How planets orbit the Sun

In the picture

1. 1The Sun: sits at one focus of the ellipse, not at its centre
2. 2The orbit: an ellipse, a slightly stretched circle
3. 3Short, wide wedge near the Sun: the planet covers a long arc in a given time, so it moves fast
4. 4Long, thin wedge far from the Sun: same area in the same time, short arc, so the planet moves slowly

Examples

• Earth is closest to the Sun in early January and moves about 3% faster in its orbit then than in early July.
• A comet such as Halley’s crawls through the outer Solar System for decades, then sweeps quickly around the Sun in a few months.

Quiz: How planets orbit the Sun (Age 17)

1. 1

   Where is the Sun located relative to a planet’s elliptical orbit?

   • At one focus of the ellipse
   • At the exact centre of the ellipse
   • On the ellipse itself, like the planet
2. 2

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

   • At aphelion, when it is farthest from the Sun
   • At perihelion, when it is closest to the Sun
   • At the same speed everywhere along its orbit
3. 3

   In equal amounts of time, what stays the same for the line joining the Sun to a planet?

   • The length of the line itself
   • The area it sweeps out
   • The length of arc the planet covers

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.