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 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. ('''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/s2 | units2 = SI units | inunits2 = 0.01 m/s2 | 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 mGal (magenta) to +30 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 cm/s2). 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 initials "SI"). In 1978 the 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 80000-3:2006, 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 Gal is equal to 0.01 m/s2.

The acceleration due to Earth's gravity at its surface is 976 to 983 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 gradient, usually measured in eotvos (0.1 μGal/m). The vertical gravity gradient near Earth's surface is ~3.1 μGal per centimeter of height ({{val|3.1|e=-6|u=s−2}}), resulting in a maximal difference of about 2 Gal (0.02 m/s2) 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 |- |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 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 μGal. These include change in ground water level by ~0.1 m, underground magma formations near a volcano, daily evapotranspiration from a deciduous forest, yearly change in ground height due to subsidence, the free oscillations of earth excited by major earthquakes, etc. For example, the maxima precision of a superconducting gravimeter is sufficient to measure groundwater level change of 1 mm, and can detect the onset of the 2011 Tohoku earthquake 510 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== * 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}}

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