Leap day: fixing the faults in our stars




Orion, the annual visitor.
Mouser, CC BY-SA

Orion, the annual visitor. Mouser, CC BY-SA

I imagine myself late one night, eight months from now, remembering the overfull recycling bin, at midnight on trash day. As I try to quietly dump wine bottles into the yellow-topped container, there striding over the eastern skyline is Orion. Back again is my ancient friend, telling me that winter is near, and that I have ridden this miraculous rock almost another full lap around my home star. Rigel shimmers its blue-white light, the twinkle in the eye (the knee, actually) of a companion who has visited me, annually, every place on Earth I have lived since childhood. Even to the Southern Hemisphere, the steady Orion came for a summer visit – cartwheeling upside down, feet over hands.

It is from these celestial cycles that our concepts of time originate, and, ultimately, from which we gain the leap day.

The sidereal year is the length of time it takes for the Earth to return to the same place with respect to the “fix’d” and “constant” stars, so that Orion appears exactly in the same place in the sky, at exactly midnight, 365.2563 days later. Stellar friends like that don’t stand you up; they keep their appointments to seven-digit precision (and more).



Right over the equator: A diagram showing the sun’s position relative to the Earth at the vernal equinox.
Tfr000, CC BY-SA

Right over the equator: A diagram showing the sun’s position relative to the Earth at the vernal equinox. Tfr000, CC BY-SA

Our Western calendar is tied to the tropical year – the time between successive vernal equinoxes. At that moment, the sun’s position in the sky is exactly where the ecliptic (the plane of the solar system and the path that the planets —> Read More

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