Standard gravitational parameter
|Body||μ (m3 s−2)|
For all objects in the Solar System, the value of μ is known to greater accuracy than either G or M. This is because μ is calculated directly from the speed of a small test mass in orbit about the body, which can be done very accurately. The mass M is then inferred by using G. The SI units of the standard gravitational parameter are m3 s−2.
Small body orbiting a central body
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 M ≫ m. 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, μ, 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 μ with similar precision.
For a circular orbit around a central body:
This can be generalized for elliptic orbits:
Two bodies orbiting each other
In the more general case where the bodies need not be a large one and a small one (the two-body problem), we define:
- the vector r is the position of one body relative to the other
- r, v, and in the case of an elliptic orbit, the semi-major axis a, are defined accordingly (hence r is the distance)
- μ = Gm1 + Gm2 = μ1 + μ2, where m1 and m2 are the masses of the two bodies.
- for circular orbits, rv2 = r3ω2 = 4π2r3/T2 = μ
- for elliptic orbits, 4π2a3/T2 = μ (with a expressed in AU; T in seconds and M the total mass relative to that of the Sun, we get a3/T2 = M)
- for parabolic trajectories, rv2 is constant and equal to 2μ
- for elliptic and hyperbolic orbits, μ is twice the semi-major axis times the absolute value of the specific orbital energy, where the latter is defined as the total energy of the system divided by the reduced mass.
Terminology and accuracy
Note that the reduced mass is also denoted by μ.
The value for the Earth is called the geocentric gravitational constant and equals 600.4418±0.0008 km3 s−2. Thus the uncertainty is 1 to 398000000, much smaller than the uncertainties in G and M separately (1 to 500 each). 7000
The value for the Sun is called the heliocentric gravitational constant or geopotential of the Sun and equals 12440018×1020 m3 s−2. 1.327
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- This is mostly because μ 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.
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