SCI-KIDS: Not easy to believe the Earth is round

The Earth is round. Adults assure us from an early age that the Earth is shaped like a basketball. Globes marked with countries and mountains remind us. Even on Google Earth, the Earth is round.

When we look outdoors, however, the Earth really looks flat and bumpy. Standing on the Virginia seashore, looking eastward over the Atlantic, the Earth looks flat as far as I can see.

This is the problem with a round Earth -- what we see of it is flat.

To ancient people, during all but the past 2,000 years, the Earth was flat.

Pythagoras, a Greek scientist and mathematician, gets credit for many things. He devised the Pythagorean rule. The sum of the squares of the sides of a right triangle equals the square of the hypotenuse.

In about 350 B.C., Pythagoras also began systematically thinking about the roundness of the Earth. He gave us three reasons to believe in a round Earth. They are as true today as they were 2,350 years ago.

First, he said, consider the stars at night. As you travel northward, you can see more of the northern sky, with stars revolving around the pole star. As you travel southward, some of the northern stars are no longer visible, but to the south, new stars appear.

Next, he said, during an eclipse of the moon, the Earth's shadow always appears in an arc on the moon. It must mean the Earth is round.

Third, Pythagoras pointed out that when ships sail away from us and move out of sight, the hull disappears first, then the lower masts and finally the upper masts. On a flat Earth, a sailing ship would appear smaller and smaller, but all of it would be in view. Since a ship's hull vanishes first, the Earth must be round.

If the Earth is really like a ball, how big is the ball?

Another Greek, Eratosthenes, who lived in Egypt in 240 B.C., worked out the size of the Earth in a clever way. He had learned that at Syrene, modern Aswan, the noon sun shone straight down the water wells, and objects cast no shadows. The sun was straight overhead.

Where he lived, in Alexandria to the north, the sun was 7 degrees from vertical on the same day. He knew the distance between the two towns, and calculated the Earth's circumference as 25,000 miles.

His result agrees quite well with modern measures, which give the Earth's circumference as 24,901.55 miles at the equator (and 40 miles less measured through the poles).

Today, space travel in Earth orbit, and to the moon, gives us evidence our eyes can believe. The Earth is round.

Although Virginia's science Standards of Learning include light and shadow in K.7, it is not until fourth grade (4.7) that students study the size and motions of the Earth, moon and sun.