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TIL of the Astronomical Unit, or AU, a unit of length equivalent to the distance between the Earth and the Sun, measuring 149,597,870.7 kilometres exactly. It's also a fundamental component in defining another unit of astronomical measurement, the parsec.

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Egx1X TIL "1" wasn't considered a number by Ancient Greeks (even Aristotle & Euclid). Numbers were multitudes of units (1 was the unit). "One is the seed of number but not number." This view didn't change til 1701 AD, when 1 was defined as a number in the first math dictionary: "Mathematicks Made Easie"
jJDa TIL that Octopus eyes do not have a blind spot and evolved separately from vertebrate eyes - an example of convergent evolution.
M7kd7 TIL that bystanders are less likely to intervene in emergency situations as the size of the group increases, and they feel less personal responsibility.
loYYa TIL of Terry Marsh, who legally changed his name to "None Of The Above X" and ran for parliament. He is also a professional chessboxer.
JYeJm TIL The American Smarties candy received their namesake “Smarties” because the creator wanted to encourage people to pursue an education.