Sir Isaac Newton's Universal Law of Gravitation states that matter in the universe attracts every other piece of matter with a force of gravity proportional to its mass. In accordance with Newton's Law, the most basic method for measuring the size and mass of a planet or sun is by observing its effects on the motion of other bodies near it, such as its satellites. By tracking the speed of a body passing close to the planet, the mass of the planet can be calculated: the heavier the planet, the faster a neighboring body moves around it. The mass of moonless planets can also be calculated in this way, by observing the planets' relationships with planets nearby.
Masses of objects outside the solar system can also be calculated by observing the mutual interactions between an object and its neighbors. Planets that have been discovered outside our solar system, as well as other types of objects such as stars and black holes, can be measured in this way. Perturbations in orbits and rotations of these objects provide essential information about a planet or star's mass and dimensions.
The basic unit for measuring distances between planets is the Astronomical Unit (AU), based on the average distance between the Earth and the sun, about 149,600,000 kilometers. These distances can be determined by bouncing radar signals off planets or moons, or through timing radar transmissions sent by spacecraft on missions into the outer solar system, which allows precise measurement of distances between those planets and their moons and the Earth or the sun.
Beyond the solar system, distances are measured in light-years, or the amount of time it takes for light from a distant object to reach Earth. For objects closer than 100 light years, a method called stellar parallax is used, based on the fact that, when viewed from different vantage points, a relatively close object shifts position against more distant ones. Since this method fails for more distant objects, other methods, such as spectral redshift variations and the relative brightness of stars called the Cepheid variables, are used.