Editing Gravity anomaly (section)

==Definition==
The gravity anomaly is the [[Anomaly (natural sciences)|difference]] between the observed [[acceleration]] of an object in [[free fall]] ([[gravity]]) near a planet's surface, and the corresponding value predicted by a model of the planet's [[gravitational field]].<ref name=Jackson1997>{{cite book |editor1-last=Jackson |editor1-first=Julia A. |title=Glossary of geology. |date=1997 |publisher=American Geological Institute |location=Alexandria, Virginia |isbn=0922152349 |edition=Fourth |chapter=gravity anomaly}}</ref> Typically the model is based on [[Idealization (science philosophy)|simplifying assumptions]], such as that, under its self-gravitation and [[Earth's rotation|rotational motion]], the planet assumes the figure of an [[ellipsoid]] of revolution.<ref name="Lowrie2007">{{cite book |last1=Lowrie |first1=William |title=Fundamentals of geophysics |date=2007 |publisher=Cambridge University Press |location=Cambridge |isbn=978-1-60119-744-3 |edition=2nd |chapter=2}}</ref> Gravity on the surface of this [[reference ellipsoid]] is then given by a simple formula which only contains the [[latitude]]. For Earth, the reference ellipsoid is the [[WGS84|International Reference Ellipsoid]], and the value of gravity predicted for points on the ellipsoid is the ''normal gravity'', ''g''<sub>n</sub>.{{sfn|Lowrie|2007|p=65}}

Gravity anomalies were first discovered in 1672, when the French astronomer [[Jean Richer]] established an observatory on the island of [[Cayenne]]. Richer was equipped with a highly precise pendulum clock which had been carefully calibrated at Paris before his departure. However, he found that the clock ran too slowly in Cayenne, compared with the apparent motion of the stars. Fifteen years later, [[Isaac Newton]] used his newly formulated universal theory of gravitation to explain the anomaly. Newton showed that the measured value of gravity was affected by the rotation of the Earth, which caused the Earth's equator to bulge out slightly relative to its poles. Cayenne, being nearer the [[equator]] than Paris, would be both further from the center of Earth (reducing the Earth's bulk gravitational attraction slightly) and subject to stronger centrifugal acceleration from the Earth's rotation. Both these effects reduce the value of gravity, explaining why Richer's pendulum clock, which depended on the value of gravity, ran too slowly. Correcting for these effects removed most of this anomaly.{{sfn|Lowrie|2007|p=44}}

To understand the nature of the gravity anomaly due to the subsurface, a number of corrections must be made to the measured gravity value. Different theoretical models will include different corrections to the value of gravity, and so a gravity anomaly is always specified with reference to a particular model. The [[Bouguer anomaly|Bouguer]], free-air, and isostatic gravity anomalies are each based on different theoretical corrections to the value of gravity.<ref>{{cite book |last1=Allaby |first1=Michael |title=A dictionary of geology and earth sciences |date=2013 |publisher=Oxford University Press |location=Oxford |isbn=9780199653065 |edition=Fourth|chapter=gravity anomaly}}</ref>