Why we have leap days | Bad Astronomy | Discover Magazine: "We have two basic units of time: the day and the year. Of all the everyday measurements we use, these are the only two based on concrete physical events: the time it takes for the Earth to spin once on its axis, and the time it takes to go around the Sun. Every other unit of time we use (second, hour, week, month) is rather arbitrary. They’re convenient, but not based on independent, non-arbitrary events."
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