| gal | |
|---|---|
 Earth's gravity measured by NASAGRACE mission, showing deviations from thetheoretical gravity of an idealized smooth Earth, the so-calledearth ellipsoid. Red shows the areas where gravity is stronger than the smooth, standard value, and blue reveals areas where gravity is weaker. (Animated version.)[1] | |
| General information | |
| Unit system | CGS units |
| Unit of | Acceleration |
| Symbol | Gal |
| Named after | Galileo Galilei |
| Conversions | |
| 1 Galin ... | ... is equal to ... |
| CGS base units | 1 cm/s2 |
| SI units | 0.01 m/s2 |
| Imperial, US customary | 0.03280840 ft/s2 |
Thegal (symbol: Gal), sometimes calledgalileo afterGalileo Galilei, is a unit ofacceleration typically used in precisiongravimetry.[2][3][4] The gal is defined as 1 centimeter per second squared (1 cm/s2). Themilligal (mGal) andmicrogal (μGal) are respectively one thousandth and one millionth of a gal.
The gal is not part of theInternational System of Units (known by itsFrench-language initials "SI"). In 1978 theCIPM decided that it was permissible to use the gal "with the SI until the CIPM considers that [its] use is no longer necessary".[3][5] Use of the gal was deprecated by the standardISO 80000-3:2006, now superseded.
The gal is a derived unit, defined in terms of thecentimeter–gram–second (CGS) base unit of length, thecentimeter, and thesecond, 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 inlatitude andelevation.Standard gravity is 980.665 Gal. Mountains and masses of lesser density within the Earth's crust typically cause variations ingravitational acceleration of 10 to hundreds of milligals (mGal).
The gradient of gravity is thegravity gradient, usually measured ineotvos (0.1 μGal/m). The vertical gravity gradient near Earth's surface is ~3.1 μGal per centimeter of height (3.1×10−6 s−2), resulting in a maximal difference of about 2 Gal (0.02 m/s2) from the top ofMount Everest to sea level.[6]
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 thetorr and its symbol, the unit name (gal) and its symbol (Gal) are spelled identically except that the latter is capitalized.
| 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 ofKater's pendulum | 7×10−3 |
| Magma entry underMount Etna in 2002 October | 4×10−4 |
| Semidiurnal and diurnalearth tide, due to sun and moon | 3×10−4 |
| Pole tide component of earth tide, due toChandler wobble | 5×10−6 |
| Precision of a superconducting gravimeter | 1×10−8 |
| Background level of the free oscillations of earth ("earth hum") | 3×10−10[8] |
| Theoretical precision of anoptomechanical gravimeter | 1×10−13[9] |
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, dailyevapotranspiration from adeciduous forest, yearly change in ground height due tosubsidence, thefree oscillations of earth excited by majorearthquakes, 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 the2011 Tohoku earthquake 510 km away from the epicenter.[7]
By combining data from many measurements, the sensitivity of gravimetry can be decreased further. 100 days of measurement with a superconducting gravimeter reached1×10−10 Gal in precision, which was sufficient to detect the hum of the earth.[8]
| Base value | (Gal, or cm/s2) | (ft/s2) | (m/s2) | (standard gravity,g0) |
|---|---|---|---|---|
| 1 Gal, or cm/s2 | 1 | 0.0328084 | 0.01 | 1.01972×10−3 |
| 1 ft/s2 | 30.4800 | 1 | 0.304800 | 0.0310810 |
| 1 m/s2 | 100 | 1/0.3048 ≈3.28084 | 1 | 0.101972 |
| 1g0 | 980.665 | 32.1740 | 9.80665 | 1 |
