The winter solstice plinth at the Open Deck Observatory. The hole in the center aligns with the Sun as it rises on the day of the December solstice!
What is the December Solstice?
Though the weather has cooled and the holidays are right around the corner, winter hasn’t officially started yet — at least, not from an astronomical standpoint. On Sunday, December 21, 2025, the Northern Hemisphere will experience its shortest day and longest night of the year. This event, known as the December solstice or winter solstice, marks the astronomical beginning of winter in the Northern Hemisphere and summer in the Southern Hemisphere.
Earth is divided into two hemispheres by an invisible line called the equator. As our planet orbits the Sun throughout the year, each hemisphere receives different amounts of sunlight. Because Earth is tilted on its axis by about 23.5 degrees, one hemisphere is always angled more directly toward the Sun while the other is angled away. This tilt causes the Sun to rise and set at different points on the horizon over the course of the year, changing the length of daylight and creating what we know as seasons.
On the day of the December solstice, the Northern Hemisphere is tilted at its farthest point away from the Sun. As a result, the Sun follows its lowest and shortest path across the sky, rising and setting at its southernmost points and producing the longest night of the year. At noon local time, the Sun also reaches its lowest maximum altitude above the horizon.
After the solstice, daylight hours begin to increase by about a minute per day as Earth continues its orbit toward the spring equinox.
Fun fact: the night of the winter solstice is also one of the best nights of the year to visit Lowell Observatory. With the Sun setting at its earliest time of the season, you’ll have an extra-long window for evening stargazing. That means more time at the telescopes, clearer views of the winter night sky, and a chance to experience Flagstaff’s famously dark skies at their best!