Why counting sunspots by eye nowadays, in the space era?
Indeed, nowadays, we can observe the Sun in many different and sophisticated ways: CCD imaging, spectrocopy, space probes, radioastronomy. These techniques allow to probe our star in various quantitative ways. However, they were created only over the past few decades. Many modern instruments are also unique and have a limited lifetime (spacecrafts). Therefore, the associated data sets are limited to just one or a few solar activity cycles, i.e. a duration that is too short to study the secular variability of this cycle.
By contrast, telescopic visual observations exist since the epoch of Galileo, more than 400 years ago. Therefore, sunspot counts and the resulting sunspot number offer the only direct record of solar activity spanning tens of 11-year solar cycles. The sunspot number series thus remains the prime reference to connect our modern observations to the distant past. As in any experiment, in order to keep results from different epochs strictly comparable, the observing technique and processing methods should not be changed. This is why this essential long-term census of sunspots is still continued visually.
By their simplicity, visual observations also bring key advantages that remain valid today and probably for some time in the future:
- simple low-cost obsevations can always be done by many observers. This allows to obtain a robust time series, with very low chances of interruption due to e.g. the local political situation or the availability of funding.
- moreover, the multiplicity of stations allows extensive cross-validation and calibration between many simultaneous observations, thus preventing unrecognized biases that may affect isolated observatories, whatever their level of sophistication.
Thus, the sunspot number also brings the power of a collective endeavour !