# Thread: Measuring Gravity On Stars and Planets?

1. How do physicists measure gravity on planets and stars which we obviously haven't landed on before?
I know they use gravitometers when inside the planets gravitational field, but let's take Mercury for example?
How can they measure it's gravity?

2.

3. Just a guess here.. but I would asume there is in equation that you would enter the suns mass, the distance from the sun, and velocity of mercury, to determine its mass. thereofre detmining its gravity...

4. Originally Posted by FutureWarp
How do physicists measure gravity on planets and stars which we obviously haven't landed on before?
I know they use gravitometers when inside the planets gravitational field, but let's take Mercury for example?
How can they measure it's gravity?
For most planets, its fairly straight forward: They have moons. How fast those moons orbit at the distances they do tells us the strength of the Planet's gravity. With that and knowing the size of the planet we can calculate its surface gravity without having ever landed on it.

Mercury doesn't have a moon. However, it's gravity does have an effect on other planet's ( by perturbing their orbit.). by carefully measuring how much Mercury perturbs Venus' orbit, we can take a pretty good guess at its mass. And even thought we have not landed on Mercury, we have sent probes the made close flybys. Just like with the planets with moons we can see just how much Mercury's gravity affects the path of the Probe, we can then use this information to get a good idea of the strength of the Mercury's gravity.

5. Originally Posted by Brandon
Just a guess here.. but I would asume there is in equation that you would enter the suns mass, the distance from the sun, and velocity of mercury, to determine its mass. thereofre detmining its gravity...
This reverses the method to determin the suns mass from gravity. For doing this you need a gravitational constant. I have never seen proof that this constant is really universal in that it applies when the gravitational field is different then for determining the constant.

Newton,s formula on itself uses G for determining Fz from mass (with a torsion balance) or mass from Fz (mass of the earth). Because the equation uses G it can,t proof G as a universal constant.

Same with the torsionbalance. It can,t do both measure G and be used as experimental proof for what it meassures with using the same formula.

6. Have you ever seen any evidence that the gravitational constant isn't constant?

7. Originally Posted by Ghrasp
Originally Posted by Brandon
Just a guess here.. but I would asume there is in equation that you would enter the suns mass, the distance from the sun, and velocity of mercury, to determine its mass. thereofre detmining its gravity...
This reverses the method to determin the suns mass from gravity. For doing this you need a gravitational constant. I have never seen proof that this constant is really universal in that it applies when the gravitational field is different then for determining the constant.

Newton,s formula on itself uses G for determining Fz from mass (with a torsion balance) or mass from Fz (mass of the earth). Because the equation uses G it can,t proof G as a universal constant.

Same with the torsionbalance. It can,t do both measure G and be used as experimental proof for what it meassures with using the same formula.

History of measurementThe gravitational constant appears in Newton's law of universal gravitation, but it was not measured until 71 years after Newton's death by Henry Cavendish with his Cavendish experiment, performed in 1798 (Philosophical Transactions 1798). Cavendish measured G implicitly, using a torsion balance invented by the geologist Rev. John Michell. He used a horizontal torsion beam with lead balls whose inertia (in relation to the torsion constant) he could tell by timing the beam's oscillation. Their faint attraction to other balls placed alongside the beam was detectable by the deflection it caused. Cavendish's aim was not actually to measure the gravitational constant, but rather to measure the Earth's density relative to water, through the precise knowledge of the gravitational interaction. In retrospect, the density that Cavendish calculated implies a value for G of 6.754 × 10−11 m3/kg/s2.[5]
The accuracy of the measured value of G has increased only modestly since the original Cavendish experiment. G is quite difficult to measure, as gravity is much weaker than other fundamental forces, and an experimental apparatus cannot be separated from the gravitational influence of other bodies. Furthermore, gravity has no established relation to other fundamental forces, so it does not appear possible to calculate it indirectly from other constants that can be measured more accurately, as is done in some other areas of physics. Published values of G have varied rather broadly, and some recent measurements of high precision are, in fact, mutually exclusive.[3][6]
In the January 5, 2007 issue of Science (page 74), the report "Atom Interferometer Measurement of the Newtonian Constant of Gravity" (J. B. Fixler, G. T. Foster, J. M. McGuirk, and M. A. Kasevich) describes a new measurement of the gravitational constant. According to the abstract: "Here, we report a value of G = 6.693 × 10−11 cubic meters per kilogram second squared, with a standard error of the mean of ±0.027 × 10−11 and a systematic error of ±0.021 × 10−11 cubic meters per kilogram second squared."[7]
Gravitational constant - Wikipedia, the free encyclopedia