Since ancient times, seasons have played a powerful role in shaping human life. They affect what crops we grow, what customs we follow, even our moods.

The ability to predict seasons by tracking the movement of the sun’s rising and setting  points during the year was key to survival in ancient times. The Babylonians and Mayans created elaborate systems for monitoring seasonal shifts. The science behind those changes in the day’s length, however, took centuries to unravel.

This March 20, when the equinox marks the arrival of spring, we can credit two past scholars with our understanding of how our spherical planet circles the sun to divide the year into four seasons.

Over 2,000 years ago, the Greek geographer Eratosthenes discovered a remarkably simple method for  accurately measuring the circumference of the earth. In the great library in Alexandria, he read that a deep vertical well near Syene, in southern Egypt, was entirely lit up by the sun at noon once a year. Eratosthenes reasoned that at this time, the sun must be directly overhead, with its rays shining directly into the well. In Alexandria, almost due north of Syene, he knew that the sun was not directly overhead at noon on the same day because a vertical object cast a shadow. Eratosthenes could now measure the circumference of the earth (sorry, Columbus) by making two assumptions — that the earth is round and that the sun’s rays are essentially parallel.

He set up a vertical post at Alexandria and measured the angle of its shadow when the well at Syene was completely sunlit. Eratosthenes knew from geometry that the size of the measured angle equaled the size of the angle at the earth’s center between Syene and Alexandria.

Knowing also that the arc of an angle this size was 1/50 of a circle, and that the distance between Syene and Alexandria was 5000 stadia, he multiplied 5000 by 50 to find the earth’s circumference. His result, 250,000 stadia (about 46,250 km), is quite close to modern measurements.

Centuries later, Nicolai Copernicus (1473-1543) altered astronomy when he proposed that the sun, not Earth, was the center of the solar system. This led to our modern understanding of the relationship between the sun and Earth.

We now know that Earth orbits the sun elliptically and, at the same time, spins on an axis that is tilted relative to its plane of orbit. This means that different hemispheres are exposed to different amounts of sunlight throughout the year. Because the sun is our source of light, energy and heat, the changing intensity and concentration of its rays give rise to the seasons of winter, spring, summer and fall.

The seasons are marked by solstices and equinoxes — astronomical terms that relate to Earth’s tilt. The vernal equinox and autumnal equinox herald the beginning of spring and fall, respectively. At these times of the year, the sun appears to be directly over Earth’s equator, and the lengths of the day and the night are equal over most of the planet.

Modern people have a number of rites to mark spring’s arrival, from surfing to seeing whether it’s true that a raw egg will balance on end during the equinox. (It may, but that has nothing to do with astronomy, as the National Oceanographic and Atmospheric Administration’s summary of Why We Have Seasons notes.

(adapted from MSNBC’s Scientific Reasons Behind Earth’s  Seasons)

Filed under: Special Features

Tags: Astronomy