The longest days arrive in Philly, and the sunsets are about to get even later

As though celebrating one of the year’s astronomical highlights, the fireflies have begun enchanting the darker spaces of the region with their electrifying flashes.

“They’re their own little meteor showers,” said Ken Butler, an organizer of the perennially sold-out Pennsylvania Firefly Festival.

At 10:42 p.m. Friday Philly time — the instant of the summer solstice when the sun’s direct light is upon the Tropic of Cancer — some of their numbers will be quietly blinking away in reasonably dark and vegetated places throughout the region.

For Butler, the solstice is a heady time when a spectacular harvest of synchronized flashing is approaching its peak. Fortuitously, this year the moon will fully cooperate with the festival by politely staying out of the way — even though it will be a “super moon.”

Here are observations on fireflies and other matters to honor “the Longest Day.”

Or is it?

Since the solstice occurs so late at night Philly time, is the longest day the 20th or the 21st? The answer: Both.

The interval between sunrises and sunsets on Friday and Saturday is just under 15 hours and four minutes. While the sunset on Saturday is a few seconds later, so is the sunrise.

While the days become almost imperceptibly shorter, the sunsets come almost imperceptibly later. Sunset time on Friday is 8:34 p.m. It will increase a few seconds a day until it gets pushed back to 8:35 on Wednesday and holds serve until the end of the month.

The solar radiation over the Northern Hemisphere is at its peak at the solstice. Yet, as its power leaks ever so gradually, it gets hotter around here. That’s because of the lag between the source of our heat and the ripening of the effects. It takes a while to prime the atmosphere and the surfaces. Consider: The sun is at its highest at noon, yet the daily high temperatures typically are recorded in the late afternoon.

Climatologically, the warmest period in Philly begins around July 4, with a daily “normal” high average temperature of 88. Yet the daily average of solar radiation is about the same as it was on June 4.

As it happens, around July 4, Earth is as far from the sun as it gets in its annual orbit. On average, the planet is about 93 million miles from the sun. On the Fourth, it will be roughly 94.5 million miles away.

But don’t spare the sunscreen. The 23.5% tilt of the Earth’s axis far outweighs the effects of distance from the sun. It happens that on Jan. 3, when Earth makes its closest approach to the sun, chances are excellent that you’ll need a coat more than sunscreen.

While that axial tilt has the sun bathing more of the Northern Hemisphere, more of the Southern Hemisphere is in shadow.

This imbalance will change, but it will take some time. The Earth’s tilt has varied over time between 22.1 and 24.5 degrees. These days, it is heading back from its maximum tilt, and by NASA’s calculation, in about 10,000 years, it will be at 22.1 degrees. That would mean more sun almost everywhere, except at the poles, where the ice should party in the darkness.

For now, the winter as we know it arrives in Tierra del Fuego at 11:42 p.m. provincial time Friday, and it will be winter for the next three months for 10% of the world’s population.

It’s hard to compare amenities, but the Northern Hemisphere does have a whole lot more buildable land.

For starters, the surface of the Southern Hemisphere is about 80% water-covered.

In their annual celestial seesaw, when the sun reaches its highest point in the sky, the moon hits its lowest. Even with that disadvantage, it does a remarkable job of casting tree shadows and peeking through the branches.

Come the winter solstice, the moon will be riding high through the leafless branches, the sun at a low. A “super moon” is coming, but it will be super for stargazers.

The moon is being kind to Ken and Peggy Butler’s firefly festival in Kellettville, in central Pennsylvania’s Allegheny National Forest, home to a rare species that flashes in synchrony. The Butlers, who live on the edge of the forest, had no idea when they moved there about 15 years ago that they had wandered into a cosmic light show that would end up being a tourist attraction. This year once again had way more applicants than campsites.

The moon will be close enough, about 228,000 miles away, to be a super moon, by the definition of the late astronomer Fred Espenak, but in this case it will be a “new super moon,” completely out of sight when the four-day festival begins Thursday. For fireflies, the less light — even moonlight — the better for their mating and other activities (some of them do eat each other). Unfortunately, said Ken Butler, artificial lighting, along with development, has been poison for fireflies, the state insect.

However, the shows go on, and they are always final acts for the players. “Once they start flashing,” said Butler, “they’re dead within a week to 10 days.”

But what a way to go.