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I was exploring some queries about the moon and came across the question Which artificial satellites in lunar orbit are currently active?. In this, the accepted answer says:

The lunar gravity field varies by as much as 1%.

I want to know why lunar gravity is not constant as Earth's is.

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    $\begingroup$ space.com/21364-moon-gravity-mascons-mystery.html Earth gravity is also not constant; 9.8metres per second squared is a rationalized figure. As an example your mass on top of Mt. Everest will be slightly different from that at the foot of the Himalayas. $\endgroup$ – Everyone Nov 23 '14 at 16:21
  • $\begingroup$ @Everyone yes I agree with you.In this question space.stackexchange.com/q/4948/1006,the first line of the answer says In general terms, once an object is in orbit, it will stay in orbit. Doesn't matter if it's debris, a satellite, rocket body, etc. So as you said the gravity is not constant so an object in the orbit have to come down once,Am I right? $\endgroup$ – SpringLearner Nov 23 '14 at 17:20
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    $\begingroup$ @Everyone, for the sake of pedantry, your mass is the same at the top and bottom of Everest. Your weight changes. $\endgroup$ – Holloway Nov 24 '14 at 9:22
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    $\begingroup$ @Trengot That's not pedantry: it's the whole point. One cannot possibly answer this sort of question without understanding what weight is and how it differs from mass. $\endgroup$ – David Richerby Nov 24 '14 at 12:17
  • $\begingroup$ Now how did I do that ... I stand corrected (still on top and at the bottom of the Himalaya range though!) $\endgroup$ – Everyone Nov 25 '14 at 4:40
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The Moon's gravity does not "decrease". The Moon is not of uniform composition. It contains a variety of materials. The different materials have different densities. These materials are not uniformly distributed. So, the gravity field around the Moon is not uniform. If you imagine a sphere of some radius exactly centered on the Moon's center of mass, gravity will not be the same at all points on the sphere. At any given point, it is constant over time, just not necessarily the same as all other points on that sphere. So, any object orbiting the Moon is not going to travel in an ideal elliptical or circular path. It will be perturbed by the irregularities in the Moon's gravity field. These effects can add up quickly, so a Lunar orbiting satellite has to make frequent adjustments to maintain an orbit.

The farther away you get from the Moon, the less significant those irregularities become, but you don't have to go very far before another factor takes hold: the Earth.

The Earth also has an irregular distribution of mass, so Earth orbiting satellites also have to contend with similar perturbations, but to a smaller degree. The GPS satellites have to account for these perturbations. Although they don't have to make adjustments to their orbits very often, the paths of the satellites are affected enough to impact system accuracy if not factored in to the descriptions of their trajectories.

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  • $\begingroup$ I would have assumed that GPS satellites don't need to worry about this as they are in a geosynchronous orbit? $\endgroup$ – DavidG Nov 24 '14 at 10:29
  • $\begingroup$ Ephemeris prediction for GPS satellites is good for 2-4 weeks, IIRC. And no, they're very much NOT in geosynchronous orbits. That wouldn't work at all. They're about 100 times closer, and still they operate at a SNR of -6 dB. Remember, they're solar powered, so they distribute about 1 kW of energy over half the earth's surface. That's not a lot per square meter. $\endgroup$ – MSalters Nov 24 '14 at 10:37
  • $\begingroup$ @DavidG they're not in geosynchronous orbit, but rather in a MEO You need at least 4 satellites to be visible in order to get your position. You also need them to be as spread out as possible across the entire sky (not just the equatorial belt) in order to get the most accurate position. If they were in geosynch, it would be harder for more extreme latitudes to have visibility (compare: Sirius radio vs XM radio orbits). See also gps.gov/systems/gps/space and hyperphysics.phy-astr.gsu.edu/hbase/gps.html $\endgroup$ – user5892 Nov 24 '14 at 15:10
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The answer appears to be mascons. "Mascons" is short for "Mass concentrations", which are regions of a celestial body where there are anomalies in density, which lead to abrupt changes in the local gravitational field of the body. They can even impact lunar satellites.

Here's an example:

enter image description here

Mascons may be formed by subsurface processes, such as changes in the Moon's crust following an impact with another small body. Hypothetical sub-surface lava flows are another explanation. However, the impact theory has gained more support, in part because of the high frequency in impacts on the Moon.

You can read more about mascons here and here.

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First off, gravitation from the Earth is not quite as uniform as you think. It varies by about 1/2 of one percent.

Secondly, and far more importantly, the bigger an object is, the more uniform is its gravitational field. Look to Jupiter and Saturn. By far the biggest contributor to Jupiter's and Saturn's non-spherical gravitational fields are the fact that Jupiter and Saturn rotate at rather high rates. After accounting for this rotation, Jupiter and Saturn are essentially "spherical."

While the Earth is not nearly as massive as Jupiter or Saturn, it is still more or less "spherical." While the gravitation deviations from that predicted by an ellipsoidal Earth are important to finding oil or minerals, they are rather small. The deviations of gravitation on the Moon from an ellipsoidal Moon are huge in comparison. Comets such as 67P/Churyumov–Gerasimenko and 25143 Itokawa provide even more extreme examples. The gravitation from these objects only looks spherical from a large distance (any finite-shaped object looks like a point mass from a sufficiently large distance). Close-up, gravitation from a comet is anything but spherical because they are so lumpy.

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