Ellipses and Kepler's Laws of Planetary Motion

Perihelion

The nearest point to the sun in the orbit of a celestial body.

Aphelion

The farthest point from the sun in the orbit of a celestial body.

Eccentricity

A measure of how an orbit deviates from being circular; a value of 0 indicates a perfect circle.

Kepler’s First Law of Planetary Motion

The orbits of planetary bodies are ellipses with the sun at one of the two foci of the ellipse.

Kepler’s Second Law of Planetary Motion

A line drawn from the sun to an orbiting body sweeps out equal areas in equal times.

Kepler’s Third Law of Planetary Motion

The ratio of the squares of the periods of two planets is equal to the ratio of the cubes of their semi-major axes.

Semi-major axis

The longest radius of an ellipse, half of the major axis.

Orbital period

The time taken for a celestial body to complete one orbit around another body.

Elliptical orbit

An orbit that is in the shape of an ellipse, characterized by its eccentricity.

Mercury’s orbital period

88 days, the shortest orbital period of any planet in the solar system.

Neptune's orbital period

165 Earth years, the longest orbital period of the planets mentioned.