What are the three laws?
First law (ellipses): each planet orbits in an ellipse with the Sun at one focus. Second law (equal areas): a line from the Sun to a planet sweeps out equal areas in equal times, so planets move faster near perihelion. Third law (harmonic law): the square of the orbital period is proportional to the cube of the semi-major axis, T² ∝ a³.
How were they found and used?
Kepler spent years on Tycho Brahe's precise observations of Mars and finally abandoned two thousand years of belief in circular orbits. Decades later, Newton derived all three from the law of universal gravitation. Astronomers still use the third law to work out exoplanet orbits and stellar masses. Mars, at a ≈ 1.52 AU, has T ≈ 1.88 years.
FAQ
- Is Earth's orbit an ellipse?
- Yes, but with an eccentricity of 0.017, it's nearly circular.
- What units work for the third law?
- With T in years and a in AU, T² = a³ for planets around the Sun.
Sources reviewed: 2026-10-25
References
- Kepler, J. (1609). Astronomia Nova.
- Kepler, J. (1619). Harmonices Mundi.