Our solar system is comprised of nine planets and various smaller objects such as asteroids and comets, all of which orbit around the brightest and warmest object, the Sun. Each planet has its own 'orbital cycle' -- that is, how long it takes to circle completely around the Sun. ‘Interior’ planets are the fast-moving planets between Earth and the Sun. ‘Exterior’ planets are those beyond Earth and have longer orbits. The planets furthest from the Sun have the longest orbits. Also, each planet has physical qualities that make it unique.
Every planet except the Sun and Moon has its own 'retrograde cycle' too. Remember, retrograde motion is an optical illusion caused by a faster planet outpacing a slower planet in their respective orbits. Think of two runners on a circular track. The runner in the inner lane usually gains distance over a runner in an outer lane. It’s the same with the planets, and the changing relationships between planets and the Earth sometimes make it seem that planets are moving backwards against the backdrop of the zodiac. The energy of the planet is directed inward during retrograde periods.
Each planet also has its own cycle of visibility, except Uranus, Neptune and Pluto, which aren’t visible without a telescope. In fact, some of our interpretations for the planets come from factors related to their visibility, such as the fleeting nature of Mercury or the limits of Saturn.
An astrological chart is cast for the moment a person draws their first breath. After a person is born, the planets continue to move through each of the zodiac signs until, sooner or later, they each return to the original degree and sign they were in at the time of birth.
However, humans don't live long enough to witness the complete revolution of every planet. We usually live to see many entire revolutions of interior planets and some of the exterior planets. The return of a planet to its natal position indicates a focus on the activities of that planet.