![]() ![]() As the earth rotates, the stars, sun, and moon appear to spin around the north celestial pole, so the north celestial pole remains fixed in the sky. The North Star is within a degree of the north celestial pole. The dish will only project a circular shadow when oriented face-on to the light, and when it is edge on, or at any other angle, it will produce a line or ellipse. You can easily see this if you point a flashlight at a wall and use a circular dish and a ball. The only shape that consistently produces a circular shadow, regardless of its orientation, is a sphere. ![]() But during a lunar eclipse, the earth’s shadow is always a circle regardless of when the eclipse occurs. For a lunar eclipse at sunrise or sunset, the earth’s shadow would be an ellipse, a line, or a rectangle. If the earth were flat and round, like a disk, it could only cast a circular shadow for lunar eclipses that occur at midnight. And because the edge of the earth’s shadow is always a portion of a circle, the earth’s shadow on the moon must also be a circle. He observed lunar eclipses and correctly realized that the cause of lunar eclipses (which can happen only with a full moon) is the shadow of the earth crawling across the moon. Aristotle (384–322 BC) was among the first to write down the fact of our planet being a round sphere. ![]() So how did people in the ancient world figure out that the earth is spherical? In the sixth century BC, Pythagoras taught that the earth was spherical, although none of his writings exist which explain how he came to this conclusion. Reasons We Know That the Earth Is Spherical Earth’s Shadow ![]()
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