The divergent numbers before 1885 were trickier. In compiling the earlier parts of their data series, Hoyt and Schatten had faced the same problem as Wolf: how do you compare sunspot counts from different observers, with different eyesight, different telescopes, and even perhaps different opinions on what constitutes a group of sunspots?
They had handled this in much the same way that Wolf did: by stringing together a daisy chain of overlapping observers, correcting the numbers up or down so that they produced the same average number of sunspots in the time periods when two observers overlapped. The problem with this approach is that if any number is way off-beam errors propagate through the series and accumulate. "It's like the children's game Chinese whispers," says Svalgaard.
Two such errors in particular came to light. The first was in sunspot records kept by the
The second error was in the final multiplier Hoyt and Schatten used to ensure their average sunspot count matched Wolf's. Soon after starting the sunspot series, Wolf became chairman of the Swiss geodetic survey and then director of its weather service. From the 1860s until his death in 1893 he was almost constantly travelling, and continued with his observations not with his large telescope in
The upshot is that Hoyt and Schatten's sunspot numbers before 1885 will be revised upwards, to bring them into line with the Wolf number. A meeting is planned for May next year in Locarno to work out precisely how big that correction should be.
Schatten is happy with what has emerged, despite the fact it has highlighted flaws in his work. "Of course, one doesn't like it when the work one does is not perfect," he says, but he thinks the outcome is the right one. A more accurate time series will allow for a better understanding of past solar cycles. "And the past is the key to the present, and the future," says Svalgaard.
Clearing up its blemishes could give the sunspot record new life, says
This problem becomes especially acute when it comes to how the sun's activity affects Earth's climate. This becomes more uncertain over longer time periods, says
With the wrinkles in the earlier data ironed out, we can have a little more confidence in the world's oldest data series. And while plans are afoot to automate the collection of the international sunspot number, for all their imperfections humans remain the most reliable observers. For now, Holmberg will continue delivering his numbers as he always has – methodically, consistently, without fail. As a scientist and a historian, he's proud his hobby can continue to be put to scientific use. "It gives me satisfaction that my data become part of something bigger." n
Eye of the beholder: sunspot counting is open to interpretation
Sunspots have long captivated popular imagination
"In the mid 1940s, the director of the
A spot of bother: have we been getting solar activity wrong?
For four centuries sunspot numbers have been telling us what the sun is up to. But parts of the historical record seem confused
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