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This question on Physics SE about apparent brightness during a solar eclipse has a log-scale graph of light intensity versus time. Parts of it are the shape I'd expect: a nearly-smooth curve going from "extremely bright" to "very dim". However, there's a great deal of apparent noise in the graph, with jumps of nearly a full division in the graph (corresponding to about a two-fold or eight-fold change in brightness, depending on if it's a natural or base-10 log). What causes these jumps?

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  • $\begingroup$ Could the diamond ring effect (if I'm using that term right) have such a great effect on the brightness measured? $\endgroup$
    – user10106
    Commented May 15, 2018 at 11:02
  • $\begingroup$ @Kozaky, the "diamond ring" effect is (mostly) a special case of a lens flare, would occur around the points marked "second contact" and "third contact", and would show up as a single jump in brightness. $\endgroup$
    – Mark
    Commented May 15, 2018 at 23:38

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Eric Pauer, who produced the chart, has an interesting web site that covers this eclipse. It is evident that the weather was not great at the time of the eclipse, with intermittent cloud, and it will have been passing clouds that caused the variations in intensity.

You can see the evidence from this page http://www.pauerhome.com/eclipse/eclipse99/1contact/1contact.htm

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  • $\begingroup$ Indeed, clouds can have a big impact on ambiant lighting, a clear sky with direct sunlight generates around 3,000 to 9,000 lumens per sq. ft., but an overcast sky generates only 100, and scattered clouds and haze will give a reading between a few hundred to a few thousand lum/sq.ft., so it can easily reduce lighting by a factor 5-10. Interestingly, the general brightness of an overcast sky is about the same as a 98% eclipse with clear skies! $\endgroup$
    – FSimardGIS
    Commented May 16, 2018 at 23:44

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