of the
northern night sky above the
Nepali
Himalayas shows the
apparent paths of the stars as Earth rotates. Among the ancient
Greeks, several of the
Pythagorean school believed in the rotation of Earth rather than the apparent diurnal rotation of the heavens. Perhaps 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 supported by
Hicetas,
Heraclides and
Ecphantus in the fourth century BCE who assumed that Earth rotated but did not suggest that Earth revolved about the Sun. In the third century BCE,
Aristarchus of Samos suggested the
Sun's central place. However,
Aristotle in the fourth century BCE 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 Earth. This was accepted by most of those who came after, in particular
Claudius Ptolemy (2nd century CE), who thought Earth would be devastated by gales if it rotated. In 499 CE, the
Indian astronomer Aryabhata suggested 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 10th century, some
Muslim astronomers accepted that the Earth rotates around its axis. According to
al-Biruni,
al-Sijzi (d. c. 1020) invented an
astrolabe called
al-zūraqī based on the idea believed by some of his contemporaries "that the motion we see is due to the Earth's movement and not to that of the sky." The prevalence of this view is further confirmed by a reference from the 13th century which states: "According to the geometers [or engineers] (
muhandisīn), the Earth is in constant circular motion, and what appears to be the motion of the heavens is actually due to the motion of the Earth and not the stars." Treatises were written to discuss its possibility, either as refutations or expressing doubts about Ptolemy's arguments against it. At the
Maragha and
Samarkand observatories, Earth's rotation was discussed by
Tusi (born 1201) and
Qushji (born 1403); the arguments and evidence they used resemble those used by Copernicus. In medieval Europe,
Thomas Aquinas accepted Aristotle's view and so, reluctantly, did
John Buridan and
Nicole Oresme in the fourteenth century. Not until
Nicolaus Copernicus in 1543 adopted a
heliocentric world system did the contemporary understanding of Earth's rotation begin to be established. Copernicus pointed out that if the movement of the Earth is violent, then the stars' movement must be 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.
Tycho Brahe, who produced accurate observations on which
Kepler based his
laws of planetary motion, used Copernicus's work as the basis of a
system assuming a stationary Earth. In 1600,
William Gilbert strongly supported Earth's rotation in his treatise on Earth's magnetism and thereby influenced many of his contemporaries. Those like Gilbert who did not openly support or reject the motion of Earth about the Sun are called "semi-Copernicans". 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.
Empirical tests Earth's rotation implies that the
Equator bulges and the
geographical poles are flattened. In his
Principia, Newton predicted this
flattening would amount to one part in 230, and pointed to the
pendulum measurements taken by
Richer in 1673 as corroboration of the change in
gravity, 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 Geodesic Mission in the 1730s established the
oblateness of Earth, thus confirming the positions of both Newton and
Copernicus. In 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 the
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 opposite directions of
cyclone rotation in the Northern and Southern hemispheres (anticlockwise and
clockwise, respectively).
Robert Hooke, following a suggestion from Newton in 1679, tried unsuccessfully to verify the predicted eastward deviation of a body dropped from a height of , but definitive results were obtained later, in the late 18th and early 19th centuries, by
Giovanni Battista Guglielmini in
Bologna,
Johann Friedrich Benzenberg in
Hamburg and
Ferdinand Reich in
Freiberg, using taller towers and carefully released weights. A ball dropped from a height of 158.5 m departed by 27.4 mm from the vertical compared with a calculated value of 28.1 mm. 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 from the top of the
Panthéon in Paris. Because of 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
clockwise per hour. Foucault pendulums now swing in
museums worldwide. == Periods ==