Five questions about the winter solstice, answered

The longest night of the year is just ahead. Here are answers to common questions about the winter solstice, and why it's been an important part of human history since our very beginnings.

1. What is the winter solstice?  

While we tend to think of the winter solstice as the shortest day and longest night of the year, it actually only occurs for a moment. The winter solstice is the precise moment when the North or South Pole is furthest away from the Sun.
 

Everyone in the Southern Hemisphere experiences the winter solstice when the South Pole is furthest from the Sun, on June 20 or 21. For those in the Northern Hemisphere, it’s when the North Pole is furthest, usually on December 21 or 22. Here at Perimeter Institute in Waterloo, Ontario, the 2025 winter solstice will happen at 10:03 a.m. on December 21.
 

The winter solstice marks the start of astronomical winter, which is based on the Earth’s position in space. Astronomical seasons are different from meteorological seasons, which are based on temperature cycles and the 12-month calendar. In the Northern Hemisphere, meteorological winter starts on December 1, weeks before astrological winter.
 

2. Why does it happen?  

The winter solstice occurs because the Earth is tilted and rotates on an axis – an imaginary line that travels through the Earth from the North to the South Pole. As the Earth completes its year-long orbit around the Sun, there is a period of time when the Earth’s North Pole is tilted away from the Sun. The exact moment the Northern Hemisphere is tilted the furthest away from the Sun is our winter solstice.
 

3. How is the winter solstice different from the other days of the year?  

When the Northern Hemisphere is maximally tilted away from the Sun on the winter solstice, the light from the Sun hits the Northern Hemisphere for the shortest duration of time of the year. This in turn makes it the shortest day of the year with the fewest hours of sunlight.
 

But take heart; the winter solstice also marks the point where the Earth’s elliptical path will bring the planet around so that the Northern Hemisphere is once again tilted towards the Sun. This means that from December 21 onwards, the days will begin to get longer again.
 

If you head outside at noon on December 21st, you will cast your longest possible shadow of the year. If you have clear pavement, try making a chalk sundial to track your shadow’s changing shape throughout the day.
 

4. Why do people across the world celebrate the winter solstice?  

The winter solstice is a time for celebration in cultures around the world because it marks both the longest night of the year and the start of increasing daylight hours. Following the winter solstice, days will get longer and longer until the summer solstice. So if you’re feeling the winter blues, don’t worry –- sunnier days are just around the corner!
 

5. How long have humans been celebrating the winter solstice? 

Archaeology reveals that humans have tracked the solstice for thousands of years. Ancient monuments like Stonehenge in England were built to carefully align with the Sun’s position during the winter solstice. Paying attention to the Sun’s movements throughout the year probably helped farmers plan and harvest crops and organize the breeding and slaughter of livestock.
 

The winter solstice has often been a time of celebration and feasting, from Yule in Germany to the Dongzhi Festival in China. Many winter holidays borrow traditions and their timing from the solstice celebrations of yore.