The sun’s solar cycles were once around three years shorter than they are today, a new analysis of centuries-old Korean chronicles reveals. This previously unknown anomaly occurred during a mysterious solar epoch known as the “Maunder Minimum,” more than 300 years ago.

The sun is constantly in a state of flux. Our home star cycles through periods of increased activity, known as solar maximum, when solar storms become more frequent and powerful, as well as spells of reduced activity, known as solar minimum, when solar storms almost completely disappear. It currently takes about 11 years for the sun to complete a solar cycle, from minimum to maximum and back again. Scientists can track the sun’s progress through a solar cycle by counting the number of sunspots on the star’s surface, which appear more frequently in the lead-up to and during solar maximum.

But just as the sun fluctuates within individual cycles, historical sunspot records show that over longer periods, spanning decades or centuries, the overall output of solar cycles can also rise and fall. The Maunder Minimum, sometimes referred to as the Grand Solar Minimum, was a period of greatly reduced solar activity between 1645 and 1715 when sunspots “effectively disappeared,” Scott McIntosh, a solar physicist at the National Center for Atmospheric Research in Colorado who was not involved in the recent research, told Live Science in an email.

During this time, the sun’s output was so low that average global temperatures also dropped, in what scientists have dubbed a “mini ice age,” according to NASA — though it was also likely linked to high levels of volcanic eruptions at the time.

Sunspot records paint a general picture of the Maunder Minimum, which is named after the English astronomer Edward Walter Maunder. But there is still much about the period that scientists don’t know. In the new study, published in the journal AGU Advances, researchers analyzed historic auroral records from Korea and found that solar cycles during the Maunder Minimum were only eight years long on average — three years shorter than modern cycles.