It’s a familiar theme in adventure movies: a shaft of sunlight streams through a hole in the cave’s ceiling, illuminating a clue to the treasure. Such scenes are inspired by theories that some ancient monuments- -Stonehenge, for example--were built to direct a beam of sunlight at a special nook on the day of the solstice.
The June solstice, almost upon us, marks the start of summer and the longest day of the year. Peer at the year’s highest sun and you’ll see it rise and set as far to the north as possible. A north window never gets sun? Watch it stream in during the day’s first and final hours. And while you’re at it, why not set up your own solstice marker? No reason the ancients should have had all the fun. Just notice where the extreme edge of sunlight strikes the room at sunrise or sunset. That exact spot won’t get direct light again until you’re a year older. Aging hippies might hang a small prism to let a rainbow of colors announce the solstice. Gadget lovers might install a solar cell to power a chime or a bell. A photoelectric cell might be used to turn on a stereo preset with the Beatles’ Here Comes the Sun.
Astronomers can now predict solstices to one-second precision. This one will occur at precisely ten seconds after 3:14 in the morning on June 21, Greenwich mean time, or at 8:14:10 P.M. on the twentieth in California--around sunset. At that moment, the South Pole stands shrouded in its deepest darkness (yes, researchers really have erected a pole there; the marker has to be repositioned every year because the antarctic ice sheet moves), while the North Pole basks in the year’s loftiest sun. If you were there you’d witness the sun’s still anemic height as equal to our planet’s tilt: 23.5 degrees. You’d also probably marvel at its perfectly sideways motion around the whole sky, a sun displaying not the slightest interest in rising or setting.
Quietly, and with little hoopla, the solstice has over the past few years started occurring while the sun is in the constellation Taurus (meaning that if we could see the stars during the day, Taurus would be directly behind the sun). Thanks to the leisurely 25,800-year wobble, or precession, of Earth’s axis of rotation, for the next three millennia the solstitial sun will remain firmly corralled within the stars of the bull, instead of in Gemini, where it celebrated the past 2,000 solstices. Taurus is such a large constellation that the solstice will spend a thousand more years there than it did in Gemini. Astrologers, however, still regard the solstice as taking place in Cancer, as it did before it got to Gemini. Such failure to keep track of precession explains why the place that sees the solstitial sun shining straight overhead is called the Tropic of Cancer. By rights, the signposts down there should now officially read Tropic of Taurus.
Since everyone in the contiguous United States lives north of the Tropic of Cancer, no mainland American city ever gets to experience an overhead sun. But in some southern states during the solstice it’s just a near miss. You can quickly tell by how many degrees the sun misses the zenith in your location by subtracting 23.5 from your latitude. Key West, for example, at latitude 24.5, sees the midday sun just one negligible degree from straight up.
Yes, a lot happens at the solstice, even if we no longer perform human sacrifices to appease angry gods. Unless, of course, you hook up that photocell so that when the in-laws walk in . . .