Today is the winter solstice (aka: midwinter), making this the darkest day of the year! Speaking from the admittedly-biased perspective of the Northern Hemisphere, I’m gonna drop some astronomical science on you, in honor of this short, dark day.
First, a question for the audience: What causes the seasons? (dig deep in your mind back to 7th grade physical earth science) You have the space of 2 random pictures from Rovaniemi to solidify your answer…
So the seasons are caused by the tilt of the earth, which changes the angle (and therefore, the intensity) of the sun’s rays hitting the earth’s surface. To be clear, the sun is always tilted with respect to the sun. Earth’s ‘celestial equator’ (0° latitude) is about 23.4° different than the orbital equator (the plane of orbit), a property known as the axial tilt. The earth maintains the same tilt direction all the time (the tilt does not flop over), so the annual rotation of the earth around the sun (orbit), in combination with the tilted nature of our planet, creates the seasons.
Another side-effect of the earth’s axial tilt is the difference in day lengths as the earth rotates around the sun. The winter and summer solstices are the times of the year when the combination of the earth’s rotation and axial tilt create the shortest and longest days of the year, respectively. The summer solstice (June 21, the longest day of the year) is when the earth’s north pole is most pointed towards the sun, basking the northern hemisphere in extra sunshine. And today, the winter solstice, is when the north pole is pointed the farthest it gets away from the sun, so the northern hemisphere has the least amount of sun-exposure.
Okay, so if you didn’t know this, or if you were wrong about it, don’t feel bad! I watched a pedagogical documentary (made in the late 80s) about how most recent graduates and professors at Harvard got this question wrong! (Actually this was a really cool documentary, called “A Private Universe“.) A big part of the problem is that our minds make a logical leap when confronted with astrological pictures like this one (from EnchantedLearning.com):
The way this is drawn makes it look like the earth gets closer and farther away from the sun, so your mind thinks: ‘it’s summer when the earth is closer to the sun, and winter when the earth is farther away from the sun, because the sun is hot!’. The earth’s orbit is very nearly a perfect circle, so getting closer or farther away from the sun has nothing to do with our climactic seasons, despite the seemingly-logical nature of this explanation (you also might wonder how that works, given that the summer and winter are opposite each other in any oval-shaped ellipsis, so they’d be the same distance from the sun…).
I’m not gonna lie – if I hadn’t known the date, I would not have been able to tell the difference in today’s light levels (turns out they’re only a few minutes different than the two bracketing days). Rovaniemi is about 8km south of the Arctic Circle boundary (latitude: 66° 33′ 44″), so depending on where you are in the city (by the rivers vs. up on the hills/bluffs), you may or may not see the sun at all today. This is because today in Rovaniem the sun will reach it’s noon-time zenith at a whopping 0.5° above the horizon (which is why where you are in the city will determine if you see it at all). The ‘sunrise’ today is at 11:08am, and the ‘sunset’ is at 1:22pm, giving this day an official length of 2 hours, 14 minutes, and 19 seconds! And that’s only really if you’re up on the hills. :)
So in honor of this shortest, darkest day, I took a picture from the back porch of the Metla building approximately every hour to show the light changes during my one-and-only (hopefully?) Arctic winter solstice! Here they are, starting with 10:20am (because it was just too dark before that point). Also, it was snowing all day long, so that’s why the pictures look really white, and often a little fuzzy.
Something I really like about Finns is that they are all very positive about today being the solstice – despite the fact that midwinter marks the first day of winter, they choose to look at the solstice as the year’s low point, because from now on out, there’s only going to be more light each day! I’ve got to try to adopt this ‘half-full’ attitude, since I’m in for about 5 more months of winter…