Tap the game to open it fullscreen. On a computer, Esc exits; on a phone, use the ✕ button.

How to play

Drag your finger (or use the arrow keys) to move the marker along the horizon. In the crystal, two lights change brightness as the marker gets nearer to or farther from the sun. When both shine equally, you are close. Tap Mark. There are five rounds, and the clouds get more restless with each one. Get four right to win.

The story behind it

In the Rauðúlfs þáttr, King Olaf uses a sunstone to see where the sun is on a day of snow and clouds. Modern experiments show a crystal could work this way, but nobody knows whether the Vikings used one. The game is a simplification: the brightness of the two lights and the marker are made up. Read the story in The sunstone: the crystal that may have helped the Vikings find the sun in a cloudy sky.