Kepler's laws of planetary motion provide examples of clearly formulated scientific laws that have been in continuous use since the 17th century. According to Jacob Silverman for HowStuffWorks, these three laws, taken together, describe the movement of planets around the sun to a high degree of accuracy.
Kepler's first law states that planets orbit stars along elliptical paths. Kepler's second law, or the law of areas, describes the way orbiting bodies change their velocity as their distance to their parent star changes. According to this law, Silverman notes that a direct relationship exists between a planet's orbital speed and its distance. If a line is drawn from a planet to its star at two points in its orbit, the area between the lines is equal to the area between any other two lines drawn elsewhere in the planet's orbit with the same length of time between them.
Kepler's third law, which is commonly taught as the law of periods, is a predictive tool that describes the relationship between a planet's orbital period and its average distance from its parent star, notes Silverman. Scientific laws such as these describe natural processes that are sufficiently regular and predictable to be used in formulating larger-scale scientific theories.