Saturday, September 25, 2010

Happy Equilux

Yep. "Equilux". Even though it sounds like a brand of vacuum cleaner, or perhaps an advanced light bulb, it isn't.

An equilux is the day on which there's exactly (or almost exactly) 12 hours between sunrise and sunset. Equilux's occur once in the Spring (a few days before the Vernal Equinox), and once in the Fall (a few days after the Autumnal Equinox).

The exact dates of the equilux vary by latitude; for Hood River, the autumnal equilux is today. There are a number of online sunrise/sunset calculators. Probably the most official one is at the US Navy site, but the one here is quicker.

Up until recently (yesterday), I thought that the "12 hour equal day/night" thing happened exactly on the Spring and Fall equinoxes. After all, equinox means "equal night". But apparently, that's a blatant lie passed down through the ages by gullible people (me). In conclusion, all is not as it seems, buyer beware, caveat emptor, and so forth.


Furthermore, while the equilux is an actual date, an equinox isn't. It's a moment in time. Specifically,
the moment at which the center of the Sun’s disk crosses the celestial equator, which is the projection of the Earth’s equator out into space;
all explained in more detail here.

Meanwhile, speaking of projections out into space...



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