Solar activity picked up in 2010 and especially 2011. Then the number of sunspots started dropping again. That was not necessarily surprising. In some previous cycles the Sun’s northern hemisphere became active first, and scientists expected a second peak in sunspots as the southern hemisphere entered its active period.

The southern hemisphere indeed began to perk up, but then leveled off and has remained that way for the past year, leading to more head-scratching. “In all honesty, it really feels like the Sun can’t make up its mind,” Dr. Biesecker said. “It’s just this flat mesa, and it’s not budging.”

If there is no second peak, and solar maximum actually occurred two years ago, then Solar Cycle 24 would be extremely odd — late to start and early to end. “What would surprise me is if it didn’t pick up over the next year,” Dr. Pesnell said.

How far back do have scientists have to look back to find a solar maximum quite as weak? As far back as Chicago Cubs fans do for a World Series championship.

Cycle 14 in the early 1900s was similarly quiet. (The Cubs won the 1908 World Series, about a year after the maximum of that solar cycle.) This time, solar scientists have Sun-watching satellites providing reams of data for them to analyze. “For the first time, we’ll be looking at a solar cycle that’s really different from the ones we’ve seen before,” Dr. Pesnell said.

Dr. Pesnell says it has already become apparent that the flow patterns within the Sun are more complicated than had been supposed.

Despite the minimal sunspots, the Sun is still going through the rest of its cycle as usual. Its magnetic field is on the cusp of flipping, as expected. At solar maximum, the magnetic fields at the poles essentially disappear for a brief time, and when they re-emerge, they are pointing in the opposite direction. If you had a compass on the Sun’s north pole and it were pointing north before solar maximum, it would be pointing south after solar maximum. (Actually, the compass would vaporize.) The north pole has already flipped; the south pole is behind but last month scientists at Stanford’s Wilcox Solar Observatory said they expected the transition to be complete soon.

“We do see indications that solar maximum is about now,” Dr. Pesnell said.