The sun is growing. And shrinking, and growing again. Every 11 years, the sun’s radius oscillates by up to two kilometres, shrinking when its magnetic activity is high and expanding again as the activity decreases.

We already know that the sun is not a static object. Its surface is regularly covered with darker areas known as sun spots and brighter areas known as flares. The first precise measurements of the sun’s size in the 18th and 19th centuries showed that it appeared to be larger when there were fewer of these …