Give thanks for the winter solstice. Without it you might not be here. (Published 2017)

[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 be on the horizon. To those of us in the Northern Hemisphere, it will seem like it barely rises—barely peeking above a city’s skyline or the snow-covered evergreen trees of a forest—before it quickly sets.

For months the arc of the sphere in the sky has collapsed, growing shorter every day.

In New York City, for example, the sun will be in the sky for just over nine hours – roughly six hours less than in June during the summer solstice. The winter solstice marks the shortest day of the year, before the sun changes course and climbs higher into the sky. (At the same time, places like Australia in the Southern Hemisphere mark the summer solstice, the longest day of the year.)

This is a good opportunity to imagine what such a day would be like if we had evolved on another planet where the sun performed a different dance across the sky. Maybe you want to be grateful for the solstices and seasons we have, or maybe we’re not here to witness them at all.

The solstices occur because most planets do not spin upright or perpendicular to their orbits.

For example, the Earth is tilted 23.5 degrees on a tilted axis. This keeps the planet’s north pole pointed toward the North Star for relatively long periods of time, even as Earth makes its annual migration around the sun. That means the Northern Hemisphere will be slightly tilted towards the sun for half the year, bathed in direct sunlight during the long, blissful days of summer, and cool for half the year as it faces slightly away from the sun during the short, cold days of winter. leans away. December 21 marks the day when the North Pole is tilted most away from the sun.

But each planet hangs at different angles.

For example, Venus’s axial tilt is so extreme – 177 degrees – that the planet is actually tilted upside down with its south pole pointing upwards. Perhaps counterintuitively, this means that there is very little tilt in the reverse spin and the hemispheres will never point dramatically toward or away from the sun. As such, the sun’s dance across the sky will remain relatively stable – shifting only six degrees over the course of a Venusian year.

If we had evolved on Venus, we probably wouldn’t have noticed solstices or seasons at all, says David Grinspoon, an astrobiologist at the Planetary Science Institute.

The same cannot be said of imaginary aliens living in the chilly 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’s poles points almost directly toward the Sun at the solstice—as if about to form a perfect bullseye. That means one hemisphere will be in the sun both day and night, while the other hemisphere will experience a frigid and dark winter and won’t catch a glimpse of the sun for that entire season.

“It really can’t get more extreme than that,” says Heidi Hammel, a planetary scientist at the Planetary Society.

Such a tilt on Earth would mean that the Arctic Circle did not begin 66 degrees north of the equator, but at the equator itself. All of North America, Europe, Asia and half of Africa would spend winters in permanent darkness and summers under constant sunlight. And on Uranus, which takes 84 Earth years to orbit the sun, these seasons last decades.

“If there were beings on Uranus – and I don’t think there are – seasonal affective disorder would be a lifelong thing,” said Dr. Hammel.

But the king of the extreme seasons is Pluto.

When NASA’s New Horizons spacecraft arrived at the dwarf planet in 2015, scientists discovered a unique world full of surface features that resemble networks of drains and even a frozen lake. But given Pluto’s low atmospheric pressure and cold surface temperature, liquids can’t flow across its surface — at least not today. “That can’t happen in the current environment,” said Alan Stern, the mission’s principal investigator. “So, neither the canals nor the lake can be that.”

Scientists now have an explanation: Seasons in Pluto’s past caused atmospheric pressure to be high enough for liquids of methane and nitrogen to flow and collect on the surface.

Changing axial tilt is the biggest cause of wildly varying seasons on Pluto. Over the course of 4 million years, Pluto’s tilt shifts back and forth between 102 and 126 degrees, causing the equivalent of an Arctic Circle to grow and shrink. This occasionally creates seasons in which the atmospheric pressure is high enough that liquid methane and nitrogen can flow.

Although astronomers are still unsure how a planet’s seasons might affect the likelihood of life on a planet, Dr. Grinspoon that such dramatic fluctuations – like those on Pluto – are likely a hindrance because they can make a planet unsuitable for long-term life. . “Life needs a continuously habitable zone to thrive,” he said. Likewise, astronomers have long suspected that life on Earth would likely not survive if it had an axial tilt more similar to that of Uranus.

So be grateful when the sun reaches its farthest point in the sky on December 21. The sun will never sink so far below the horizon that it immerses half the globe in a month-long night and the other half in an equally long summer. Also, the Earth’s tilt does not change drastically over millions of years, thanks to the influence of the moon. Instead, the sun appears to trot back and forth between the extremes, like the pendulum of a great clock, keeping the planet cozy as it steadily counts down the years.