Fri, 20 Jun 2003

Summer Solstice

There was some discussion at work today, on a break, about the Summer Solstice. Tomorrow is the longest day of the year in our neck of the woods.

I thought I had a reasonably good understanding of the earth’s tilted axis and it’s effects on the Sun’s path through the sky from my observation point here at 47.63 degrees north latitude. But someone asked, “Is it the longest day of the year everywhere in the Northern Hemisphere, or just everywhere north of the Tropic of Cancer? And, if it’s the longest day of the year at some dividing line, how can it be the shortest day of the year just on the other side of that line?”

It wasn’t until I got home and found a few quiet minutes to close my eyes and ponder the question that it all made sense.

Yes. Tomorrow is the longest day of the year everywhere in the Northern Hemisphere, and conversely, the shortest day of the year everywhere in the Southern Hemisphere. And you can stand on the dividing line between the two.

Right on the equator, every day, year-round, is the same length. Twelve hours of light and twelve hours of darkness. A few feet north, the days get just a little (as in infinitesimal fractions of a second) longer as the North Pole tilts toward the Sun, peaking at the Summer Solstice. The farther from the equator you get, the more pronounced the effect, until, at the North Pole, the Sun does not rise, or set. It circles sky, the 23 degrees above the horizon.

At the equator, although the Sun makes a northward arc as it travels across the sky, it rises due east and sets due west. At our latitude, this time of year, the Sun rises is the northeast and sets in the northwest, arcing southward as it travels across the sky. It’s the extra time it spends morning and evening, north of us, that gives us the extra hours of daylight.

Now that I’ve resolved my own confusion over the matter, I should be able to get a few hours sleep before the long day begins …unless I start worrying about when North, South, East, and West should be capitalized and when they shouldn’t.

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