Kepler’s First Law states simply that Mars travels in an elliptical orbit, with the Sun at one focus of the ellipse. Although he chose to list it first, Kepler only came to this conclusion after figuring out his "second" law, which says that if you draw a line from the Sun to Mars, and wait a fixed amount of time, that line will sweep out a certain area as Mars moves along its orbit. This finding was generalized to include all planets in 1621.
Kepler’s First Law states simply that Mars travels in an elliptical orbit, with the Sun at one focus of the ellipse. Although he chose to list it first, Kepler only came to this conclusion after figuring out his "second" law, which says that if you draw a line from the Sun to Mars, and wait a fixed amount of time, that line will sweep out a certain area as Mars moves along its orbit. This finding was generalized to include all planets in 1621.