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The Earth’s Rotation

ibn tufail 74Earth’s rotation is the rotation of the solid Earth around its own axis. The Earth rotates from the west towards the east. As viewed from the North Star or polestar Polaris, the Earth turns counter-clockwise.

The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where the Earth’s axis of rotation meets its surface. This point is distinct from the Earth’s North Magnetic Pole. The South Pole is the other point where the Earth’s axis of rotation intersects its surface, in Antarctica.

The Earth rotates once in about 24 hours with respect to the sun and once every 23 hours 56 minutes and 4 seconds with respect to the stars (see below). Earth’s rotation is slowing slightly with time; thus, a day was shorter in the past. This is due to the tidal effects the Moon has on Earth’s rotation. Atomic clocks show that a modern day is longer by about 1.7 milliseconds than a century ago, slowly increasing the rate at which UTC is adjusted by leap seconds
Among the ancient Greeks, several of the Pythagorean school believed in the rotation of the earth rather than the apparent diurnal rotation of the heavens. The first was Philolaus (470-385 BCE) though his system was complicated, including a counter-earth rotating daily about a central fire.

A more conventional picture was that supported by Hicetas, Heraclides and Ecphantus in the fourth century BCE who assumed that the earth rotated but did not suggest that the earth revolved about the sun. In the third century, Aristarchus of Samos suggested the sun’s central place.

However, Aristotle in the fourth century criticized the ideas of Philolaus as being based on theory rather than observation. He established the idea of a sphere of fixed stars that rotated about the earth. This was accepted by most of those who came after, in particular Claudius Ptolemy (2nd century CE), who thought the earth would be devastated by gales if it rotated.

In 499 CE, the Indian astronomer Aryabhata wrote that the spherical earth rotates about its axis daily, and that the apparent movement of the stars is a relative motion caused by the rotation of the earth. He provided the following analogy: “Just as a man in a boat going in one direction sees the stationary things on the bank as moving in the opposite direction, in the same way to a man at Lanka the fixed stars appear to be going westward.”

In the Middle Ages, Thomas Aquinas accepted Aristotle’s view and so, reluctantly, did John Buridan[9] and Nicole Oresme[10] in the fourteenth century. Not until Nicolaus Copernicus in 1543 adopted a heliocentric world system did the earth’s rotation begin to be established. Copernicus pointed out that if the movement of the earth is violent, then the movement of the stars must be very much more so. He acknowledged the contribution of the Pythagoreans and pointed to examples of relative motion. For Copernicus this was the first step in establishing the simpler pattern of planets circling a central sun.

This was not accepted immediately even by many astronomers due to the widespread conformance to Aristotle and the Bible. Tycho Brahe, who produced accurate observations on which Kepler based his laws, used Copernicus’s work as the basis of a system assuming a stationary earth. In 1600, William Gilbert strongly supported the earth’s rotation in his treatise on the earth’s magnetism[12] and thereby influenced many of his contemporaries. Those like Gilbert who did not openly support or reject the motion of the earth about the sun are often called “semi-Copernicans”.[14] A century after Copernicus, Riccioli disputed the model of a rotating earth due to the lack of then-observable eastward deflections in falling bodies; such deflections would later be called the Coriolis effect. However, the contributions of Kepler, Galileo and Newton gathered support for the theory of the rotation of the Earth.

The earth’s rotation implies that the equator bulges and the poles are flattened. In his Principia, Newton predicted this flattening would occur in the ratio of 1:230, and pointed to the 1673 pendulum measurements by Richer as corroboration of the change in gravity.[16] But initial measurements of meridian lengths by Picard and Cassini at the end of the 17th century suggested the opposite. However measurements by Maupertuis and the French Geodetic Mission in the 1730s established the flattening, thus confirming both Newton and the Copernican position.

In the Earth’s rotating frame of reference, a freely moving body follows an apparent path that deviates from the one it would follow in a fixed frame of reference. Because of this Coriolis effect, falling bodies veer slightly eastward from the vertical plumb line below their point of release, and projectiles veer right in the northern hemisphere (and left in the southern) from the direction in which they are shot. The Coriolis effect is mainly observable at a meteorological scale, where it is responsible for the differing rotation direction of cyclones in the northern and southern hemispheres.

Hooke, following a 1679 suggestion from Newton, tried unsuccessfully to verify the predicted eastward deviation of a body dropped from a height of 8.2 meters, but definitive results were only obtained later, in the late 18th and early 19th century, by Giovanni Battista Guglielmini in Bologna, Johann Friedrich Benzenberg in Hamburg and Ferdinand Reich in Freiberg, using taller towers and carefully released weights.[n 1] A ball dropped from a height of 158.5 m (520 ft) departed by 27.4 mm (1.08 in) from the vertical compared with a calculated value of 28.1 mm (1.11 in).

The most celebrated test of Earth’s rotation is the Foucault pendulum first built by physicist Léon Foucault in 1851, which consisted of a lead-filled brass sphere suspended 67 m from the top of the Panthéon in Paris. Because of the Earth’s rotation under the swinging pendulum the pendulum’s plane of oscillation appears to rotate at a rate depending on latitude. At the latitude of Paris the predicted and observed shift was about 11 degrees clockwise per hour. Foucault pendulums now swing in museums around the world.


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