Analemma

This image is a composite of photographs showing the position of the Sun at the same time each day, over the course of a year (an analemma). This illustrates what the first astronomers were watching as they traced the position of the Sun in the sky, and realized that it was linked to the changing of the seasons.

The apex at the upper left represents the summer solstice, when the sun traces its highest arc in the sky, and the day is longest. This marks the start of summer. The nadir at the lower right represents the winter solstice, when the sun traces its lowest arc in the sky, and the day is shortest. This marks the start of winter. The crossing point of the figure eight represents the equinoxes, marking the start of autumn and spring.

The seasons are caused by the varying amount of light illuminating each hemisphere of the Earth throughout the year. The amount of light reaching each hemisphere varies because the Earth's axis is tilted. The axis is the line through the center of the Earth and the north and south poles. It stays pointed in the same direction all year, as the Earth rotates around the sun. During the winter solstice in the northern hemisphere, the north pole is pointed the farthest away from the Sun, causing the Sun to stay low in the sky, and the days to be short. At the same time, the south pole is pointing its closest toward the sun, so the sun is high in the sky, and the days are long. Thus, winter in the northern hemisphere is summer in the southern, and vise versa.

The solstices represent the start of the winter and summer, and not their middle, because it takes time for the light from the longer spring and summer days to heat the air and the earth, and time again for the ground to cool in the fading light of autumn and winter.

The realization that the motion of the sun and the changing seasons are related may well be the first triumph of science. Once astronomers knew when the seasons would occur, they also could determine when to hunt migratory animals, when to collect wild foods, and when to sow and reap crops. Thus, science took root as an enabling force for technology and civilization. At the same time, the regular cycles of the sun, moon, and stars became deeply entwined with most human religions. It is ironic to think that two cultural forces that are often seen as being at odds were probably born of the same intellectual impulses --- a desire to bring order to the world.