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.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.