The Sun is one of the steadiest of all stars. Even so, its energy output does vary — by less than a tenth of one percent. This variation takes place over an 11-year cycle that’s tied to changes in the Sun’s magnetic field. But cycles aren’t the same. The most intense yet seen peaked in the early 1960s. And right now, we’re in the middle of the weakest cycle in more than a century.

The Sun is a big, spinning ball. Because it’s made of gas, different layers and different latitudes rotate at different rates. That causes the Sun’s magnetic field to get twisted and tangled. Lines of magnetic force poke through the surface, creating dark sunspots, giant eruptions, and other activity. When the cycle peaks, the Sun can be covered with dozens of sunspots.

Astronomers have been tracking sunspots for more than 400 years. In the late 1600s and early 1700s, they saw almost no sunspots at all — a period known as the Maunder Minimum. It corresponded to a period of unusually cold weather in North America and Europe. Four of the five busiest cycles they’ve seen have happened in the last 75 years or so.

By looking at tree rings, and at ice cores from Antarctica and Greenland, scientists have estimated the intensity of solar cycles going back more than 11,000 years. And they’ve found that the peaks of the mid-20th century are the strongest in at least 8,000 years.

Over the last few cycles, though, that intensity has faded. More about that tomorrow.

Script by Damond Benningfield