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For several objects in the Solar System, the value of ? is known to greater accuracy than either G or M. The SI units of the standard gravitational parameter are . However, units of are frequently used in the scientific literature and in spacecraft navigation.
Small body orbiting a central body
Log-log plot of period T vs semi-major axis a (average of aphelion and perihelion) of some Solar System orbits (crosses denoting Kepler's values) showing that a³/T² is constant (green line)
The central body in an orbital system can be defined as the one whose mass (M) is much larger than the mass of the orbiting body (m), or . This approximation is standard for planets orbiting the Sun or most moons and greatly simplifies equations. Under Newton's law of universal gravitation, if the distance between the bodies is r, the force exerted on the smaller body is:
Thus only the product of G and M is needed to predict the motion of the smaller body. Conversely, measurements of the smaller body's orbit only provide information on the product, ?, not G and M separately. The gravitational constant, G, is difficult to measure with high accuracy, while orbits, at least in the solar system, can be measured with great precision and used to determine ? with similar precision.
for elliptic and hyperbolic orbits, ? is twice the semi-major axis times the negative of the specific orbital energy, where the latter is defined as the total energy of the system divided by the reduced mass.
In a pendulum
The standard gravitational parameter can be determined using a pendulum oscillating above the surface of a body as:
where r is the radius of the gravitating body, L is the length of the pendulum, and T is the period of the pendulum (for the reason of the approximation see Pendulum in mathematics).
Geocentric gravitational constant
GM?, the gravitational parameter for the Earth as the central body, is called the geocentric gravitational constant. It equals .
The value of this constant became important with the beginning of spaceflight in the 1950s, and great effort was expended to determine it as accurately as possible during the 1960s. Sagitov (1969) cites a range of values reported from 1960s high-precision measurements, with a relative uncertainty of the order of 10-6.
During the 1970s to 1980s, the increasing number of artificial satellites in Earth orbit further facilitated high-precision measurements,
and the relative uncertainty was decreased by another three orders of magnitude, to about (1 in 500 million) as of 1992.
Measurement involves observations of the distances from the satellite to Earth stations at different times, which can be obtained to high accuracy using radar or laser ranging.
Heliocentric gravitational constant
GM☉, the gravitational parameter for the Sun as the central body,
is called the heliocentric gravitational constant or geopotential of the Sun and equals .
The relative uncertainty in GM☉, cited at below 10-10 as of 2015, is smaller than the uncertainty in GM?
because GM☉ is derived from the ranging of interplanetary probes, and the absolute error of the distance measures to them is about the same as the earth satellite ranging measures, while the absolute distances involved are much bigger.
^ ab"IAU Astronomical Constants: Current Best Estimates". iau-a2.gitlab.io. IAU Division I Working Group on Numerical Standards for Fundamental Astronomy. Retrieved 2021., citing Ries, J. C., Eanes, R. J., Shum, C. K., and Watkins, M. M., 1992, "Progress in the Determination of the Gravitational Coefficient of the Earth," Geophys. Res. Lett., 19(6), pp. 529-531.
^This is mostly because ? can be measured by observational astronomy alone, as it has been for centuries. Decoupling it into G and M must be done by measuring the force of gravity in sensitive laboratory conditions, as first done in the Cavendish experiment.
^Sagitov, M. U., "Current Status of Determinations of the Gravitational Constant and the Mass of the Earth", Soviet Astronomy, Vol. 13 (1970), 712-718, translated from Astronomicheskii Zhurnal Vol. 46, No. 4 (July-August 1969), 907-915.
^Lerch, Francis J.; Laubscher, Roy E.; Klosko, Steven M.; Smith, David E.; Kolenkiewicz, Ronald; Putney, Barbara H.; Marsh, James G.; Brownd, Joseph E. (December 1978). "Determination of the geocentric gravitational constant from laser ranging on near-Earth satellites". Geophysical Research Letters. 5 (12): 1031-1034. Bibcode:1978GeoRL...5.1031L. doi:10.1029/GL005i012p01031.
^Pitjeva, E. V. (September 2015). "Determination of the Value of the Heliocentric Gravitational Constant from Modern Observations of Planets and Spacecraft". Journal of Physical and Chemical Reference Data. 44 (3): 031210. Bibcode:2015JPCRD..44c1210P. doi:10.1063/1.4921980.