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The axis of an equatorial mount is parallel to Earth's surface when observing stars at Earth's equator – but perpendicular to it when observing from one of Earth's poles. Equatorial mounts were specifically developed for a spherical and rotating Earth. If Earth were flat, an equatorial mount would not make sense.
Image from space: The curved surface of the spherical planet Earth. Spherical Earth or Earth's curvature refers to the approximation of the figure of the Earth to a sphere.The concept of a spherical Earth gradually displaced earlier beliefs in a flat Earth during classical antiquity and the Middle Ages.
The concept of a spherical Earth dates back to around the 6th century BC, [2] but remained a matter of philosophical speculation until the 3rd century BC. The first scientific estimation of the radius of the Earth was given by Eratosthenes about 240 BC, with estimates of the accuracy of Eratosthenes's measurement ranging from −1% to 15%.
The Old Bedford River, photographed from the bridge at Welney, Norfolk (2008); the camera is looking downstream, south-west of the bridge. The Bedford Level experiment was a series of observations carried out along a 6-mile (10 km) length of the Old Bedford River on the Bedford Level of the Cambridgeshire Fens in the United Kingdom during the 19th and early 20th centuries to deny the curvature ...
The monk Bede (c. 672–735) wrote in his influential treatise on computus, The Reckoning of Time, that Earth was round. He explained the unequal length of daylight from "the roundness of the Earth, for not without reason is it called 'the orb of the world' on the pages of Holy Scripture and of ordinary literature.
A great many other ellipse parameters are used in geodesy but they can all be related to one or two of the set a, b and f. A great many ellipsoids have been used to model the Earth in the past, with different assumed values of a and b as well as different assumed positions of the center and different axis orientations relative to the solid ...
Earth's circumference is the distance around Earth. Measured around the equator, it is 40,075.017 km (24,901.461 mi). Measured passing through the poles, the circumference is 40,007.863 km (24,859.734 mi). [1] Treating the Earth as a sphere, its circumference would be its single most important measurement. [2]
The Earth's rotation rate is still slowing down, though gradually, by about two thousandths of a second per rotation every 100 years. [1] Estimates of how fast the Earth was rotating in the past vary, because it is not known exactly how the moon was formed. Estimates of the Earth's rotation 500 million years ago are around 20 modern hours per ...