Glossary / Space
Kepler's laws
Three laws of planetary motion: planets move on ellipses with the Sun at one focus, sweep faster when closer to it, and have orbital periods tied to orbit size.
From Tycho Brahe's precise observations of Mars, Johannes Kepler published the first two laws in Astronomia nova (1609)[1] and the third in Harmonices mundi (1619). The motion of the heavens became something that could be written as a formula.
In 1687 Newton derived all three from universal gravitation, explaining why the orbits are ellipses. Orbit calculations for satellites and spacecraft still build on this.
Related terms
- Hohmann transferMoving between two circular orbits by riding an ellipse tangent to both, with just two burns — a fuel-efficient way to transfer.
- Lagrange pointFive points where the gravity of two bodies, such as the Sun and Earth, balances the motion of orbiting around them, so a small object can hold its position.
- Gravity assistUsing a planet's gravity and orbital motion to change a spacecraft's speed or direction without fuel as it flies past.
On the timeline
Sources
- J. Kepler, “Astronomia nova,” Heidelberg (book) (1609).
- J. Kepler, “Harmonices mundi,” Linz (book) (1619).