Wednesday, June 20, 2012

The Longest Day of the Year

Long Days, Long Nights, Summer Days, Summer Nights

In honor of the soon-to-arrive summer solstice, my wife and I were at Gladstone's, the famous "EAT FISH" place between Malibu and Santa Monica, trying to decide how long the longest day really is, and, inversely, in winter, is the longest night the same duration as the longest day?

I said one thing, she said another.  As usual, she was right.

She also ordered a prettier cocktail than mine, as shown here in this gold-bordered Polaroid.

Why gold?  Just a whim.  This film is now manufactured in Holland by the "Impossible Project," and they release "editions" of film in colored formats.  I prefer black borders but this one happens to be gold.  Love the retro look of the colors, though, on the print itself.  Sorry about the seasick horizon line.  My fancy dancy Nikon has a built-in artificial horizon (like airplanes have) so you can level everything just-so on the tripod.  My Polaroid camera by contrast has a dim, off-axis viewfinder and a plastic lens.  It is what it is.

Anyway, the days are long now and so the question is, how long?

Here is what the good men and women of the US Naval Observatory have to say:

"The longest day is longer than the longest night, and the shortest day is longer than the shortest night, for the reason that sunrise occurs when the upper edge of the disk of the Sun appears on the horizon, and sunset is at the moment when the upper edge disappears below the horizon. These are the instants of first and last direct sunlight; but at these times the center of the Sun's disk is still 50 minutes of arc vertically below the horizon, because the semidiameter of the sun is 16 minutes of arc, and in addition the Sun is seen 34 minutes of arc above its actual geometric position on account of atmospheric refraction. Consequently, the length of every day exceeds the time that the center of the Sun is geometrically above the horizon by the intervals of time required for the Sun to move through these extra amounts of 50 minutes of arc in altitude at both rising and setting, or 100 minutes of arc altogether; this shortens the night by the same amount.

On any two dates when the center of the Sun is at equal distances north and south of the equator, as it is at the summer and winter solstices, the center is geometrically below the horizon on one date for the same length of time as it is above the horizon on the other date; but on both dates the upper edge of the disk is visible longer than the center is geometrically above the horizon by the time required for 100 minutes of arc of motion in altitude, and therefore the day that is the longer exceeds the night on the other date by twice this amount, i.e., by the time equivalent of 200 minutes of arc in altitude. This excess is greater the higher the latitude because the path of the Sun at rising and setting is at a smaller angle with the horizon, and more time is required in this slanting motion to cover a given vertical distance."

There you go: you read it here first.

Short version?  My wife was right, Malibu is a nice place to have lunch, and if you're driving (as I was), make sure you just have just one cocktail only.

The link to the rest of the "longest day" discussion is here, along with moonrise data, leap year tables, and the answer to the question, "when did asteroids become minor planets"?

If you have seen Men in Black III, you know that not leaving a tip can put the Earth at risk for being wiped out by a rogue comet, so at Gladstone's I made sure to do my part towards preserving the planet for another few days --- and to make sure my server went home that night with a living wage.

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