First Law
Kepler's first law of planetary motion states that planets orbit the sun in an ellipse, with the sun at one of the ellipse's two foci. This law is also known as the law of ellipses.
An ellipse is a shape that resembles a flattened circle. The amount of flattening is expressed by the ellipse's eccentricity, which is a number between 0 and 1. A perfect circle has an eccentricity of 0.
Kepler's first law means that the motion of the planets around the sun follows an elliptical path, not a circle. As the planet goes around its orbit, the distance between the planet and the sun is constantly changing.
Second Law
Kepler's second law of planetary motion, also known as the law of equal areas, describes the speed of a planet traveling in an elliptical orbit around the sun.
The law states that a line between the sun and the planet sweeps out equal areas in equal times. This means that the planet speeds up as it nears the sun and slows down as it moves away from it.
According to Kepler's second law, the speed at which planets move in space is constantly changing. For example, the Earth travels at an average velocity of 29.8 kilometers per second, but it travels at a faster rate of 30 kilometers per second at perihelion and a slower rate of 29.3 kilometers per second at aphelion.
Third Law
Kepler's third law of planetary motion states that the square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.
Kepler's third law implies that the time it takes for a planet to orbit the sun increases rapidly with the radius of its orbit.
Kepler's third law also states that a radius vector joining any planet to the sun sweeps out equal areas in equal lengths of time.
The orbit of a planet is an ellipse with the Sun at one of the two foci. A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time. The square of a planet's orbital period is proportional to the cube of the length of the semi-major axis of its orbit.
