Why does the moon change its shape?
The moon does not change its shape by itself. How much of it we can see that actually changes. The moon is not luminous (not produces its own light) so we can only see the moon when the sun is shining on it. The moon orbits the earth in 29.5days; it is also the same time which moon takes to rotates on its own axis. The portion of it we see clearly depends on its position relative to the earth and the sun. The different shapes that we see of the lit-up part of the moon are called the phases of the moon. And there are 8 such phases.
If you ever look up at the moon in the night sky, say before you go to bed, you’ve probably noticed that as the days and weeks go by, the moon looks like it is changing shape. Some nights it is a little slimmer and some nights a few weeks later, it is a big bright circle. Well the moon doesn’t actually change shape but our view of it does. Why you ask?
Well the moon, just like the earth has a day side and a night side. A part that is facing the sun and a part that is not. So, when we see a little sliver of the bright moon, we are seeing a little part of the moon that is facing the sun. While the rest of the side that is facing us is dark. And the size and the shape of that sliver of the moon that we see changes because the moon orbits or moves around the earth.
It takes the moon about one month to make one full orbit around the earth. And as it moves around our planet the day time side of the moon, that is the side that is lit up by the sun, is facing in different directions as we see it from the earth. This means that every night we get a slightly different view of the moon. And during its month-long trip in orbit around the earth, the bright part of the moon appears to grow and then shrink from right to left.
The different shapes that we see of the lit-up part of the moon are called the phases of the moon. And there are 8 such phases.
The first phase is the new moon. This is when the night time side of the moon, the side that is facing away from the sun is what we can see from the earth. The moon is still up there but because it is the dark side that is facing us, we can’t see it during the new moon phase.
The next phase is called the crescent phase. This is when the moon looks like a little sliver, kind of like the tip of your finger nail.
After the crescent moon comes the quarter moon. This is when the moon looks like a half circle.
And then the next phase is called the gibbous moon. A gibbous moon is when it is facing just a little sliver of light and its almost full. And that’s what is next.
The moon is at its brightest when it is in the full moon phase. That is when the fully lit up side of the moon is facing us and the sun so that we can see a full bright circle.
But we are only halfway through our phases. After the full moon, night after night, a little part of the moon appears to shrink. And just how the moon grew from a sliver to a full circle from right to left, the sunny side of the moon will shrink from right to left too.
So, after the moon is full it starts to shrink back to a gibbous moon. You will notice that the left side is shining bright instead of the right. After the gibbous phase the moon will look like half a circle and it reaches the quarter moon phase again. And after a few days, it shrinks down to a little sliver again, the second crescent moon. After that the tiny sliver shrinks all the way down until the lit-up side of the moon can’t be seen at all. Now the moon is a new moon again. It is the same old moon, but it has gone through a month-long cycle of phases and it is ready to start all over.
Like I said, it takes a month for the moon to go through all 8 phases. In fact, the word ‘month’ comes from the word moon because it takes a month for the moon to grow the new moon phase to the full moon phase and back.
So tonight, before you got to bed, take a look outside your window. If the sky is clear, can you see the moon. If you can’t, what phase is it in? Is the bright part on the right side or on the left side? As the days go by, keep an eye on it and watch how the moon looks like it is changing shape. You can even make drawings to keep track of how it is changing its shape from day to day. Now you know that the moon does not really change shape. We just see different amounts of the sunny side of the moon as it makes its monthly trip around the earth.