Why Is the Night Sky Dark?

Olbers’ paradox

An interactive lesson with 6 steps, about 5 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• In an infinite, eternal, static universe, the night sky would be blazing bright.
• Each shell of stars contributes equal light: dimming (1/r²) and numbers (r²) cancel.
• A finite age and cosmic expansion keep the night sky dark.

Steps

1. A forest of stars
2. Shells of stars
3. Lines of sight
4. Is expansion enough?
5. A poet’s answer
6. How far to a star?