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!

How to tell the earth is a sphere…

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So how can you tell the Earth is a sphere? The ancient Greeks worked it out ( I don’t remember the name of the person who did).

The experiment was to put a stick in the sand at or near the equator, so it would have a tiny shadow directly underneath the it at mid day as the Sun was directly overhead.

If you place a stick upright, at 90° to the Eaths surface either North or South of the equator, the stick casts a shadow at mid day. (Look at the hands of a clock the hands move round in the same way). Say the equator is 3pm and the Sun is overhead, you would get no shadow, then as you go further away from the stick at the equator you get increasing angles (see diagram). Eventually at the poles the shadows would be at their longest.

So how can you tell its a sphere? The angles add up. You can calculate the curvature of the planet from these simple experiments. The ancient Greeks got very close to calculating the circumference of the Earth. Their calculations were only a small percentage out on their measurement.