Earth and Space

Day and night: how do we know?

Physics Narrative for 11-14 Supporting Physics Teaching

The Earth spins

Although the Earth spinning explanation is relatively straightforward, justifying it is not so simple. The common-sense notion is that it is the Sun that moves. After all, we do not appear to be moving and if we jump up we land on the same spot on the Earth's surface.

A crucial piece of evidence that led people to believe in the idea that the Earth spins was provided by a long and heavy pendulum called Foucault's pendulum. This was first hung in the Pantheon in Paris in 1855. The length and mass of the pendulum means that it will keep swinging for over a day. During that time, the plane in which it swings appears to turn. As it is suspended by a friction free pivot, the only simple explanation for this effect is that the ground underneath the pendulum is turning. When it was first shown people were invited, in Foucault's own words, to come and watch the world turn.

The night sky

The other piece of significant evidence comes from long exposure photographs of the night sky. To create these photographs, the camera is pointed at the northern pole star. All the stars come out as long circular trails as if they are all turning around the North Star.

The same effect is observed with the camera pointing at the Southern Celestial Pole, as shown.

There are two possible explanations for this effect. Either all the stars are turning around a single point or the Earth on which the camera is fixed is turning.

The second explanation is correct. The axis on which the Earth spins is currently pointing at the North Star and so as the Earth rotates all of the stars appear to move on circular paths around that star. This apparent circular motion of the stars can easily be seen on a clear night if you pick out the North Star and then follow the progress of the other stars around it as the night progresses.

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