The 2025 Winter Solstice is Sunday, December 21, at 7:03 am PST.  This is when we experience the shortest day, and longest night, of the year in the Northern Hemisphere.  And it’s the time when the winter season officially starts.  Many cultures have had, and still have, celebrations around this time: people feel depressed by the darkness and need a holiday to cheer them up.

You might ask why our planet has such dramatic changes in the length of the day and night?  If you’ll permit me to put it in informal language, it’s because the Earth is not orbiting the Sun with its head held high – it is leaning over as it goes around by about 23 degrees. So on one side of our orbit, we lean into the Sun – that’s the summer solstice.  On the other side of the Sun, we lean away from the Sun – that’s Sunday’s winter solstice.

So why is the Earth leaning?  Venus and Jupiter are not; they do perfectly well going around the Sun with their rotation axis pointing straight up.  The answer lies in the ancient past.  Planet Earth got hit by a stray planet, very early in the violent history of the solar system, and like, many accident victims, couldn’t ever straighten out again. Back then, probably around 4 billion years ago, there were many more planets and mini-planets, formed from the great cloud of material around the Sun that gave rise to all the worlds of the solar system.  These “extra” planets had irregular orbits, and some fell into the Sun, while others collided or exchanged energy with more regular planets, changing their own orbits and the worlds they collided with.

Our Earth got dealt a glancing blow by a sizeable world, a collision that may have changed the impactor’s orbit and sent it into the Sun or out of the solar system entirely. But, billions of years later, we are still stuck with a leaning planet and the seasons the tilt of our axis causes.

Now I might mention that many people think the seasons are caused by the Earth’s orbit not being a circle, but an skinny ellipse, and that we are further from the Sun when it is winter.  But if that were so, then Australia would have the same seasons as the U.S. (since both sides of the Earth would be further from the Sun.)  Instead, the North and the South hemispheres have opposite seasons; only the tilt can cause that.

The differences between the North and South get more extreme as you travel away from the mid-latitudes where we live, and toward the poles. The North Pole has had no sunrise since October. The South Pole, on the other hand, continues to bask in the glow of the midnight sun, which won’t set until March.

Whatever holiday you are celebrating or avoiding in late December, bear in mind you have had good company for millennia.  Some scholars think the winter solstice is the oldest continuing holiday celebration in the world.  Happy holidays and here is to a better 2026 for all of us.