[This article, which was originally published for 2017’s winter solstice, has been updated for 2023. Sign up for The Times Space Calendar here.]
On December 21, or Thursday this year, the sun will hug the horizon. To those of us in the Northern Hemisphere, it appears to rise slightly — barely peeking above the city skyline or the snow-covered evergreens of the forest — before it quickly sinks.
For many months, the arc of the orb in the sky falls, getting shorter every day.
In New York City, for example, the sun is in the sky for just over nine hours — about six hours less than during the June summer solstice. The winter solstice marks the shortest day of the year, before the sun turns and climbs higher in the sky. (At the same time, places like Australia in the Southern Hemisphere mark the summer solstice, the longest day of the year.)
It’s a great opportunity to imagine what a day might look like if we evolved on another planet where the sun would have a different dance in the sky. Maybe you want to be thankful for the solstices and seasons we have, or maybe we’re not here to witness them.
Solstices occur because most planets do not rotate vertically, or vertically in their orbits.
The Earth, for example, tilts 23.5 degrees on a tilted axis. This leaves the planet’s North Pole pointing toward the North Star for quite some time, even as the Earth makes its year-round transition around the sun. That means the Northern Hemisphere will spend half the year tilted slightly toward the sun, bathed in direct sunlight during the long, happy summer days, and half the year cooling as it moves slightly away from the sun during the short and very cold winter days. December 21 marks the day when the North Pole is most tilted away from the sun.
But each planet bends at different angles.
The axial tilt of Venus, for example, is so extreme – 177 degrees – that the planet is essentially upside down pointing at its South Pole. Perhaps counter-intuitively, that means it tilts very little in its reverse rotation and its hemispheres never point noticeably toward or away from the sun. Because of this, the sun’s dance in the sky remains relatively stable – changing only six degrees during a Venus year.
If we evolved on Venus, we probably wouldn’t notice solstices or seasons, said David Grinspoon, an astrobiologist at the Planetary Science Institute.
The same cannot be said for the imaginary aliens living within the cold blue clouds of Uranus.
“Uranus is wild,” said Dr. Grinspoon.
An axial tilt of 98 degrees causes the ice giant to turn on its side. So, while one of Earth’s poles leans slightly toward the sun at the solstice, one of Uranus’ poles is directed almost directly toward the sun at the solstice – seemingly poised to make a perfect bullseye. That means one hemisphere will be warmed by the sun both day and night, while the other will experience a cold and dark winter and not see the sun for that entire period.
“It really can’t get any more extreme than that,” said Heidi Hammel, a planetary scientist at the Planetary Society.
This tilt of the Earth means that the Arctic Circle does not start at 66 degrees north of the Equator, but at the Equator itself. All of North America, Europe, Asia and half of Africa will spend winters in permanent darkness and summers under constant sunlight. And on Uranus, which takes 84 Earth years to orbit the sun, these periods last for decades.
“If there were creatures on Uranus — and I don’t think there are — seasonal affective disorder would be a lifelong thing,” said Dr. Hamel.
But the king of extreme weather is Pluto.
When NASA’s New Horizons spacecraft arrived at the dwarf planet in 2015, scientists discovered a strange world overflowing with surface features like networks of drainage channels and even a frozen lake. But because of Pluto’s low atmospheric pressure and cold surface temperatures, liquids cannot flow to the surface – at least not yet. “That can’t happen in the current environment,” said Alan Stern, the mission’s principal investigator. “So, it can’t be the channels or the lake.”
Scientists now have an explanation: periods in Pluto’s past pushed the atmospheric pressure high enough to allow liquids of methane and nitrogen to flow and pool on the surface.
The change in axial tilt is the biggest driver of the wildly varying seasons on Pluto. Over the course of 4 million years, Pluto’s slouch swings back and forth between 102 and 126 degrees, causing its equivalent of an Arctic Circle to grow and shrink. That occasionally creates periods where the atmospheric pressure is high enough that liquid methane and nitrogen can flow.
Although, astronomers remain uncertain how a planet’s seasons might affect its likelihood of harboring life, Dr. Grinspoon that such dramatic swings – like those on Pluto – are likely to be a hindrance because they can make a planet uninhabitable for long periods of time. . “Life needs a continuous habitable zone to thrive,” he said. Similarly, astronomers have long suspected that life would be unlikely to survive on Earth if it had an axial tilt more like that of Uranus.
So, as the sun reaches its furthest point in the sky on December 21, be thankful. The sun will never set below the horizon with half the world plunged into a moonlit night and the other half into an equally long summer. Earth’s tilt also hasn’t changed much over millions of years, thanks to the influence of the moon. Instead, the sun appears to swing between extremes, like the pendulum of a great clock, keeping the planet comfortable as it continues to count its years.
