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They count days since January 1, 4713 BC, a pretty-much arbitrary date that the scholar Joseph Scaliger chose in 1583.

His books and essays frequently resurrect fascinating if half-forgotten polymaths, such as astrologer-mathematician Girolamo Cardano, Renaissance architect Leon Battista Alberti, philologist Joseph Scaliger and groundbreaking philosopher of history Giambattista Vico.

The Julian calendar, which Scaliger named after his father, counts the days since Day 0.

Scientists tracking supernovae, cometary orbits, and other celestial phenomena would still have to add or subtract a year manually when they crossed the a.d.-b.c. barrier if a sixteenth-century astronomer named Joseph Scaliger hadn’t got sick of the whole business and devised a calendar for astronomers that doesn’t skip a year.

Scaliger chose Day 0 as January 1, 4713, b.c.;