Still, something like the Dalton Minimum  two solar cycles in the early 1800s that peaked at about an average of 50 sunspots  lies in the realm of the possible, Dr. Hathaway said. (The minimums are named after scientists who helped identify them: Edward W. Maunder and John Dalton.)

With better telescopes on the ground and a fleet of Sun-watching spacecraft, solar scientists know a lot more about the Sun than ever before. But they do not understand everything. Solar dynamo models, which seek to capture the dynamics of the magnetic field, cannot yet explain many basic questions, not even why the solar cycles average 11 years in length.

Image ANOTHER VIEW These photographs show an ultraviolet view of the Sun on the same days: July 19, 2000, left, and March 18, 2009, right. Most solar physicists do not think anything odd is going on with the Sun. Credit... NASA

Predicting the solar cycle is, in many ways, much like predicting the stock market. A full understanding of the forces driving solar dynamics is far out of reach, so scientists look to key indicators that correlate with future events and create models based on those.

For example, in 2006, Dr. Hathaway looked at disturbances in the Earth's magnetic field that are caused by the Sun, and they were strong. During past cycles, strong disturbances at minimum indicated strong fields all over the Sun at maximum and a bounty of sunspots. Because the previous cycles had been shorter than average, Dr. Hathaway thought the next one would be shorter and thus solar minimum was imminent. He predicted the new solar cycle would be a ferocious one, consistent with a short cycle.

Instead, the new cycle did not arrive as quickly as Dr. Hathaway anticipated, and the disturbances weakened. His revised prediction is for a smaller-than-average maximum. Last November, it looked like the new cycle was finally getting started, with the new cycle sunspots in the middle latitudes outnumbering the old sunspots of the dying cycle that are closer to the equator.

After a minimum, solar activity usually takes off quickly, but instead the Sun returned to slumber. “There was a long lull of several months of virtually no activity, which had me worried,” Dr. Hathaway said.