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Partial Solar Eclipse Afterthoughts

Today’s eclipse viewing went really well at McMaster University. The weather was perfect with clear skies and there was a very strong turnout by the general public. I would like to acknowledge the hard work of the Astronomy/Astrophysics students who organized this event and successfully held the floor down during this very busy viewing. There was one thing that stayed with me after chatting with people from the general public. Many were surprised (and some, disappointed) that there was no obvious change in the light level of their surroundings. After all, who wouldn’t want to experience a mock-up apocalyptic version of reality?

Reason: Among many, the key point is that the human eye has a logarithmic response to lighting conditions. What this simply means is that big changes in the light level have to occur for it to be obvious to a human being. The next question then is, what sort of “big” changes are we talking about?

It turns out that there needs to be a three-fold increase or decrease in the apparent magnitude1 of the light source for humans to detect that there is some obvious change in the light level of their surroundings. In the case of an eclipse, the source is the sun. When the sun is totally unobstructed, it has a base visual magnitude. With the use of some high school math2 – you know, use some trigonometric tricks, logarithm rules and massage some equations – it can be deduced with a bit of work that the apparent visual magnitude of the obstructed sun during a solar eclipse is a third of the base value of the unobstructed sun when 97.3% of its apparent area is blocked by the moon. This then explains why:

  1. In places where only ~ 70% of the sun was covered, only very subtle changes were apparent to observers
  2. If you observed a total eclipse, the most dramatic changes in light level occur within a few minutes of totality, when the sun is totally obstructed by the moon.

I hope you’ve found this description helpful if you were previously in the dark, or, in the case of those gathered today at McMaster, not. Yes, you will have to forgive my interesting humour when it makes its way to a post.

Bis zum nächste mal!

Dalini viewing the solar eclipse at McMaster University – August 21st, 2017.

Partial solar eclipse captured with solar shade during viewing at McMaster University – August 21st, 2017














1The apparent magnitude of a celestial object is a number that is a measure of its brightness as seen by an observer on Earth.

2If you are interested in reviewing the calculation involved in deducing this result, please refer to the attached pdf document: Brightness_Of_Solar_Eclipse

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