Earth and the Moon


How was Earth first measured?

The study of the size and shape of Earth is called geodesy. People have studied geodesy for millennia. As early as two thousand years ago, the Greek-Egyptian astronomer and mathematician Eratosthenes (276–194 B.C.E.) used the shadow of the Sun to compute that Earth was a sphere about twenty-five thousand miles in circumference. This was impressively close to the modern value.

This scientific knowledge was lost and rediscovered several times throughout history, as civilizations rose and fell. By the middle of the fifteenth century, for example, most Europeans living away from the ocean thought that Earth was flat, although sailors and scholars were fully aware that Earth was a sphere. However, the size was still uncertain; Christopher columbus, for example, thought Earth was much smaller than it actually is. He was convinced he could get to India by traveling west from Spain faster than by traveling east. (He, of course, ran into the Caribbean islands and the Americas instead.)

Finally in the seventeenth and eighteenth centuries, Europeans were able to develop techniques to measure the size and shape of Earth accurately. Dutch physicist, astronomer, and mathematician Willebrord Snell (1580–1626), who is best remembered today for Snell’s law, explaining the angle of refraction (bending) of light through different materials, extended these mathematical ideas to figure out how to measure distances using trigonometry. He used a large quadrant (a circular arc divided into 90-degree angles) to measure angles of separation between two points. From this he could calculate distances between them and measure the radius of Earth.

The German mathematician and scientist Karl Friedrich Gauss (1777–1855) also worked on this problem; as director of the Göttingen Observatory from 1807 until the end of his life, Gauss became interested in geodesy. In 1821 he invented the heliotrope, an instrument that reflects sunlight over great distances to mark positions accurately while surveying.


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