Gal (unit) - AbsoluteAstronomy.com

The gal, sometimes called galileo, (symbol Gal) is a unit of acceleration

Acceleration

In physics, acceleration is the rate of change of velocity with time. In one dimension, acceleration is the rate at which something speeds up or slows down. However, since velocity is a vector, acceleration describes the rate of change of both the magnitude and the direction of velocity. ...

used extensively in the science of gravimetry

Gravimetry

Gravimetry is the measurement of the strength of a gravitational field. Gravimetry may be used when either the magnitude of gravitational field or the properties of matter responsible for its creation are of interest...

.
The gal is defined as 1 centimeter per second squared (1 cm/s²).

The gal is not part of the International System of Units

International System of Units

The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...

(known by its French

France

The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

-language initials “SI”). However, in 1978 the CIPM

International Committee for Weights and Measures

The Interglobal Committee for Weights and Measures is the English name of the Comité international des poids et mesures . It consists of eighteen persons from Member States of the Metre Convention...

decided that it was permissible to use the gal “with the SI until the CIPM considers that [its] use is no longer necessary.”

The gal is a derived unit, comprising the centimeter-gram-second (CGS) base unit of length, the centimeter, and the second

Second

The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....

, which is the base unit of time in both the CGS as well as the modern SI system.
In SI base units, 1 Gal is precisely equal to 0.01 m/s².

The acceleration due to Earth’s gravity (see Standard gravity
Standard gravity
Standard gravity, or standard acceleration due to free fall, usually denoted by g0 or gn, is the nominal acceleration of an object in a vacuum near the surface of the Earth. It is defined as precisely , or about...

) at its surface is 976 to 983 Gal; the variation being due mainly to differences in latitude

Latitude

In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

and elevation

Elevation

The elevation of a geographic location is its height above a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface ....

. Mountains and masses of lesser density within the Earth's crust typically cause variations in gravitational acceleration

Gravitational acceleration

In physics, gravitational acceleration is the acceleration on an object caused by gravity. Neglecting friction such as air resistance, all small bodies accelerate in a gravitational field at the same rate relative to the center of mass....

of tens to hundreds of milligals (mGal). The gravity gradient

Gradient

In vector calculus, the gradient of a scalar field is a vector field that points in the direction of the greatest rate of increase of the scalar field, and whose magnitude is the greatest rate of change....

(variation with height) above Earth’s surface is about 3.1 µGal per centimeter of height , resulting in a maximum difference of about 2 Gal (0.02 m/s²) from the top of Mount Everest to sea level.

Unless it is being used at the beginning of a sentence or in paragraph or section titles, the unit name gal is properly spelled with a lowercase g. As with the torr

Torr

The torr is a non-SI unit of pressure with the ratio of 760 to 1 standard atmosphere, chosen to be roughly equal to the fluid pressure exerted by a millimetre of mercury, i.e., a pressure of 1 torr is approximately equal to 1 mmHg...

and its symbol, the unit name (gal) and its symbol (Gal) are spelled identically except that the latter is capitalized. The unit should not be confused with the identical all-lowercase abbreviation

Abbreviation

An abbreviation is a shortened form of a word or phrase. Usually, but not always, it consists of a letter or group of letters taken from the word or phrase...

for gallon

Gallon

The gallon is a measure of volume. Historically it has had many different definitions, but there are three definitions in current use: the imperial gallon which is used in the United Kingdom and semi-officially within Canada, the United States liquid gallon and the lesser used United States dry...

(gal).

The gal is named after Galileo Galilei

Galileo Galilei

Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...

, a physicist who made the first measurements of the Earth’s gravity.

It can also be used to measure peak ground acceleration

Peak ground acceleration

Peak ground acceleration is a measure of earthquake acceleration on the ground and an important input parameter for earthquake engineering, also known as the design basis earthquake ground motion...

in earthquakes.

See also