Sphericity Of Earth’s

We all have heard that some people believed Earth was flat. Yet Earth’s sphericity, or roundness, is not as modern an idea as many think. For instance, more than two millennia ago, the Greek mathematician and philosopher Pythagoras (ca. 580–500 B.C.) determined through observation that Earth is spherical (sphericity). We do not know what observations led Pythagoras to this conclusion. Can you guess at what he saw to deduce Earth’s roundness (sphericity)?

He might have noticed ships sailing beyond the horizon and apparently sinking below the water’s surface, only to arrive back at port with dry decks. Perhaps he noticed Earth’s curved shadow cast on the lunar surface during an eclipse of the Moon. He might have deduced that the Sun and Moon are not just the flat disks they appear to be in the sky, but are spherical, and that Earth must be a sphere as well (sphericity).

Earth’s sphericity was generally accepted by the educated populace as early as the first century A.D. Christopher Columbus, for example, knew he was sailing around a sphere (sphericity ) in 1492; this is one reason why he thought he had arrived in the East Indies.

Sphericity model

Earth as a Geoid Until 1687, the spherical-perfection model was a basic assumption of geodesy, the science that determines Earth’s shape and size by surveys and mathematical calculations. But in that year, Sir Isaac Newton postulated that Earth, along with the other planets, could not be perfectly spherical. Newton reasoned that the more rapid rotational speed at the equator, the equator being farthest from the central axis of the planet and therefore moving faster would produce an equatorial bulge as centrifugal force pulled Earth’s surface outward. He was convinced that Earths is slightly misshapen into an oblate spheroid, or more correctly, an oblate ellipsoid (oblate means “flattened”), with the oblateness occurring at the poles.

Sphericity : Satellite observations

Earth’s equatorial bulge and its polar oblateness are universally accepted and confirmed with tremendous precision by satellite observations. The "geoidal epoch" is our modern era of Earth measurement because Earth is a geoid, meaning literally that "the shape of Earth is Earthshaped." Imagine Earth’s geoid as a sea-level surface that extends uniformly worldwide beneath the continents. Both heights on land and depths in the oceans measure from this hypothetical surface. Think of the geoid surface as a balance among the gravitational attraction of Earth’s mass, the distribution of water and ice along its surface, and the outward centrifugal pull caused by Earth’s rotation. Sphericity Figure below gives Earth’s average polar and equatorial circumferences and diameters.

Earth’s dimensions. Earth’s equatorial and Earth polar circumference (a) and Earth diameter (b). The dashed line is a perfect circle for reference to Earths geoid.
Earth’s dimensions. Earth’s equatorial and Earth polar circumference (a) and Earth diameter (b). The dashed line is a perfect circle for reference to Earths geoid.

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