Today, 2:06 p.m. EDT on June 21st, is the summer solstice for the Northern Hemisphere. That means that this is the very worst time of year for astronomy, with just four hours of full darkness at my latitude. Things will be getting better for the next six months; it's all downhill from here.

The Skygazer's Almanac plots celestial events on an hourglass shape that shows how long the sky is dark at any time of year.

Tony Flanders

This is also called Midsummer Day, an odd name considering that it marks the beginning, not the middle, of summer. That reflects the way that seasons in the temperate zones are perpetually off balance. We're now getting more heat from the Sun than at any other time of year, but the ocean just a few miles from me is still too cold for all but the hardiest people to swim in. And the cold ocean keeps the air cool as well. By late August, when the ocean is warmest, solar radiation will be declining rapidly, heralding the onset of autumn. So as far as the Sun's heat is concerned, this is indeed the middle of summer. But in terms of air temperature, it's just the beginning.

But why is this called the solstice? It's from the Latin word solstitium, meaning literally the Sun's standstill. It's quite an old word, but it's a surprisingly modern concept. First, it requires careful observation to note that the midday Sun climbs ever higher throughout winter and spring and then moves back down during summer and fall. The ancient Greek word for the moment when the Sun's upward motion changes to downward is trope, meaning "turning" — the root of the English word "tropic."

But calling the year's longest day a standstill requires a finer level of sophistication. The Sun doesn't just reaches its endpoint, turn around, and dash back at full speed, like a ball bouncing off a wall. Instead, it slows down as it approaches Midsummer Day, then pauses, then slowly gathers steam as it starts back down. A mathematician would say that the first derivative is continuous; the rate of change changes smoothly.

So, for the last few weeks, the nights have been getting shorter, but the change from one night to the next has become ever less perceptible. And now the world takes a deep breath while the motion of the seasons comes to a momentary halt. This is the Midsummer's Night when Shakespeare's play takes place, when the barriers between nature and magic, between reality and fantasy, disappear. Tomorrow, on the second-longest day of the year, we awake again to everyday, changing reality.


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Tony Flanders

July 26, 2007 at 11:50 am

just wonderful! Enlighting, educating, everything...

Is there any relation between the "solsticio" and the story in the bible?

- Adolfo Rufatt


Tony answers: No doubt the idea of the solstice was well-established when the Book of Joshua was written down, and it's conceivable that the word or the idea suggested this story. But the event described in the Bible is clearly something quite different. The actual solstice happens when the Sun's north/south motion on the celestial sphere stops. In the Biblical account, the enitre celestial sphere stops rotating -- including the Sun, the Moon, and presumably also the stars.

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Tony Flanders

July 26, 2007 at 11:51 am

Tony Flanders,

It would indeed have required extraordinarily sophisticated measurement for the ancients to have observed how the rate at which the sun's noon declination increased, decreased as such declination approached its maximum. But that isn't what the ancients were observing. They did not have sextants with which to measure such a thing. The "standstill" they observed was in the movement of the point at which the sun rose along the northeastern horizon. This could be easily marked with stakes, and the observer will easily note how the daily movement of the rising point decreases as we near the solstice. In fact, the motion of the rising point is virtually undetectable for several days before and after the solstice. It is in this sense that the sun "stands still." Once the solstice point was determined, the alignment could be incorporated into a structure such as Stonehenge, which would serve to mark the solstices ever after.

- David Fried


Tony responds: Actually, it's easy to measure the Sun's declination quite accurately just by marking the tip of a shadow at noon. (Eratosthenes did that, among others.) But I imagine that you're right that the original idea of solstice had to do with the Sun's point of rising and setting. That requires no equipment at all -- just an unobstructed horizon with interesting features to serve as markers.

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John Stetson

July 26, 2007 at 11:52 am

Hello Tony,

If you were to use a gnomon (as the ancient Egyptians did) to measure time, you could easily find "local noon" when the shadow is shortest on any given day. If you were to mark that spot every day throughout the year at that time (or any chosen time), you would have an analemma. One of my students did create an analemma by making daily observations throughout a year. As the summer (and winter) solstice arrived, the space between the observations became very small; in the days before and after the solstice there appeared to be little or no movement when observing with the gnomon-and-shadow method. That is to say that man may have observed this sense of the sun "standing still" for all of the history of western civilization. Also, the shape of the analemma, the figure eight, gave us the mathematical symbol for infinity. (I have sent picture of Cary marking his analemma on the summer solsticve in the "gallery" section of this website along with the completed analemma. Best regards

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