Kepler’s First Law states that planets move in elliptical orbits, with the Sun at one focus of each ellipse. It is therefore also called the law of ellipses.
Johannes Kepler reached this conclusion while studying Tycho Brahe’s exceptionally accurate observations of Mars. Kepler had expected the planet’s path to be circular, because ancient and Renaissance astronomers often considered circles the ideal celestial shape. Small discrepancies remained, however, and Mars’s relatively eccentric orbit made them impossible to ignore.
The First Law does not say that the Sun sits at the exact center of an orbit. It lies at one of the ellipse’s two foci. The orbit of Earth is close to circular because its eccentricity is small, but it is technically an ellipse. Kepler’s Second Law concerns equal areas swept out in equal times, while the Third Law relates a planet’s orbital period to its average distance from the Sun.