Everything in the solar system moves: planets orbit the Sun, moons orbit planets, and every body spins on its axis. Understanding these motions was one of the first great successes of science, and it is still an active field.
The laws of motion
- Kepler (early 1600s): planets move on ellipses with the Sun at one focus, sweep equal areas in equal times, and their orbital periods are linked to their distances.
- Newton (1687): universal gravitation explains Kepler’s laws and lets astronomers calculate orbits precisely.
- Einstein (1915): general relativity explained the small anomaly in Mercury’s orbit that Newton’s theory could not.
Precise orbit calculations led to the discovery of Neptune in 1846, predicted from irregularities in the orbit of Uranus. See the history of astronomy.
Measuring how planets rotate
- Tracking surface features: clouds on Jupiter, markings on Mars.
- Radar: bouncing radio waves off Venus and Mercury, whose surfaces are hidden or hard to see. In 1965, radar showed that Mercury rotates three times for every two orbits.
- Doppler shifts: one edge of a spinning planet moves toward us and the other away, shifting the wavelength of reflected light or radar.
- Magnetic fields and radio emissions: for gas giants with no solid surface, the rotation of their magnetic field is a clue, see planetary magnetic fields.
- Ring seismology: Saturn’s day was pinned down in 2019, at about 10 hours 33 minutes, by studying waves in its rings caused by vibrations inside the planet.
Odd rotations
- Venus spins backward and extremely slowly: a day on Venus (243 Earth days) is longer than its year (225 days).
- Uranus is tilted about 98 degrees, spinning on its side, probably after a giant impact.
- The Moon and many other moons are tidally locked, always showing the same face, see moon phases.
Resonances and long-term changes
Bodies tug on each other. Jupiter’s moons Io, Europa and Ganymede are locked in a 1:2:4 orbital resonance that heats Io’s volcanoes. Jupiter’s gravity shapes gaps in the asteroid belt. Over millions of years, small effects add up: Earth’s tilt and orbit vary slightly, which influences ice ages.
Modern tools
Spacecraft tracking, laser ranging to reflectors left on the Moon by Apollo astronauts, and powerful computer simulations allow orbits to be predicted with great precision. Such calculations are what let space agencies send probes to precise targets and track near-Earth asteroids, see comets and asteroids.
