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Julian Day Clock. 2013.


My dad turned eighty recently, and for his birthday I made him a clock that tells the time in Julian days. He’s an astronomer, and astronomers use Julian days to mark astronomical events. The standard clocks we all use repeat themselves—4pm tomorrow is much like 4pm today—but for astronomers, time is always pressing onwards, and the scale is so vast, that they need a system that is as unambiguous as possible.

Julian days were first proposed in 1583 by French scholar Joseph Scaliger. On January 1, 4,713 BC, several multiyear calendar cycles—the Indiction, Metonic, and Solar—all aligned. This was Julian day 1. Today, as I'm writing this, it’s Julian day 2,456,370. There are no time zones with Julian days; each day starts and ends at noon Greenwich Mean Time.

This Julian clock calculates the time using GPS satellites, so it never needs to be set, and it has no controls or buttons. It’s made from 1/4" thick acrylic, with 3D-printed scaffolding on the interior to keep all the components in place.

I’ve made the code, as well as the design documents, available at github.