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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