Saturday, March 19, 2011

Solar Dates for 2011 (Kind Of)

I like to keep track of when the Equinoxes, Solstices, and "Seasonal Ides" (some folks call these the cross-quarter days) are. I went to the US Naval Astronomy site and got the Equinox and Solstice dates, then took the average between the them to compute the Ides. All times are in Universal Time (UT), which is the time at Greenwich.

Spring Equinox 3/20/2011 23:21:00
Spring Ides 5/6/2011 8:18:30
Summer Solstice 6/21/2011 17:16:00
Summer Ides 8/7/2011 13:10:30
Fall Equinox 9/23/2011 9:05:00

It turns out the Ides dates are not as exact as the Solstice and Equinox dates. I'm guessing that since the Earth moves in an ellipse, taking an average between two dates does not jive with the sun's apparent position in the sky.

Technically, the Ides should be when the meridian sun is at a height of 44.06 (Eugene's latitude) plus or minus 11°43' (half the distance in degrees between a solstice and an equinox). Going back to plugging in the date 5/6/2011 for Eugene, the computed table says that when the sun is near the azimuth of 180, it's altitude is ... 62.6 (higher than the mid-point, 55.-uh-75, by about seven degrees). Fiddling with dates, around April 20, 2011, the sun will pass through 55.6 at noon. (April 21, it overshoots slightly.)

Oh well; so much for using The Gregorian Calendar to calculate solar events.

On a completely different topic, I looked in the mirror this morning and there's no denying it: my current hair arrangement makes me look like the Thunderbird Mail icon. I think it's time for a trim.
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