Explainer: All about orbits
Explainer: All About Orbits
An orbit is a route that one space object repeatedly takes around another.
Comets — such as Neowise C/2020 F3, seen here — travel around the sun in very elliptical orbits.
ANTON PETRUS/MOMENT/GETTY IMAGES PLUS
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Even in ancient times, stargazers knew that planets differed from stars. While stars always appeared in the same general place in the night sky, planets shifted their positions from night to night. They appeared to move across the backdrop of stars. Sometimes, planets even appeared to move backward. (This behavior is known as retrograde motion.) Such strange movements across the sky were hard to explain.
Then, in the 1600s, Johannes Kepler identified mathematical patterns in the planets’ movements. Astronomers before him had known that the planets orbited, or moved around the sun. But Kepler was the first to describe those orbits — correctly — with math. As if putting together a jigsaw puzzle, Kepler saw how the pieces of data fit together. He summed up the math of orbital motion with three laws:
- The path a planet takes around the sun is an ellipse, not a circle. An ellipse is an oval shape. This means that sometimes a planet is closer to the sun than at other times.
- A planet’s speed changes as it moves along this path. The planet speeds up when passing closest to the sun and slows as it gets farther away from the sun.
- Each planet orbits the sun at a different speed. The more distant ones move more slowly than those closer to the star.
Orbits, orbits everywhere. This image shows the orbits of 2,200 potentially hazardous asteroids orbiting the sun. The orbit of the binary asteroid Didymos is shown by a thin white oval, and Earth’s orbit is the thick white path. The orbits of Mercury, Venus, and Mars are labeled as well.CENTER FOR NEAR-EARTH OBJECT STUDIES, NASA/JPL-CALTECH
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