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In physics, **gravity** (or **gravitation**)
is the name given to a force that attracts masses together
generally according to the product of their masses and the reciprocal of the
square of the intervening distance (**law of gravitation**).
It was theorized by Isaac Newton
who observed the force drawing objects toward Earth and that drawing
planets toward the Sun and moons toward planets
could all be explained
by a single law, according to his estimates of feasible masses of the
Sun, planets, and moons. Newton posited it as universal, i.e.,
that in other situations masses would affect each other according to
the same law, its effect between everyday objects being negligible
because of their relatively tiny mass.

Albert Einstein recast the theory as space itself being sucked into
each mass (general relativity, GR), calibrating his formula to virtually match
Newton's excepting extreme circumstances,
but with some consequences, such as the effect of a massive
object on passing EMR: GR predicted a degree of light bending
unexplainable by Newton's laws.
The term **Newtonian gravity** is used when it is necessary to
distinguish it from GR.

These theories are phenomenally successful: for example, their reliability and precision has made space navigation as we know it possible, and the theorized degree to which it bends light has been observed. Yet they have failed to explain some observations:

- Galaxies within galaxy clusters do not orbit in accordance to gravitational theory, given the apparent masses of the clusters' constituent galaxies and gas.
- Stars in galaxies do not orbit it in accordance to gravitational theory, given the apparent masses of the galaxies' constituent stars and clouds.
- Gravity would make the universe accelerate inwardly (or given its current expansion, would make that expansion decelerate), but observations suggest it is doing the opposite.

Scientists have sufficient faith in gravitational theory that they cite it to assert galaxies and galaxy clusters must include matter that has yet to be detected (dark matter), and that something otherwise-undetected must be providing an outward force throughout the universe (dark energy). Alternately, attempts have been made to further refine gravitational theory to explain these observations (such as modified Newtonian dynamics and DGP gravity).

The term **gravity** has a different, but related use:
in studies of the detail of the effects of the gravity of Earth
and/or other bodies, the terms *gravity* and *gravitation* are often
used with distinct meanings: *gravitation* to indicate the universal
force and *gravity* to indicate a body's gravitational effects,
i.e., to indicate the downward force-per-unit-mass (which amounts
to acceleration) experienced at different positions in relation to
the body, such as at a particular point on its surface. In this
usage, the word *gravity* is also meant to include the effects of
inertia from the body's rotation (centrifugal force), the two
together being what would be measured by an accelerometer.
Gravimetry is the measurement of this acceleration.

http://en.wikipedia.org/wiki/Gravity

http://en.wikipedia.org/wiki/Gravimetry

accretion

accretion disk

admittance

advection

alternative cosmologies

areal coordinate system

atmosphere

atmospheric escape

atmospheric tide

baryon acoustic oscillations (BAO)

barycenter

black hole thermodynamics

binary star

binding energy

Birkhoff's theorem

black hole (BH)

broad-line region (BLR)

Bouguer anomaly

Brunt-Väisälä frequency

Castro

cold dark matter (CDM)

Ceres

Chandrasekhar limit

Chern-Simons gravity

complex crater

computational astrophysics

conic section

convection zone

corotation torque

cosmic string

cosmic web

cosmological constant (Λ)

Coulomb's law

critical density (ρ

curvature

Darcy velocity field

dark energy

dark flow

dark matter

dark matter annihilation

dark matter halo

DGP gravity

dark matter filament

Doppler shift

Effelsberg 100-m Radio Telescope

Einstein-de Sitter model

electron degeneracy

electroweak

escape velocity (V

Europa

evaporation

extra-solar planet

star formation feedback

Fermi sea

free-air anomaly

free-fall time

f(R) gravity

fuzzy dark matter (FDM)

galactic tide

galaxy cluster (CL)

geodesic

Goddard gravity model (GGM)

gravitational instability (GI)

giant star

general relativity (GR)

GRAIL

gravimetry

gravitational collapse

gravitational constant (G)

gravitational instability model

gravitational lensing

gravitationally bound

gravitational potential (Φ)

gravitational potential model

graviton

gravity anomaly

gravity sounding

gravity wave

gravitational wave (GW)

GW detection (GW)

Hale Telescope

helium rain

Hill radius

hypermassive neutron star (HMNS)

homologous collapse

Hubble expansion

Hubble time (t

hydrodynamic equations

hydrostatic equilibrium

internal gravity wave

inverse square law

ion engine

isolation mass

J

Jeans length

Jeans parameter (λ)

JUICE

Juno

Keplerian disk

Keplerian orbit

Kepler's laws

Kelvin-Helmholtz mechanism

Kelvin-Helmholtz timescale (KH timescale)

Lagrangian point

Lambda-CDM model (ΛCDM)

Lane-Emden equation

libration

liquid mirror telescope

long-period comet

Lovelock gravity

Mach's principle

mass

maximum iron fraction

Maxwell-Boltzmann distribution

mirror support cell

mixing length theory

MMX

modified Newtonian dynamics (MOND)

multi-messenger astronomy

N-body problem

N-body simulation

neutron star (NS)

Newton's laws

nutation

open cluster (OC)

orbital resonance

orbital speed

pointing error (PE)

planet

planetary differentiation

planetesimal

planet formation

potential energy (PE)

power law

parameterized post-Newtonian formalism (PPN formalism)

precession

protostar

Psyche

radio science

repulsive dark matter (RDM)

redshift (z)

reduced mass

relativity

red-giant branch (RGB)

Richardson number (Ri)

Roche limit

Roche lobe

scalar-tensor gravity

supercritical fluid (SCF)

Schrödinger-Poisson equation

self-gravitation

gravitational singularity

Solar and Heliospheric Observatory (SOHO)

Solar Orbiter (SolO)

sounder

special relativity (SR)

speed of light (c)

spherical harmonics

spiral density wave

standard gravitational parameter (μ)

stationkeeping

stellar cluster (SC)

stellar core

stellar dynamics

stellar evolution

stellar mass determination

stellar stream

stellar structure

string theory

strong-field gravity

strong force

superfluid

surface gravity

symmetry breaking

Trace Gas Orbiter (TGO)

theoretical modified GR metrics

tidal capture

tidal force

tidal migration

tide

timescale (t)

time standard

Titan

Theory of Everything (TOE)

theory of figures (TOF)

Toomre Q parameter (Q)

topological defect

terrestrial time (TT)

T-Tauri star (TTS)

transit timing variations (TTV)

variable star

virial parameter

virial theorem

warp

wide binaries (WB)

wavefront error (WFE)

wormhole