Parallax

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The photo is an illustration and it doesn’t actually show parallax.

Do you know what parallax is?

Hold out your thumb in front of you and close one eye. Place it so that it covers something, perhaps a flower outside, or something in front of you. Maybe the moon.

Now without moving your hand open your eye and close the other one.

What do you see? The moon or the object appears to have moved! That is because your eyes are seperate. They are the base of a very long thin triangle and believe it or not you can measure distance that way.

Now stick a piece of wood in the soil on the equator, and a similar one a few miles north or south of it. At midday on the equator the sun will be directly overhead and there will be no, or hardly any shadow. But the one miles away will cast a shadow. The further away from the equator the longer the shadow. If you know the distance between the sticks, and the angle the shadow casts by the other stick. (measure the angle from the top of the stick and the end of the shadow) you can actually work out the distance to the Sun (which is casting the shadow). In this way the ancient Greeks did this and also worked out the size of the Earth approximately. You can use this idea if you look at a star at one end of the Earth’s orbit and six months later the other side of the orbit. That’s how they work out distances to stars. Amazing what you can work out by using your eyes, a couple of sticks and your thumb!

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This was a phrase I thought of this afternoon, then realised it was impossible.

The Universe is apparently infinite, so you can’t have a centre, you can’t have a top, bottom, side, front or back?

Obviously we know that stars are a certain distance away, and if you look at star maps you can identify individual stars. But did you know they move? Stars move around the centre of the galaxy. Some faster than others. The distance to a star is measured by parallax. Hold your hand out at arms length, look at the tip of your thumb with one eye, then the other. Notice how the background seems to move. That’s because each eye is at the base of a very long thin triangle and your thumb is at the tip. Now think of the stars. How do you measure their parallax.? The answer is take the measurements at opposite sides of the Earth’s orbit, six months apart.

What was found was the stars are not fixed. They are moving due to the matter around them and their relative speeds. So stars in one position a thousand years ago might have moved quite a distance.

That’s not all, the Galaxies that contain the stars are moving too. Some, like our galaxy, the Milky Way, and the Andromeda galaxy are due to collide in the next few billion years. We won’t be around to see it, but it has been modeled.

Then looking further out Galaxies seem to be flying away from each other at a rapid rate. You would think gravity would hold them together, but something in the Universe, (dark matter, dark energy) is pushing it apart.

This was measured by noticing the galaxies light from further away appears redder ( red shifted). In other words travelling away from us.

To explain, if you hear a fire engine siren it sounds different moving towards you than moving away. That is because the sound waves get squashed up as they move towards you, and stretch as they move away. Light does the same thing, as an object moves towards you it squashes up and changes to a bluer colour. As it moves away it stretches and we see a redder colour.

I’m not a scientist, so I can only hope what I’ve written makes sense. All I know is that something might be in an exact place now but because everything is in constant motion you just can’t say where the ‘left side of the universe’ actually is!

Mallaband-Brown

Tag: Parallax

Left side of the Universe