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The geoid (/ ˈ dʒ iː. ɔɪ d / JEE-oyd) is the shape that the ocean surface would take under the influence of the gravity of Earth, including gravitational attraction and Earth's rotation, if other influences such as winds and tides were absent.
Earth's rotation axis moves with respect to the fixed stars (inertial space); the components of this motion are precession and nutation. It also moves with respect to Earth's crust; this is called polar motion. Precession is a rotation of Earth's rotation axis, caused primarily by external torques from the gravity of the Sun, Moon and other bodies.
Geodesy or geodetics [1] is the science of measuring and representing the geometry, gravity, and spatial orientation of the Earth in temporally varying 3D.It is called planetary geodesy when studying other astronomical bodies, such as planets or circumplanetary systems. [2]
The gravity of Earth, denoted by g, is the net acceleration that is imparted to objects due to the combined effect of gravitation (from mass distribution within Earth) and the centrifugal force (from the Earth's rotation).
The pendulum was introduced in 1851 and was the first experiment to give simple, direct evidence of the Earth's rotation. Foucault followed up in 1852 with a gyroscope experiment to further demonstrate the Earth's rotation. Foucault pendulums today are popular displays in science museums and universities. [1]
Also available was an advanced least squares method called collocation that allowed for a consistent combination solution from different types of measurements all relative to the Earth's gravity field, measurements such as the geoid, gravity anomalies, deflections, and dynamic Doppler. The new world geodetic system was called WGS 84.
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The Moon's gravity helps stabilize Earth's axis, causes tides and gradually slows Earth's rotation. Tidal locking has made the Moon always face Earth with the same side. Earth, like most other bodies in the Solar System, formed 4.5 billion years ago from gas and dust in the early Solar System.