Description: The three laws formulated by Johannes Kepler in the beginning of the 1600s were the first to describe the orbits of the planets as not perfectly circular. The first law states that the planets orbit the Sun in an elliptical shape with the Sun at one of the focal points. The second law says that the area covered by a line between the planet and the Sun is the same in a given time interval of the orbit. According to the third law, the square of the time (T²) a planet takes to move around the Sun is proportional to the cube of its semi-major axis (a³, semi-major axis is a length which characterizes the size of the planet's orbit around the Sun). Kepler found these three laws by studying the observations of Mars carried out by his mentor Tycho Brahe. He used the laws to make the most precise calculation of the orbits of the planets known to his time.

Related Terms:

• Ellipse
• Orbit
• Planet
• Solar System

Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

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