because it reflects the Sun’s light. And just as we have half the planet lit and the other in darkness at any one time, the Sun is always shining on half the Moon. This means there is no dark side of the Moon, just a side facing away from the Sun.

From Earth though, it looks like the Moon goes through different lit-up phases. If you are in the Northern Hemisphere, the Moon’s right side grows with light from a crescent to a semicircle and then a gibbous (more than a semicircle but less than a full circle) shape until finally it becomes a glorious full moon. This process is called ‘waxing’ and takes about a fortnight. And then the opposite happens on the left side as the portion lit up shrinks to nothing, known as a ‘new moon’. This process is called ‘waning’.

As the view from the Southern Hemisphere is the other way around, the Moon waxes from the left and wanes to the right. This shows clearly that it’s not the Moon but our vantage point on Earth that changes, and explains the phases we see.

For instance, a new moon happens when the Moon is between the Earth and the Sun. The whole of the far side of the Moon is in sunshine, but we see (or rather don’t see) the side that’s in full shadow. During a half moon, we see half of the Moon’s day side. The other half is shining out in the opposite direction to Earth. The fact we can sometimes spot a faint outline of the dark sections of the Moon when it’s not full is because some of the light from the Sun is bouncing off Earth and lightly illuminating our lunar companion. The effect is called 'Earthshine'.

Mercury, Venus and Mars also have phases similar to the Moon which can be spotted at certain times with a telescope. And the Earth has phases too. When Apollo astronauts looked back on Earth from the Moon, they didn’t just see a globe, they marvelled at crescent, gibbous, full and new Earths depending on which phase our planet was going through.

In sync

So, the appearance and position of the Moon change based on the way the Earth and Moon orbit the Sun and the fact that the Earth spins round once every 24 hours. But with all this spinning and orbiting, it begs the question: why don’t we see the Moon spinning too? In fact, though we always see the same side of the Moon, the Moon is spinning. It’s just spinning at exactly the same rate as its orbit ­– one revolution every 27 days. Effectively, its day is as long as its year.

This is no coincidence. It’s called ‘synchronous rotation’ and is a result of the gravitational tug of war between the Earth and the Moon. In the distant past, the Moon was rapidly spinning close to the Earth. But then gravity from Earth’s huge mass began to take effect. Tidal forces allowed the Moon to drift away from the Earth in its orbit, and slowed its spin. Eventually, the Moon reached a point where one side was always facing the Earth.

The same forces that caused the Moon to always show the same face to Earth continue to influence the Moon and Earth today. The Moon still drifts further away a few centimetres a year. What is more surprising though is that the Moon and Earth’s gravitational tug of war is slowing down the Earth’s rotation a tiny bit too. If it were not for the Sun’s eventual demise, this would continue until the Earth and Moon were ‘tidally locked’. In this situation, both the Moon and Earth would spin at the same rate of 47 days, showing the same face to each other for the rest of their existence.

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