Also just right is the length of day and night, a result of earth’s spin. If the rotation period were substantially longer, the side of the earth facing the sun would bake while the other froze. Conversely, if days were shorter, perhaps just a few hours long, earth’s rapid spin would cause relentless gale-force winds and other harmful effects.

The earth is 25,000 miles [40,000 km] in circumference and rotates once every 24 hours. Thus, points at or near the equator move at about 1,000 miles [1,600 km] an hour. (The poles, of course, just spin on the spot.)