# Gravity

From ExoDictionary

**Gravity**

- Viewed from a frame of reference fixed in the earth, force imparted by the earth to a mass which is at rest relative to the earth. Since the earth is rotating, the force observed as gravity is the resultant of the force of gravitation and the centrifugal force arising from this rotation and the use of an earthbound rotating frame of reference. It is directed normal to sea level and to its geopotential surfaces. See virtual gravity, geopotential height, standard gravity.

*The magnitude of the force of gravity at sea level decreases from the poles, where the centrifugal force is zero, to the equator, where the centrifugal force is a maximum but directed opposite to the force of gravitation. This difference is accentuated by the shape of the earth, which is nearly that of an oblate spheroid of revolution slightly depressed at the poles. Also, because of the asymmetric distribution of the mass of the earth, the force of gravity is not directed precisely toward the earth's center. The magnitude of the force of gravity is usually called either gravity, acceleration of gravity, or apparent gravity.* - = acceleration of gravity.
- By extension, the attraction of any heavenly body for any mass; as
*Martian gravity.*

## References

*This article is based on NASA's Dictionary of Technical Terms for Aerospace Use*