Back at his flat, the historian of science sets up a modest telescope and, taking due care not to burn his eyes, points it directly at the sun. He spends 5 minutes or so counting, and uploads a number to a server in
Satellite engineers use this number, updated daily, to predict how the sun's future activity will affect their spacecraft. Climate scientists use it to pick out the sun's long-term effects on Earth's climate. Electricity companies use it to anticipate solar storms that could affect their grids. It is the international sunspot number: the world's oldest continuous data series, and one of its most important. "It is probably, apart from the Dow Jones index, the most used time series ever," says Leif Svalgaard, a solar physicist at
But there is a problem. There is not one sunspot number, but two.
In the past couple of decades, a rival series has revealed the existence of mysterious blemishes in the official sunspot record that cast doubt on its accuracy. That is embarrassing for the scientists involved and problematic for those who rely on the record's accuracy. What to do?
Sunspots are dark splotches that mark cooler patches on the solar surface. They correlate with areas of intense magnetic activity that are breeding grounds for violent outbursts of matter and radiation from our star. If it weren't for the protective hull of Earth's atmosphere and magnetic field, these solar flares and coronal mass ejections would rapidly fry life on our planet.
Observers peering through the first telescopes in the early 1600s were blissfully unaware of all this when they started to systematically record dark spots on the solar surface. Many at the time rejected the idea that God's celestial orb could be anything less than perfect, and assumed these blotches must be shadows of other bodies orbiting the sun. It was Galileo who championed the view that they were features on the sun itself.
In the 1840s, the Swiss astronomer
Counting sunspots is not as easy as it sounds. Sunspots tend to cluster together in groups, and individual spots within a group can be difficult to discern. To take account of these uncountable spots, Wolf came up with a formula to calculate the "relative sunspot number", which he defined as 10 times the number of sunspot groups, plus the number of clearly distinguishable individual spots. Since different observers with different telescopes tended to count slightly different numbers of sunspots, Wolf used overlapping periods to assign correction factors to the numbers from each new observer, and so ensure these numbers were consistent with his own.
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