Earth’s axial tilt is the reason for the seasons—and December’s solstice.
getty
An important global moment arrives at 10:27 p.m. EST on Thursday, 21 December 2023 when the sun shines above the Tropic of Capricorn in the southern hemisphere.
Winter solstice north of the equator and summer solstice south of it signals the official beginning of a new season.
Confused? You shouldn’t be. It’s all a simple story about how to live on a planet with a tilted axis—and a chance to think about how and why the intensity of the sun waxes and wanes throughout the year, the changing seasons and how celestial mechanics set the tone for life on Earth.
Here’s everything you need to know about this week’s solstice—and why it matters.
Solstice Explained
Astronomically speaking, it’s the beginning of winter in the northern hemisphere and of summer in the southern hemisphere. In the north, it’s the shortest day and the longest night of the year while in the south it’s the longest day and the shortest night of the year.
This is all and only a story about how our planet spins. The axis on which Earth rotates is tilted by 23.5 degrees, so during our annual orbit around the sun, different parts of Earth receive sunlight for different lengths of time. It was probably caused by an impact billions of years ago.
During this week’s solstice, the northern hemisphere is tilted away from the sun, so the sun hangs lowest in the sky. Meanwhile, in the southern hemisphere, the sun sits over the Tropic of Capricorn, an imaginary line at 23.5 degrees south of the equator, giving that hemisphere its full glare and staying in the sky for longer.
Pivotal Moment
Solstice marks a pivotal moment in the sun’s apparent movement. “On this day, the sun’s path seems to pause and change direction, a phenomenon that gives rise to the term ‘solstice’,” said Dr Minjae Kim, Research Fellow, Department of Physics, University of Warwick in the U.K., in an email. The word originates from the Latin “solstitium”, meaning “sun stands still.” “This apparent standstill occurs as the sun reaches its southernmost point against the backdrop of stars,” said Kim.
The days will now become longer in the northern hemisphere until they reach 12 hours of daylight and 12 hours of darkness at the March equinox (equal night). At the equinox, the Earth’s axis is side-on to the sun.
By the June solstice, the situation is reversed, with the sun sitting above the Tropic of Cancer, 23.5 degrees north of the equator. It creates summer in the northern hemisphere and winter in the southern hemisphere.
Sunrise and sunset points vary throughout the year—but reach an apex on the solstice.
Effects Of Solstice
“The Earth’s axial tilt is the fundamental cause of these seasonal variations,” said Kim. “It not only influences the differences in temperature between seasons but also causes the length of day and night to change throughout the year.”
The sun rises and sets at different points on the horizon throughout the year. People who watch sunsets and sunrises know this instinctively, but most of us do not. The solstices represent the extreme points. For example, in the northern hemisphere on the winter solstice, the sun will rise in the southeast and set in the southwest.
These are the furthest points you can see a sunrise and set in an entire year. Wait six months until the summer solstice in June and, from the northern hemisphere, the Sun will rise in the northeast and set in the north-west. The halfway point is, you guessed it, the equinox, which occurs in both September and March. During the equinox, the sun rises and sets due east.
How To Observe Solstice
Although it’s an important way marker in the orbit of the Earth around the sun, not many people celebrate the solstice, particularly in winter, when it occurs very close to other festivals.
A good way to mark the event is, of course, to watch the sun rise or set on the day of the solstice—something ancient monuments such as Stonehenge appear to align to—but there are other ways. One is to simply go outside after dark, and do some stargazing, which this year will bring you excellent views of a very bright Jupiter high in the south-eastern night sky. As a bonus, it will be visible immediately after sunset.
Solstices And Equinoxes In 2024
Here are the dates of the following equinoxes and solstice in 2024, together with the cross-quarter days—the calendar way markers between these events—some of which may surprise you:
- Cross-quarter day: February 2 – Groundhog Day
- March equinox 2024: March 19
- Cross-quarter day: May 1 – May Day
- June solstice 2024: June 20
- Cross-quarter day: August 1 – Lammas
- September equinox 2024: September 22
- Cross-quarter day: October 31 – Halloween
- December solstice 2024: December 21
Wishing you clear skies and wide eyes.