The armillary sphere dates back to ancient times when astronomers created their own vision of the earth and the heavens. The idea still works today and our own design creates a sundial with a special feature of two balls that mark the passage of the seasons by their shadows. One of the balls reads summer solstice. This is the high point of the year when the sun is highest in the sky.
The balls are carefully positioned at the meridian point on two tropical rings, the upper one on the Tropic of Cancer and the lower one on the Tropic of Capricorn. When the sun is at noon, the highest point of the day, the ball on the Tropic of Cancer casts its shadow right across the sundial to the opposite tropical ring, exactly at summer solstice. This year the actual time of the solstice is 9.13am GMT (10.13am BST) on 21st June 2022. You will find a full explanation of summer solstice at the Royal Museums Greenwich.