June 21st is an important day in the history of our planet, for it was on this day, in about 240 B.C., that we humans figured out not only the shape but the size of the Earth.
This discovery resulted from the curiosity of one man - Eratosthenes (air-uh-TAHS-thin-eez) - who was the director of the great library at Alexandria, in northern Egypt. He happened to be reading a scroll in which the author stated that at midday in the city of Syene (sigh-EEN), on the longest day of the year (June 21st), vertical sticks and vertical columns cast no shadows whatsoever.Well, Eratosthenes thought he would check out this observation for himself, so when June 21st came around, he pounded a long stake straight up and down into the ground and waited for noon to arrive. As midday approached, the shadow from the stake grew shorter and shorter, but it never disappeared. At noon, his vertical stake still cast a shadow, but the scroll said that, in Syene, vertical stakes didn't cast shadows. How could this be?
June 21st is the day of the summer solstice, the day on which the sun's most direct rays fall on the latitude we call the Tropic of Cancer, 231/2 degrees north of the equator. This is the farthest latitude north on the planet that these direct rays will fall, so the sun is never directly overhead at any time in the year anywhere in the United States, except Hawaii. Still, these rays are more direct, and the sun is higher in our sky, than at any other time, so we receive more of the sun's energy during the summer, even though we are about 3 million miles farther away from the sun than we will be in January.
The town of Syene (which was located near the modern city of Aswan) lay right on the Tropic of Cancer, which is why the noonday sun cast no shadows there, for the sun was directly overhead on June 21st. But it was not quite overhead in Alexandria, which lay about 500 miles to the north, so Eratosthenes reasoned that the Earth's surface must be curved; it could not be flat.
It is helpful for us and for our children to follow Eratosthenes' reasoning by picturing the Earth as he did. Draw a circle to represent the Earth, and extend a short line at the top to represent a vertical stake driven into the surface. Now draw another stake at about one 1 o'clock position, and extend both lines with a ruler so that they meet at the center of the Earth. Next, draw some parallel lines down from the top of the page to represent the sun's rays falling on the Earth.
Eratosthenes reasoned that the angle that the sun's rays struck his stake in Alexandria would be the same angle formed by the lines extending the two stakes into the center of the earth. So he marked the tip of his stake's noontime shadow, extended a straight pole from the tip of the shadow to the top of the stake and measured the angle at the top of the stake. This gave him the difference in degrees (the angle) between Syene and Alexandria, and after he hired soldiers to step off the exact distance between the two cities, he figured the circumference of the Earth within 1 percent of its actual measure.
If you and your children measure the noontime angle (use 1 p.m. because of daylight time) of your own vertical stake on June 21st, just as Eratosthenes did, you can add that angle to 231/2 degrees and determine your own latitude on Earth.Dr. William F. Russell's books for parents and children include "Classics to Read Aloud to Your Children" and "Classic Myths to Read Aloud." Send your questions and comments to him at Family Learning, 2400 E. Main St., Suite 266, St. Charles, IL 60174-2414.