Happy Winter Solstice!

The winter solstice is the day in the year with shortest amount of daylight. What’s happening today is all related to how the Earth is orbiting, rotating, and tilting and it’s also all connected to the changing seasons throughout the year. Here is the “reason for the seasons” and why today is a special day.

Even though we can’t feel it, the Earth is in constant motion. It’s spinning and orbiting and moving really fast. It is spinning (rotating on its axis) at about 1000 miles per hour, which takes us all the way around every 24 hours. This is what gives us night and day. We have day when our side of the Earth is facing the sun and night when our side is facing away from the sun. As we’re spinning, we’re also moving in an orbit around the sun at about 67,000 miles per hour. This takes us on complete orbit every 365.25 days, which gives us a year with seasons. (Side note: that extra 0.25 days is why we have a leap year every 4 years.) If the earth’s axis were pointed exactly up and down (perpendicular to the orbital plane), where we are in the orbit around the sun wouldn’t make much difference (in terms of the seasons or how much daylight we have). The earth’s axis isn’t exactly up and down, though, it is tilted a little bit. This is called axial tilt and it’s the angle between a planet’s rotational axis and a line perpendicular to the orbital plane of the planet (the straight up and down line).

The axial tilt for Earth is about 23.5 degrees. The tilt doesn’t change as we orbit the sun, so what this means is that different parts of the Earth are tilted closer to the sun or away from the sun at different times of the year depending on where we are in the orbit. We have summer and more hours of daylight when our hemisphere is tilted towards the sun and winter and fewer hours of daylight when our hemisphere is tilted away from the sun. When one hemisphere is tilted towards the sun, the other is tilted away. Right now, the southern hemisphere is tilted towards the sun and having summer months while the northern hemisphere is tilted away and having winter.

The solstices happen when each hemisphere has its maximum tilt towards (summer solstice) or away (winter solstice) from the sun. For the northern hemisphere, the summer solstice is around June 21 each year and the winter solstice is around December 21. The southern hemisphere is opposite. (The equinoxes are around March 21 and September 21 and are halfway between the solstices when the northern and southern hemispheres have about the same tilt towards the sun.)

Today we are at our maximum tilt. It’s our shortest day of daylight. As we continue our orbit, each day will take us closer to warmer weather and more daylight (until June 21). Some traditions viewed the winter solstice as a day to focus on renewal and celebrate the return of the sun. I think it’s also a day to marvel at Science, all that we already know, and all that we have yet to discover.

‘Tis the season – thanks to axial tilt!