View source for Gal (unit)

{{Short description|Centimeter–gram–second unit of acceleration}}
{{distinguish|gallon}}
{{Infobox unit
| bgcolor = 
| name = gal
| image = Gravity anomalies on Earth.jpg
| caption = Earth's gravity measured by NASA [[Gravity Recovery and Climate Experiment|GRACE]] mission, showing deviations from the [[theoretical gravity]] of an idealized smooth Earth, the so-called [[earth ellipsoid]]. Red shows the areas where gravity is stronger than the smooth, standard value, and blue reveals areas where gravity is weaker. ('''[[:Image:GRACE globe animation.gif|Animated version]].''')<ref>{{cite web|last=NASA/JPL/University of Texas Center for Space Research|title=PIA12146: GRACE Global Gravity Animation|url=http://photojournal.jpl.nasa.gov/catalog/PIA12146|work=Photojournal|publisher=[[NASA Jet Propulsion Laboratory]]|access-date=30 December 2013}}</ref>
| standard = [[CGS units]]
| quantity = Acceleration
| symbol = Gal
| symbol2 = 
| namedafter = [[Galileo Galilei]]
| extralabel = 
| extradata = 
| units1 = CGS base units
| inunits1 = 1 cm/s<sup>2</sup>
| units2 = SI units
| inunits2 = 0.01 m/s<sup>2</sup>
| units3 = Imperial, US customary
| inunits3 = {{cvt|1|cm/s2|ft/s2|disp=out|sigfig=7}}
}}
[[Image:Southern ocean gravity hg.png|thumb|right|231px|[[Gravity anomalies]] covering the Southern Ocean are shown here in false-color relief. Amplitudes range between −30&nbsp;mGal (magenta) to +30&nbsp;mGal (red). This image has been normalized to remove variation due to differences in latitude.]]
The '''gal''' (symbol: Gal), sometimes called '''galileo''' after [[Galileo Galilei]], is a unit of [[acceleration]] typically used in precision [[gravimetry]].<ref>Barry N. Taylor, ''Guide for the Use of the International System of Units (SI),'' 1995, [[NIST]] Special Publication 811, [http://physics.nist.gov/Pubs/SP811/appenB9.html#ACCELERATION Appendix B].</ref><ref name="SIbrochure">[[BIPM]] ''SI brochure'', 8th ed. 2006, [http://www.bipm.org/en/si/si_brochure/chapter4/table9.html Table 9: Non-SI units associated with the CGS and the CGS-Gaussian system of units] {{webarchive|url=https://web.archive.org/web/20071018031134/http://www.bipm.org/en/si/si_brochure/chapter4/table9.html |date=2007-10-18 }}.</ref><ref>Some sources, such as the [http://www.unc.edu/~rowlett/units/dictG.html University of North Carolina] {{Webarchive|url=https://web.archive.org/web/20120218191001/http://www.unc.edu/~rowlett/units/dictG.html |date=2012-02-18 }}, the [http://www.esa.int/esaLP/ESAK4XZK0TC_LPgoce_0.html European Space Agency], and [http://www.conversiontables.info/index.php5?page=conversion_factors&item=Acceleration%20%5Bgeneric%5D ConversionTables.com] {{webarchive|url=https://web.archive.org/web/20090519183743/http://www.conversiontables.info/index.php5?page=conversion_factors&item=Acceleration%20%5Bgeneric%5D |date=2009-05-19 }} state that the unit name is "galileo". The NIST and the BIPM are here considered as more authoritative sources regarding the proper unit name.</ref> The gal is defined as 1 centimeter per second squared (1&nbsp;cm/s<sup>2</sup>). The '''milligal''' (mGal) and '''microgal''' (μGal) are respectively one thousandth and one millionth of a gal.

The gal is not part of the [[International System of Units]] (known by its [[French language|French-language]] initials "SI"). In 1978 the [[International Committee for Weights and Measures|CIPM]] decided that it was permissible to use the gal "with the SI until the CIPM considers that [its] use is no longer necessary".<ref name="SIbrochure"/><ref>''NIST Guide to SI Units''; Section 5, ''Units Outside the SI''; Subsection 5.2: ''[http://physics.nist.gov/Pubs/SP811/sec05.html#5.2 Units temporarily accepted for use with the SI]''.</ref> Use of the gal was deprecated by the standard [[ISO 80000-3|ISO 80000-3:2006]], now superseded.

The gal is a derived unit, defined in terms of the [[centimeter–gram–second]] (CGS) base unit of length, the [[centimeter]], and the [[second]], which is the base unit of time in both the CGS and the modern SI system. In SI base units, 1&nbsp;Gal is equal to 0.01&nbsp;m/s<sup>2</sup>.

The acceleration due to Earth's gravity at its surface is 976 to 983&nbsp;Gal, the variation being due mainly to differences in [[latitude]] and [[elevation]]. [[Standard gravity]] is 980.665 Gal. Mountains and masses of lesser density within the Earth's crust typically cause variations in [[gravitational acceleration]] of 10 to hundreds of milligals (mGal).

The gradient of gravity is the [[Gravity gradiometry|gravity gradient]], usually measured in [[Eotvos (unit)|eotvos]] (0.1&nbsp;μGal/m). The vertical gravity gradient near Earth's surface is ~3.1&nbsp;μGal per centimeter of height ({{val|3.1|e=-6|u=s<sup>−2</sup>}}), resulting in a maximal difference of about 2 Gal (0.02&nbsp;m/s<sup>2</sup>) from the top of [[Mount Everest]] to sea level.<ref>[http://www.geo.ucalgary.ca/~wu/Goph547/Gravimeters.pdf Gravity Measurements] {{webarchive|url=https://web.archive.org/web/20090306192459/http://www.geo.ucalgary.ca/~wu/Goph547/Gravimeters.pdf|date=2009-03-06}}. University of Calgary. Retrieved November 21, 2009.</ref>

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]] and its symbol, the unit name (gal) and its symbol (Gal) are spelled identically except that the latter is capitalized.

== Examples ==
{| class="wikitable"
|+Examples of Gal<ref name=":0">{{Cite journal |last1=Van Camp |first1=Michel |last2=de Viron |first2=Olivier |last3=Watlet |first3=Arnaud |last4=Meurers |first4=Bruno |last5=Francis |first5=Olivier |last6=Caudron |first6=Corentin |date=2017 |title=Geophysics From Terrestrial Time-Variable Gravity Measurements |url=https://onlinelibrary.wiley.com/doi/abs/10.1002/2017RG000566 |journal=Reviews of Geophysics |language=en |volume=55 |issue=4 |pages=938–992 |doi=10.1002/2017RG000566 |bibcode=2017RvGeo..55..938V |issn=1944-9208}}</ref>
!Effect type
!Effect size (Gal)
|-
|Various places on the surface of earth
|976–983
|-
|Standard gravity
|980.665
|-
|[[Gravitation of the Moon|Surface gravity of moon]]
|161–164
|-
|Change between Mount Everest's peak to sea level
|~2
|-
|Precision of [[Kater's pendulum]]
|{{val|7e-3}}
|-
|Magma entry under [[Mount Etna]] in 2002 October
|{{val|4e-4}}
|-
|Semidiurnal and diurnal [[earth tide]], due to sun and moon
|{{val|3e-4}}
|-
|[[Pole tide]] component of earth tide, due to [[Chandler wobble]]
|{{val|5e-6}}
|-
|Precision of a superconducting gravimeter
|{{val|1e-8}}
|-
|Background level of the free oscillations of earth ("earth hum")
|{{val|3e-10}}<ref name=":1">{{Cite journal |last=Tanimoto |first=Toshiro |date=2001-05-01 |title=Continuous Free Oscillations: Atmosphere-Solid Earth Coupling |url=https://www.annualreviews.org/content/journals/10.1146/annurev.earth.29.1.563 |journal=Annual Review of Earth and Planetary Sciences |language=en |volume=29 |pages=563–584 |doi=10.1146/annurev.earth.29.1.563 |bibcode=2001AREPS..29..563T |issn=0084-6597|url-access=subscription }}</ref>
|-
|Theoretical precision of an [[Optomechanics|optomechanical]] gravimeter
|{{val|1e-13}}<ref>{{Cite journal |last=Qvarfort |first=Sofia |last2=Serafini |first2=Alessio |last3=Barker |first3=P. F. |last4=Bose |first4=Sougato |date=2018-09-11 |title=Gravimetry through non-linear optomechanics |url=https://www.nature.com/articles/s41467-018-06037-z |journal=Nature Communications |language=en |volume=9 |issue=1 |pages=3690 |doi=10.1038/s41467-018-06037-z |issn=2041-1723|pmc=6133990 }}</ref>
|}
Many geophysical effects produce variation in surface gravity on the order of 0.1 to 1&nbsp;μGal. These include change in ground water level by ~0.1&nbsp;m, underground magma formations near a volcano, daily [[evapotranspiration]] from a [[Deciduous|deciduous forest]], yearly change in ground height due to [[subsidence]], the [[Normal mode|free oscillations]] of earth excited by major [[Earthquake|earthquakes]], etc. For example, the maxima precision of a superconducting gravimeter is sufficient to measure groundwater level change of 1&nbsp;mm, and can detect the onset of the [[2011 Tōhoku earthquake and tsunami|2011 Tohoku earthquake]] 510&nbsp;km away from the epicenter.<ref name=":0" /> 

By combining data from many measurements, the sensitivity of gravimetry can be decreased further. 100 days of measurement with a superconducting gravimeter reached {{val|1e-10|u=Gal}} in precision, which was sufficient to detect the hum of the earth.<ref name=":1" />

==Conversions==
{{Acceleration conversions}}

==See also==
* [[Gravity of Earth|Earth's gravity]]
* [[Eotvos (unit)]]
* [[g-force]] (''g'') 
* [[Gravimetry]]
* [[Gravity]]
* [[Gravitational constant]] (''G'')
* [[Gravitational field]]
* [[Gravity gradiometry]]

==References==
{{Reflist}}
{{Scientists whose names are used as units}}
{{CGS units}}

{{DEFAULTSORT:Gal (Unit)}}

[[Category:Units of acceleration]]
[[Category:Centimetre–gram–second system of units]]
[[Category:Gravimetry]]