See speck run

The question of whether the sunspot number two hundred years ago was accurately determined has been solved 150 years ago. Solar activity creates more ultraviolet light that in turn creates and maintains the Earth’s ionosphere during the daytime as an electrical conducting layer(s). During the day the ionosphere heats up and thermal winds blow this electrical conductor across the Earth magnetic field which causes an electrical current to be induced. An electrical current as its own magnetic field and as the Earth rotates under this current [which is fixed under the Sun] an observer on the surface will see a compass needle be deflected about a 1/5th of a degree to one side in the morning and then returning to its normal position when night falls. This diurnal variation was discovered by George Graham in London in 1722 and was studied by several workers in the 1740s, 1760s, and from 1780 on to the present day. Back then, 1/5th of a degree could be readily measured and we have thus reliable data about the deflection of the ‘magnetic needle’ since the middle of the 18th century. It turns out the amount of deflection is a sensitive measure of the Extreme UV flux from the Sun, we therefore know what the EUV flux has been the past ~250 years. Modern data shows that the sunspot number [until now] is a good proxy for the EUV flux, so if we know one, we know the other. We therefore know what the ‘real’ sunspot has been the past 250 years. By comparing the numbers reported by a given observer [over some time] to what we know they should have been we can determine the ‘correction’ factor [the ‘k’ in the equation SSN = k * (10*g+s)] for that observer. It therefore does not matter how good his telescope was or his eyesight or if he only counted big spots or included the specks, or whatever other quirks there might be. We can determine what his count would have been had he used modern instruments and techniques.
This much was already known to Rudolf Wolf in 1859, who used this technique in his inter-calibration of different observers to produce his famous composite Wolf Number. Later observers had forgotten or had come to distrust the method using the magnetic needle [basically the ‘fault’ of the famous geophysicist Sidney Chapman who pointed out that relation Wolf had found between the sunspot number and the diurnal variation of the Declination – the angle between true North and the direction in which the compass needle pointed varied from station to station and with time such as to render the method suspect]. As a result the sunspot number calibration since then has changed when the primary observer changed without anybody noticing.
With our modern understanding of how the ionosphere is created and maintained we now know that the important parameter is not the Declination as Wolf thought [and as Chapman derided], but the additional magnetic force in the East-West direction created by the stationary ionospheric currents overhead as the observer rotates with the Earth. There is a simple relation between the East-West force and the Declination [F = H cos D], so one can readily compute one from the other, which means that the geomagnetic method becomes correct and useful if one uses the East-West deflection instead.
The result of all this is that there is a reliable way of gauging solar activity [e.g. as measured by the EUV] going back 250 years. And that we can then determine the calibration of the sunspot number from that.
There is one fly in the ointment: the sunspot number may not be a good proxy for solar activity at all times. Livingston and Penn have found that in the last decade sunspots have become harder and harder to see [and in fact predict them to become invisible by 2015]. The magnetic field of the spots [albeit a bit weaker] will still be there, the solar cycle will still operate, cosmic rays will still be modulted, 10Be in ice cores will still vary, there will still be an interplanetary magnetic field, there will still be a solar wind, comet tails will still point away from the sun, there will still be aurorae, and magnetic storms, and flares, and CMEs, etc [all possibly slightly subdued]. This means that the sunspot number will no longer be a good measure of solar activity. Perjaps this happened during the Maunder Minimum, as well. At the ongoing Space Weather Workshop, Ken Tapping, who measures the 10.7 cm radio flux from the sun, that in the past was an accurate proxy for the sunspot number [actually the other way around], has just presented this paper:
Title: The Changing Relationship Between Sunspot Number and F10.7
Abstract: Sunspot Number and the 10.7cm solar radio flux are the most widely-used indices of solar activity. Despite their differing nature and origins at different places in the Sun, these two indices are highly-correlated to the point where one can be used as a proxy for the other. However, during Solar Activity Cycle 23 we started to see a small but definite change in this relationship….
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This may be an indication that L&P are correct [or at least some change is happening], but it is still to early to tell. The coming cycle will be most interesting if these things are happening.
It also means that prediction of the sunspot number becomes a bit fuzzy. What almost all methods predict is the magnetic field expressed by the sunspot number proxy. The implicit assumption is, of course, that that relation does not change with time. If it does, the predictions will be of the ‘effective’ sunspot number [i.e. the ‘old’ number that such and such magnetic field would give].
All of this is somewhat speculative, but makes the coming cycle extremely important for our understanding of the cycle.