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The gal (symbol: Gal), sometimes called galileo after Galileo Galilei, is a unit of acceleration typically used in precision gravimetry.Barry N. Taylor, Guide for the Use of the International System of Units (SI), 1995, NIST Special Publication 811, Appendix B.BIPM SI brochure, 8th ed. 2006, Table 9: Non-SI units associated with the CGS and the CGS-Gaussian system of units .Some sources, such as the University of North Carolina , the European Space Agency, and ConversionTables.com state that the unit name is "galileo". The NIST and the BIPM are here considered as more authoritative sources regarding the proper unit name. 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". NIST Guide to SI Units; Section 5, Units Outside the SI; Subsection 5.2: Units temporarily accepted for use with the SI. Use of the gal was deprecated by the standard , now superseded.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 (), resulting in a maximal difference of about 2 Gal (0.02 m/s2) from the top of Mount Everest to sea level. Gravity Measurements . University of Calgary. Retrieved November 21, 2009.
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 of Gal !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 | |
| Magma entry under Mount Etna in 2002 October | |
| Semidiurnal and diurnal earth tide, due to sun and moon | |
| Pole tide component of earth tide, due to Chandler wobble | |
| Precision of a superconducting gravimeter | |
| Background level of the free oscillations of earth ("earth hum") | |
| Theoretical precision of an Optomechanics gravimeter |
By combining data from many measurements, the sensitivity of gravimetry can be decreased further. 100 days of measurement with a superconducting gravimeter reached in precision, which was sufficient to detect the hum of the earth.
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