Kepler’s Laws of Planetary Orbits

In the picture

1. 1The Sun: sits at one focus of the ellipse, off-centre
2. 2The elliptical orbit: the oval path every planet follows
3. 3Short, wide wedge near the Sun: a large stretch of orbit covered in a given time, so the planet moves fast
4. 4Long, thin wedge far from the Sun: the same area swept in the same time, but along a short stretch of orbit, so the planet moves slowly

Examples

• Earth is closest to the Sun in early January and moves slightly faster in its orbit then than in early July.
• Halley’s Comet races past the Sun in a few months but spends decades drifting slowly through the outer solar system.

Quiz: Kepler’s Laws of Planetary Orbits (Age 18)

1. 1

   Where does the Sun sit in a planet’s orbit?

   • At one focus of the ellipse, off-centre
   • On the orbit itself, at the point closest to the planet
   • At the exact centre of the ellipse
2. 2

   At which point in its orbit does a planet move fastest?

   • It moves at the same speed all the way round
   • When it is closest to the Sun, at perihelion
   • When it is farthest from the Sun, at aphelion
3. 3

   What does Kepler’s equal-areas rule state?

   • A planet travels equal distances along its orbit in equal times
   • Every planet’s orbit encloses the same total area
   • A line from the Sun to the planet sweeps out equal areas in equal times

The answers, each with its explanation, are in the app. This lesson has left the free week, so its quiz is part of the subscription.