The Summer Solstice arrives Friday, June 20, and it's a funny mixture of natural events and government rulings. At a minute to 8 p.m. our time, the sun hovers straight up, as seen from the empty ocean north of Hawaii: its brief annual visit to the legendary Tropic of Cancer.

It's widely misunderstood where in the heavens the sun sits at the time of the Solstice. Traditionally it was the zodiacal sign of Cancer; hence "Tropic of Cancer" as the earthly place where the solstitial sun stands overhead. But despite the lingering name, the solstitial sun drifted out of Cancer 2,000 years ago. In 1990 that highest-of-the-year position crossed over yet again - this time from Gemini to Taurus, where it'll remain for the next 800 generations of skywatchers.

The exact solstitial point is still very near the Taurus/Gemini boundary; so close, in fact, that the sun exits the Bull constellation just six hours later. But don't put too much stock in any constellation boundary. Just as the ancient Greeks sliced the heavens into a dozen imaginary "signs" with identical 30-degree swaths of the sky, regardless of whether they matched the star patterns they were supposed to represent, modern astronomers have been nearly as whimsical. Today's official constellation boundaries were internationally agreed upon only in 1929, so that the precise place where one constellation ends and the next begins is pretty arbitrary.

So what about Friday's Solstice is of Nature, and what is created by us humans? What's real is that the day is longest, night is shortest, sun is highest, sunset occurs at the year's rightmost point on the horizon and twilight lasts longest: a situation that essentially doesn't budge all weekend.

What's unreal or arbitrary is that it's the moment when summer begins - because we could have decreed the "start of summer" during the year's statistically hottest week a month from now, or at the date of the earliest sunrise, which was last weekend. Or we could have started summer at the year's exact midpoint, which is noon on July 2. Obviously we've already had very hot weather, so temperature is no guide; back in 1996 the only above-90 readings around here happened in May, and then it never reached 90 again even once during the entire official summer.

Here's what's wacky about this Solstice: It's the earliest summer in 116 years. The Solstice hasn't happened on the 20th, Greenwich Time, since 1892. It's a direct consequence of the once-each-400-years Leap Year omission in the year 2000 - part of the grand Gregorian Calendar scheme. Solstices will continue to slip earlier throughout this 21st century at four-year intervals, only to be "set right" again by the resumption of century Leap Years in 2100.

But that's more information than nearly everyone wants. The real bottom line is: Days are longest and summer is finally here. And in case you wondered, the latest sunset - the brightest evening - is next weekend.