You won’t feel or see a thing, but at 11:32 p.m. Philadelphia time Sunday, the Northern Hemisphere will experience a cosmic event that has captured the human imagination since the early stirrings of civilization — the summer solstice — a profound astronomical and meteorological bookmark.
The instant that the sun beams directly over the Tropic of Cancer this year will be far more dramatic in say, New Delhi, when it will be close to noon, or at Stonehenge, in Wiltshire, England, where it is always a big deal. (Yes, the sun will never set on the myth of the role of the Druids, and you can watch the Stonehenge celebration via livestream.)
But archaeologists have long known that Stonehenge, where the stones are oriented toward the angle of the solstice sunrise, was late to the solar-fascination game. Another stone arrangement in Nabta Playa, near the Egyptian-Sudanese border, is believed to have predated it by at least 1,000 years.
For that matter, other pagan cultures have used such structures as astronomical monuments for thousands of years, according to M. McKim Malville, a professor emeritus of astrophysical and planetary sciences at the University of Colorado.
Just what happens at the solstice?
Envision a deep well somewhere along the Tropic of Cancer — at about latitude 23.5 or roughly 1,600 miles north of the Equator. At the time of the summer solstice in the Northern Hemisphere sunlight would penetrate all the way to the surface of the water.
That would be evidence of direct light from the sun, unobstructed by shadow.
It so happened that Pliny, that Roman know-it-all, over 2,000 years ago wrote that such a well did exist in Egypt near that latitude, and verified that the well was fully illuminated.
The sun never makes a direct hit farther north than it does at the instant of the solstice. It then begins the ponderous southward progression until it beams directly over the Tropic of Capricorn at the moment of the Northern Hemisphere’s winter solstice in December.
The long, hot summer
Summer is the longest astronomical season in the Northern Hemisphere, lasting about 94 days.
Counterintuitively, the Earth is significantly farther from the sun in June than it is in January, when its closest approach of the year induces the planet to accelerate. That’s why February loses 2.25 days annually.
If you think summers are long here, check out Uranus, where each season lasts 21 Earth years.
The long, not-so-cold winter
Winter is the longest astronomical season in Earth’s Southern Hemisphere, yet it is snow-deprived relative to the other half of the planet, notes Tyler Roys, international forecaster with AccuWeather Inc.
South of the Equator, snow mostly is confined to the higher elevations, and is sparse in heavily populated regions, he says.
A big reason is liquidity: Land masses occupy a minority of the Southern Hemisphere, and the prevailing winds off all those sea surfaces have a tempering effect.
Nor do our neighbors to the south who experience snow share Americans’ interests in the white stuff, said Roys, adding that in fact, they aren’t real big on measuring it and keeping snow records.
It is known that Ushuaia, in Tierra del Fuego, the southernmost city in Argentina, about 6,500 miles from Philly, does get its share of snow, and if you’re jonesing for winter action, some snow is possible there Monday, says AccuWeather.
Speaking of the seasons
No snow is expected around here in the months ahead, and the government’s Climate Prediction Center updated outlook issued Thursday says the odds strongly favor a hot summer.
Although the sun is at maximum strength at the solstice, on average the hottest period of the summer is about two weeks away.
The Earth is one, massive, erratic radiator that uses the sun’s energy to heat the overlying atmosphere, which at times appears to take some pleasure in ignoring the solar signals.
Summery weather has been known to start weeks before the solstice, and snow has fallen after the spring equinox. That’s one reason the weather community divides the “meteorological” seasons into tidy three-month increments. By that standard, summer started June 1.
The longest days of the year
Meteorologists can only envy the astronomers, who are never wrong. Equinoxes, sunsets, moon phases, eclipses, they nail them all.
They have calculated that Sunday is in a statistical tie with Saturday for the longest day of the year at 15 hours and 42 seconds in Philadelphia, with the sun rising 13 seconds later but also setting 13 seconds later on Sunday. Add the morning and evening twilight, and we get less than eight hours of darkness.
The sunrise-sunset period won’t dip below 15 hours until Friday.
The lighter side
Feeling more upbeat these days? Join the club, say researchers at Cornell University.
Analyzing the content of Twitter messages posted by 2.4 million people in 84 countries, they concluded that day-length has a positive effect on mood. That would be more or less the reverse of the negative symptoms of Seasonal Affective Disorder, brought on by the loss of light in fall and winter.
The findings were published a decade ago, but Michael Macy, the sociology professor who was the lead author, said they were reinforced by a more recent study using music download data from Spotify.
The folks in Fairbanks, Alaska, might be downright giddy: On Sunday they are getting just under 22 hours of daylight.