Orbital motion

How does an orbit work?

At every instant a planet has a velocity — where it is already going — and gravity adds a small pull straight toward the Sun. Bend a path by the same small amount at every step and it closes. That closed path is the orbit.

Two arrows, one rule: velocity tangent to the path, gravity toward the Sun. On a circle the bend exactly matches the curve.

One speed at each distance

At any distance from the Sun there is one speed at which the bending exactly matches a circle: the square root of the pull times the distance (Kepler and Newton, the same fact). Too slow, and the path dips closer. Too fast, and it swings wide. Either way it traces an ellipse and comes back.

Get more on this

Change one part, change the shape

Slow a planet at one point and it does not spiral in. It drops onto a longer ellipse that returns to the same point. To drop something into the Sun from Earth’s distance you would have to strip away almost all of its 29.8 km/s. Nature almost never does that.

See it live

Slow a planet and watch the ellipse

On the hub: Orbital velocity simulator · Big questions · Solar system simulator

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