December 22: the light of a solar eclipse

Annular solar eclipse (Matsudo, Chiba, Japan), by t-mizo. Image source: Flickr (Creative Commons BY 2.0)

On this day in 1870, Pierre Jules César Janssentook a flight in a hot air balloon to study a solar eclipse. To give some background on Janssen, alongside J. Norman Lockyer, albeit separately, he discovered the element helium when observing an eclipse on 18 August 1868, in Guntur, India. I was…

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