Thursday, June 21, 2007

Solstice Morning

Today is the summer solstice -- the longest day of the year. I watched for the first moment that the sun peeked over our local hills (at 6:05 AM), and measured the azimuth as 36° from due north (I used a Brunton surveyor's compass, the "Classic" model, to do this). If the hills weren't there, sunrise would have been slightly earlier and the compass reading would have been a slightly smaller azimuth.

Tonight I will measure the azimuth at sunset as well. The hills are slightly lower there, so I'm expecting the azimuth to be something like 325° instead of the exact mirror of 36° (324°). But little errors like that aside, basically the sun will be traversing 360° - (2 x 35°) = 290° today -- the most it ever does at our latitude (33°). At noon today, the sun will be at the highest elevation ever seen at our latitude: 23° + (90° - 33°) = 80°.

Update and bump:

At 1:00 PM (instead of noon, because of daylight saving time), I measured the elevation of the sun, using the clinometer built into the Brunton surveyor's compass. I took 5 readings and averaged them. The result: 10.4° -- as predicted, within the limits of my instrument's accuracy.

Update and bump, II:

Just before sunset (at 7:28 PM) I measured the azimuth of where the sun will set. In doing so, I realized that I had made dumb error this morning when measuring the azimuth of the sunrise: I set the 14° deviation for magnetic north into the compass backwards, introducing a 28° error. The azimuth this morning should have been 64°, not 36°. The azimuth of the sunset should be 360° - 64° = 296°. That agrees very well with the 295° I just measured (with the magnetic north deviation set correctly!). This morning I computed the sun's azimuthal transit incorrectly as well, it should have been 360° - (2 x 64°) = 232° of azimuthal transit.

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