A Planet Dims Its Star

In the picture

1. 1The planet in front of the star: a dark silhouette blocking a small patch of light
2. 2The flat line before the transit: the star’s full, steady brightness
3. 3The falling slope: the planet sliding onto the edge of the star
4. 4The flat-bottomed dip: the planet fully in front, and its depth gives the planet’s size

Examples

• In 2012, Venus crossed the face of the Sun and could be seen through special filters as a small black dot.
• A fly walking across a lampshade barely changes how bright the room is, but a sensitive light meter would notice.

Quiz: A Planet Dims Its Star (Age 16)

1. 1

   Why does a star look slightly dimmer for a few hours when a planet transits it?

   • The star briefly cools down and gives off less light
   • The planet’s gravity bends some of the starlight away from us
   • The planet passes in front of the star and blocks a tiny fraction of its light
2. 2

   Two planets transit identical stars, and planet A causes a deeper dip in brightness than planet B. What does this tell you?

   • Planet A is bigger than planet B
   • Planet A is closer to its star than planet B
   • Planet A moves more slowly than planet B
3. 3

   Seen from far away, roughly how much would Jupiter and Earth each dim the Sun during a transit?

   • Jupiter by about 1%, Earth by only about 0.01%
   • Both by about the same amount, roughly 1%
   • Jupiter by about 10%, Earth by about 1%

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.