Gravitational Forces

The planets move almost in circular elliptical orbits based on the force of gravity. The sun's gravitational pull is the most powerful gravitational force in the solar system. The other heavenly bodies have a much smaller gravitational force on one another called perturbations.

The planets orbit the sun in the same counterclockwise direction. A planet's year is the time required for it to complete one full orbit around the sun. This is referred to as the planet's revolution. All the planets rotate about their own axes as they revolve around the sun

The planets are grouped according to their physical properties. The inner planets (Mercury, Venus, Earth, and Mars), are called the terrestrial (meaning earthlike) planets. They are dense and small in size. They have solid, rocky crusts and interiors of metal. Except for Mercury, they have gaseous atmospheres from which lighter elements have escaped because of the low gravitational force.

The Jovian planets (Jupiter, Saturn, Uranus, and Neptune) all have great volume and mass but relatively low density (thickness). Pluto has sometimes been classed with the terrestrial planets, but it is more properly considered a special case. Most of the nine major planets have one or more moons.