Not Quite A Circle

We all know that out in space things go around other things. They circle round and round. Our planet, the Earth, goes around the Sun. Our Moon travels around us. The moons of Jupiter orbit around it and Jupiter itself rotates around the Sun just like we do.

The words we use to describe this movement are a bit misleading. They can make you think that things travel around other things in perfect circles. Around. Round. Orbit. Circle. The fact that the planets and our Sun are sphere-shaped also gets us into the same mindset. So let me tell you something that might surprise you; the planets in our solar system don’t go around the Sun in circles like the drawing at the top of this post. They follow a different shaped path shown in the drawing below:

The shape of these paths are called ellipses. An ellipse is like an oval shape. In space, things go around other things in elliptical orbits not circular orbits. There are a few important things to say about this. Let’s use the Sun and the Earth as an example but the same rules apply to all things in orbit around other things.

Firstly, the Sun is not in the centre of the ellipse. It’s over to one side. If we think of us on the Earth going around the Sun once every year this means that we are at different distances to the Sun depending upon where we are on that path. Because the path is so big and the ellipse is nearly circular we don’t noticeably see the Sun getting bigger or smaller in the sky. However, there are times the Moon is a little bit closer or further away on its elliptical orbit around us and it is possible to measure the difference in size. You may have heard some people get carried away and talk about a ‘super moon’ on nights when the Moon is full and also closest to the Earth - but take it from me, it isn’t very noticeable.

Secondly, the shape of the ellipse can be different. It can be very long and narrow or almost circular. Comets have very long narrow paths, whereas most of the planets have nearly circular ellipses. The long narrow elliptical paths have their centre very far to one side, whereas the circular ones have it almost in the middle. This is why comets come from so far away out in space and go so close around the Sun, sometimes close enough to burn up completely.

Thirdly, the reason for the elliptical shape is gravity. The Sun is heavier and is pulling the Earth towards it. Because the Earth has speed of its own in another direction it doesn’t simply get pulled into the Sun, but these two forces cause the elliptical orbit that it travels upon. This also means that the speed of travel is not the same all the time. Half the time during the orbit our Earth is falling towards the Sun and is speeding up, whereas, after it has passed it at the closest distance it ever gets to it, it flies away from the Sun and starts to slow down. The second half of Earth’s orbit is spent slowing down until it reaches the most distant point away from the Sun when it begins the cycle again speeding up and falling towards it.

So how fast does the Earth travel around the Sun? It depends upon what day you ask and where in it’s orbit it happens to be. It is closest to the Sun (and fastest) around 3rd January each year and furthest away (and so slowest) around 4th July. Instead people give the simple answer as an average of all those possible speeds, which is just under 30 kilometres every second or 66 000 miles per hour. If it helps, thats fast enough to get to the Moon in 3 and half hours! But remember because the Earth travels on an ellipse not a circle half of the days of the year we go faster than that and half of the year we go slower.

The last thing to say is that with elliptical paths you can see that it is much easier for two or more orbits to cross one another, increasing the chance of space objects bumping into each other at high speeds. This makes for a much more exciting Universe and explains why, everywhere we look around the Solar System, things have and still are bumping into other things. Next time you look at the craters on the Moon think about how they got there.

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