Today is the longest day of the year—in the Northern Hemisphere, anyway. Fifteen hours and forty minutes of daylight in New York. Seventeen and a half in Copenhagen and Moscow. Twenty-one and change in Reykjavík. Twelve hours, eight minutes, and twenty-four seconds in Kampala, just north of the equator. (The day will be one second shorter there starting next week.) The solstice is the one day when every point north of the Arctic Circle sees at least twenty-four hours of continuous sunlight, but farther north, in Deadhorse, Alaska, on the edge of the Arctic Ocean, the sun rose on May 15th and won’t set again until July 28th. I was in the region once, in late June, with an encampment of biologists on the shore of a glacial lake, and even at two-thirty in the morning, a sliver of sun was visible on the horizon, and the light in the sky was as pale and glassy as the surface of the water. It’s immortality we’re reaching for—the farthest we’ll ever get, once a year, from perpetual night.
So it’s a good moment to note how good we have it here on Earth. There are longer days in our solar system, but none are quite so pleasant. If “day” refers to the time it takes for a planet to rotate exactly once on its axis (a sidereal day), then the Venusian day is the longest, lasting two hundred and forty-three Earth days. That’s even longer, by nineteen Earth days, than a Venusian year, which is the time it takes the planet to orbit the sun. If, instead, “day” refers to the period between sunrise and sunset (a solar day), Neptune’s is the longest: the gas giant orbits the sun on its side, such that one pole or the other receives daylight for forty-two years non-stop.
Farther out in the universe, the days are longer still. Since 1995, some thirty-five hundred extrasolar planets have been discovered, but scientists only gained the ability to measure their spin rates in 2014. A great many of the known ones, though, orbit very close to their host stars and are probably tidally locked, with one side of the planet perpetually facing the star, just as our moon always presents the same face to Earth. “This leads to an infinitely long day, since if you are on the night side, you will never see the sun,” Konstantin Batygin, an astrophysicist at Caltech, told me. (Last January, Batygin and the astronomer Mike Brown, also at Caltech, announced the possible existence of a ninth planet in the solar system, a relictual ice giant so distant that it orbits our sun once every twelve thousand to twenty thousand years.) Last August, scientists discovered Proxima b, an exoplanet just 4.3 light-years away, which is about as close to us as any extrasolar planet will ever come. It, too, is probably tidally locked, its day eternal. But, even being so near, Proxima b would take us eighty thousand years (some thirty million days) to reach—a very long day’s journey into day.
Summer is a separate matter. A planet’s seasons are shaped by two factors: the eccentricity of its orbit—whether it’s closer to the sun at some times of the year than at others—and the tilt of its axis. Earth’s orbit is essentially circular, so the effect on our climate is negligible. But the planet itself leans twenty-three degrees to the side; as we orbit, there comes a day when the North Pole is maximally tilted toward the sun and the Northern Hemisphere sees more daylight than it will all year. That’s today, the summer solstice. (Below the equator, it’s the winter solstice, of course, and in six months our situations will reverse.) If we weren’t off-kilter, we’d have no summer nor any seasons at all. Every day would be as long as every other, and changes in the weather would be driven more by the local geography—latitude, elevation, that mountain range to the west that keeps the rain from falling—than by shifts in the jet stream, or the massive blooms of Pacific plankton in the winter that fuel El Niño, or the decline in sunlight that triggers autumn leaves to change color. Mercury, Venus, and Jupiter, standing all but upright, are seasonless. Sad.
Perhaps the weirdest summer of all unfolds on HD 131399Ab, an extrasolar gas giant that was discovered last July by Daniel Apai, an astronomer at the University of Arizona, and his colleagues. The planet belongs to a system with three stars but orbits only one of them, the biggest, which is eighty per cent larger than our sun. The other two stars orbit each other and, together, like a spinning dumbbell, orbit the big one. The view from HD 131399Ab would be spectacular if not for the ferocious winds, the lack of solid ground, and a steady rain of liquid iron. For much of the year, which lasts five hundred and fifty Earth years, the three stars appear close together in the sky, giving the planet “a familiar night side and day side, with a unique triple sunset and sunrise each day,” Kevin Wagner, one of the discoverers, remarked at the time. But as HD 131399Ab progresses in its orbit and the stars drift apart, a day arrives when the setting of one coincides with the rising of the other, and a period of near-constant daylight begins—a solstice of sorts, the start of a summer that will last about a hundred and forty Earth years.
As with so much in Earth’s prehistory, the tilt of our planet is a cosmic fluke, most likely the result of a series of collisions—with comets, asteroids, nascent moons—in the early days of the solar system. Our own moon has helped keep this tilt, along with our summer, stable over time. But the moon is drifting away from us—a couple of inches every year—which means that our axis will eventually shift. In two billion years, summer as we know it will dissolve for good. (Also by then, the oceans will have boiled away, ruining any prospect of a beach vacation.)
But our axis is already changing, thanks to us. Because Earth is not a perfect sphere, it wobbles very slightly as it rotates, too little to physically matter but enough for scientists to measure. Since 2000, though, the axis has shifted in a distinctly eastward direction, toward the British Isles, at a rate of about seven inches a year, twice as fast as before. Partly it’s due to the loss of ice sheets in Greenland and Antarctica, which redistribute the planet’s mass. But, last April, scientists figured out the bigger reason: the loss of water mass in Eurasia, as aquifers are depleted and drought settles in. Who knew that a being so small could move an object so big? There is some solace in it: of all the cosmic events that could alter our seasons for the worse, climate change is the one we have the most potential to manage. But we’ll need to hurry; summer is coming.
