A resource of Biophilia: Pittsburgh, #bioPGH is a weekly blog and social media series that aims to encourage both children and adults to reconnect with nature and enjoy what each of our distinctive seasons has to offer. 

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Tomorrow, December 21, will be the shortest day of the year! This morning, the sunrise felt incomprehensibly late at 7:39 a.m., and it will set at 4:56 p.m., which means Pittsburgh will only see nine hours and sixteen minutes of daylight. It’s the winter solstice! Let’s take a deeper dive into the day, beyond the shortened sunlight!

We call it a “solstice” when the Earth is angled at its most northerly or southerly position during its orbit around the sun. If you’ll recall from our chat about the spring equinox, our seasons on Earth are the result of the planet’s tilt on its rotational axis. As we travel around the sun, the Earth is tilted 23.5 degrees. What exactly does this mean? Well, picture that an imaginary line runs from top to bottom through the center of the Earth, connecting our North and South Pole. Now, imagine that this pole-pole line is perpendicular to the Earth’s path in its orbit around the sun — this would be our planet with no axial tilt. However, our tilted axis means that our imaginary line connecting the north and south poles is angled by 23.5 degrees. 

Photo credit: National Weather Service

Now that we have the Earth’s tilt in mind, we can look more closely at the solstice. As the Earth progresses in its elliptical orbit around the sun (and remember, the complete orbit takes one year!), this axial tilt keeps the Northern Hemisphere closer to the sun than the Southern Hemisphere for part of the year, and farther from the sun the rest of the year.  On December 21, at approximately 4:20 a.m. this year, the Northern Hemisphere will be at its farthest possible angle away from the sun, kicking off winter and giving us the shortest amount of daylight for the year. And the reverse is true for the Southern Hemisphere, as they are as close as possible to the sun. (The solstice is at slightly different days and times each year because it technically takes our planet 365.24 days to orbit the sun.) We both mark this as the official change in seasons. Our days in the Northern Hemisphere will grow longer and in the Southern Hemisphere, days will grow shorter. To visualize it, here is a video below from NASA:

If we take a step back and think about it, that little tilt of the planet has a pretty big impact! It drives our seasons, our daily amount of sunlight, our circadian rhythms, and many other things related to those concepts. It’s no wonder that cultures and civilizations from around the world and across history have faithfully observed the solstices and equinoxes. These events were followed a by a change in season that would have a tremendous impact on the experience and even survivability of the days ahead! (For example, my own Swedish ancestors and family celebrate Santa Lucia Day wearing candle crowns near solstice as a way to combine to combine a celebration of faith with a celebration of brighter days coming ahead.)

Today, we in Pittsburgh can note the solstice without the same concern for winter’s harshness that our great-great ancestors may have had, but it’s still important to watch the skies and understand our natural world. We’re all a part of it! So I hope you all have a lovely holiday season, and you're able to celebrate family and friends with joy - and spend some time outside for wonder. See you all in 2025!

Continue the Conversation: Share your nature discoveries with our community by posting to Twitter and Instagram with hashtag #bioPGH, and R.S.V.P. to attend our next Biophilia: Pittsburgh meeting.

Photo Credits: Header and cover, Pixabay CC0; planet image, National Weather Service, public domain