Kepler's laws
Johannes Kepler's three rules: planets move on ellipses, sweep equal areas in equal times, and take longer the farther out they are.
In the early 1600s Johannes Kepler, working from Tycho Brahe's careful observations of Mars, found three rules that describe how planets move. They are still the starting point for orbit calculations.
First, a planet's orbit is an ellipse, a slightly squashed circle, with the Sun at one of its two focus points rather than at the centre. Earth's orbit is very nearly circular, with an eccentricity of about 0.0167, but comets can follow long, thin ellipses. Second, a line from the Sun to the planet sweeps out equal areas in equal times, so a planet moves fastest when closest to the Sun and slowest when farthest away. Third, the square of the time a planet takes to go round is proportional to the cube of its average distance: a planet four times farther out takes eight times as long. Kepler published the first two laws in 1609 and the third in 1619.
Isaac Newton later showed that all three follow from his law of gravity. That let Edmond Halley calculate the orbit of the comet now named after him and predict its return. Real paths are never perfect ellipses, because the planets tug on each other and on comets; such tugs can bring a comet back weeks or months early or late.
A common misconception is that the seasons come from the elliptical orbit. Earth's orbit is too nearly circular for that; the tilt of its axis causes the seasons.
Coming up in future stories
- Kepler Saved His Mother From the Witch Hunters 1615–1621 · Leonberg, Württemberg
- The Comet That Kept Its Appointment 1758 · Prohlis near Dresden, Saxony
- The Planet That Einstein Deleted 1859–1915 · Orgères-en-Beauce, France