Radiometric dating uses naturally occurring radioactive elements such as carbon-14 within the rock or fossil to estimate its approximate age. The half-life of the element must also be known. The amount of the radioactive trace element is compared to, in a ratio, with that of the number of the decay product elements. This approximate age can then be relatively compared to those of other rocks or fossils.
Gravity is the force by which a planet or other body draws objects toward its center. The force of gravity keeps all of the planets in orbit around the sun.
