I've just recorded the luminosity during the sun eclipse, here is the resulting curve :

Only green curve is relevant, blue curve should be ignored (it's actually the temperature).

Luminosity graph

The sky was uniformly cloudy, so even if I couldn't see the eclipse directly, I am pretty sure the luminosity was well diffused, so it's unlikely a specific cloud disturbed the sensor.

Thus I was wondering, why were there these little bumps on each side of the curve ? Thanks.

  • 1
    $\begingroup$ How do you know the sky was uniformly cloudy? $\endgroup$
    – ProfRob
    Mar 20, 2015 at 17:49
  • $\begingroup$ I am not sure, but still, the curve seemed pretty regular before and after the eclipse. Actually I was thinking this could be a consequence of refraction of some kind, I'd love to read someone about this guess. $\endgroup$
    – Harijoe
    Mar 20, 2015 at 19:40

1 Answer 1


It is not possible to precisely answer this interesting old question, since no details are given about the apparatus and method used to measure the luminosity, but it is most likely that the little bumps are simply noise that is symmetric by coincidence.

To evaluate the chances of this happening we would at least need data from before and after the eclipse. That would give a sense of the magnitude and frequency of any variations in the cloud clover and any other luminosity noise (e.g. people walking by and casting shadows on the apparatus). In general one should always be careful not to present too small a region of any data set, since cutting out the wings of the curve to focus on the “interesting” features throws away background information that helps us understand those features. It is also important to clearly define the scales on the axes, e.g. is the vertical scale linear or logarithmic? Is the bottom horizontal line zero luminosity?

If we assume that this data was simply taken with a photovoltaic cell, then we would expect to see something similar to measurements by the UK Rosliston Astronomy Group of the same eclipse. Their data have fluctuations that appear to be similar in size to the two little bumps in the figure here, they just aren’t symmetric around the central dip, and are likely consistent with cloud "noise". Looking at other data from Sussex and Warsaw also helps give a feeling for what would be expected, and there is no sign of symmetric bumps on the sides of the main dip.

Another possible set of data to compare with are the many videos and time-lapse photos of this eclipse available online. Analysing film is tricky since modern cameras often auto-compensate for changes in light levels, but if nothing else the videos emphasize that the viewing conditions were not great over much of Europe that day.

Over-interpretation of noise is something that has tripped up a great many scientists. Confirmation from independent measurements is essential to be sure one is dealing with a real phenomenon.


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