Equinox

An orbit is the path an object takes in response to two laws. Inertia causes the object to move in a straight line. Gravity causes the object to fall toward some other object. The combined effect of the two laws is to cause the object to move in a fixed path, or orbit. The two objects can be the Space Shuttle or Moon around the Earth, the Apollo command module around the Moon, or a planet around the Sun. The more massive object is called the primary and the less massive one is called the secondary.

http://www.uwgb.edu/dutchs/AstronNotes/HowSolSysWorks.HTM

Kepler's Laws

An orbit isn't a circle. The object moves fast close to the Sun, loses speed as it pulls away and gravity pulls it back, and then begins falling inward, gaining speed again. The astronomer Johannes Kepler first worked out the laws of planetary motion in the early 1600's.

Kepler's First Law is that planets move in elliptical orbits with the Sun at one focus. There are many ways to draw ellipses. The top diagram shows one way. Stick two pins in a board, loop a string loosely around them, then pull the string tight with a pencil and keep it tight as you move the pencil around the pins. Each pin is a focus of the ellipse. The lower figure shows why the focus is called that. If you put a light at one focus and line the ellipse with mirrors, all the light reflects to the other focus. Some telescope designs take advantage of this fact. It also works with sound. An elliptical room, called a whispering gallery, enables two people at the foci to converse in whispers while people not at the foci hear nothing. What's at the other focus of an orbit? Nothing. It's called the empty focus and has no physical significance.

Kepler's Second Law is that a line from the planet to the Sun sweeps out equal areas in equal times. In the diagram above, the wedges are of different shapes but their areas are equal.