On the cycle of day and night

Since ancient times people have thought of explanations for the cycle of day and night.

People have said: “The sun rises each morning over the horizon in the east, throughout the day it travels in a great arc across the sky, and in the evening it sinks beneath the western horizon only to reappear in the east, several hours later, when the night’s sleeping is done. Therefore the sun travels in a circular path around the earth, one circle per day, repeated endlessly; this is the cause of the cycle of day and night.”

But even in ancient times, some people also said: “Perhaps it is the earth that travels around the sun in a circular path once per day, and the sun stays still at the center, but since we do not notice the earth’s own motion, it seems to us the sun moves instead.”

And there is yet a third possibility: it could be that the cycle of day and night is caused by the earth spinning on its own axis so that, to us here on earth, it seems the sun and the whole cosmos rotates around us once per day.

We have three possible answers. Each answer explains the cycle of day and night just as well as the others, so how shall we tell which is right?

(It’s number 3. But the interesting thing is not so much knowing the answer, as knowing how the answer can be found.)

The answer is to be found in the night sky. Those who watch the sky night after night know that it is not just the sun that appears to move in a great circle across the sky once per day; the stars do so as well. In the course of a clear night we can see the stars to the west sink below the horizon, while new stars rise over the horizon in the east. Night after night the constellations move in a great circle across the sky, only to return the next evening to (very nearly) the same position they had 24 hours before. But the stars that travel furthest nightly across the sky are those that lie along the east-west plane, directly above the equator of the earth, while the stars in the north follow shorter paths, never going across the center of the sky, and the star directly above the North Pole, Polaris, hardly seems to move at all.

From this we can infer that the third possibility is the correct one; the earth rotates once per day, and the axis of its rotation is an imaginary line drawn between the North and South Poles, so that the sun, the stars, and indeed the whole cosmos appear to circle majestically around us. The earth’s daily rotation causes the cycle of day and night, and it also causes the nightly movement of the stars from east to west across the sky.

But there are many more cycles yet to be explained, especially the cycle of the seasons, and the phases of the moon and the tides.