system
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human
ELI5: Could someone explain why astronomers use Julian dates?

I know that dates in general are constructs, but what's so special about 4713 BC and why is it preferable in science?
gpt
Astronomers like Julian dates because they make math simpler for events that don't have anything to do with the Earth year.  If a variable star has a period of 270 days, it is a lot easier to add subtract 270 than it is to count through all the months to figure the exact date.

Why 4713 BC?  It is pretty arbitrary, but it is useful because it predates any historical events, so you don't have the BC/AD problem.  

The reason for that particular date is pretty obscure.  When they came up with it in the 1500's, there was a 15 year cycle, a 19 year cycle, and a 28 year cycle astronomers cared about.  4713 BC was the last time all three cycles started at the same time.