The Solar System, including the Earth, is our most immediate laboratory for observing the consequences of gravity. The gravitational field in the Solar System is dominated by the Sun, which is far more massive than any of the planets.
In orbits that are relatively close to the Sun are the four smallest planets: Mercury, Venus, Earth, and Mars. Further out, there are four much larger planets: Jupiter, Saturn, Uranus, and Neptune. The Solar System contains a number of other objects, such as comets, asteroids, moons, and man-made spacecraft. By observing the motion of these objects, and in some cases by interacting with them, we can learn a great deal about the behaviour of gravity.
The study of experimental and observational gravity in the Solar System took of in earnest during the latter half of the 20th century. While astronomers had been tracking the motion of the planets for centuries, the development of new technologies and methods in the 20th century allowed observations and experiments to be carried out in ways that had never previously been possible. In order to try and present the results of this work in some kind of sensible order, I will classify them into experiments that probe the foundational assumptions of gravity theories; experiments that probe Newton’s law; and experiments that probe the subtle efects that result from Einstein’s theory.
Testing foundational assumptions
There are a number of foundational assumptions that go into the modern theory of gravity. These include the fact that the rest mass of an object should be independent of its position, and independent of its motion with respect to other bodies. Other assumptions are the idea that the speed of light should be the same in every direction, and that all objects should fall at the same rate (in the absence of non-gravitational forces). Over the course of the past century all these ideas have been tested to extremely high accuracy. I’ll outline some of the best of these tests here, before we move on to thinking about experiments that test the particulars of Newton’s and Einstein’s theories of gravity.