Clockwise either way | Day 23

If the clock were invented on the Southern Hemisphere, we’d all be twisting our bottle caps shut anticlockwise.

Just like the measurement of time, the movement of the hands across the clock face — and consequently the name of circular motion directions — is based both in history and convention. While we know these days that the Earth rotates around the Sun, when you’re standing in one spot on the surface of our planet, it does indeed look like the Sun might be the one doing the rotating. This is known as the apparent motion of the Sun. And, due to this motion, the shadows of objects move throughout the day, too.

The principle of the moving shadow lies at the heart of how a sundial works, which is basically an early type of the clocks we know today. A flat, horizontal disk with a stick pointing northwards from the centre would see the shadow moving ‘clockwise’ through the day, as the sunlight moves from east to west — but that only applies to the Northern Hemisphere, where the sundial first came about. In the Southern Hemisphere the dial would have to be pointing south, and the shadow would move the other direction, too.

Hence, the whole idea of clockwise and anticlockwise is relative to the viewpoint of the plane of rotation. That is, if you’re standing behind a clock with a transparent face, the hands will appear to be moving anticlockwise from your point of view. It is the same with the rotation of the Earth itself – anticlockwise when viewed from the North Pole, and clockwise from the South Pole.

Now, when it comes to bottle caps, it appears to be simply convention. Perhaps something to do with most people being right-handed?

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