Each year, around June and December, both hemispheres of the Earth experience the solstice — the longest and/or shortest day or night of the year. This astro-geographic phenomena is really remarkable. You might wonder why it happens, or how is it so predictable; and even what does it mean? Let’s learn more.
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Our Geodesic Position
The solstice is technically defined as either of the two periods in a year — the summer and the winter — of which the sun reaches it’s highest or lowest point in the sky at noon; and its usually marked by longer and shorter days. Like we said, it’s remarkable, yet pretty easy to envision how the solstices happen each year.
Now, imagine you are standing several meters away from a flashlight shining straight on you as you spin a basketball at your fingertips. If the ball is perfectly balanced — that is, if its axis of rotation is perpendicular to the beam of light — then the upper and lower halves of the ball will receive the same amount of light as it spins. But, what if the ball is slightly tilted at it’s axis — at an angle greater or less than 90 degrees — then the ball won’t receive the same amount of light evenly; and either of the two sides will never be totally illuminated.
You’re guessing where all this is going: the tilted basketball is the Earth, and the flashlight is the sun. This is what happens to the Earth and results in a solstice. It’s slightly tilted at the poles to an angle of about 23.5 degrees or so, and as a result sunlight does not shine evenly on both hemispheres. That’s what we term as a solstice. There are two solstices in a year: the summer solstice and the winter solstice.
When Is The Solstices?
The solstices — both the summer and winter — occur on the day that either of the hemispheres is leaning farthest away from the sun. What this means is that, when its the summer solstice in one hemisphere, its the winter solstice in the other hemisphere, and vice versa. This is because the Earth is always tilted at it’s axis in the same direction even as it orbits, the solstices are always six months away from each other, seeing as they come up at halfway points on the revolution of the Earth around the sun.
Technically, the solstice occurs when the sun is directly over the Tropic of Capricorn, or 23.5 degrees south latitude. The winter and summer solstices, the seasons, and the changing length of daylight hours throughout the year are all due to one fact: Earth spins on a tilted axis. The tilt — possibly caused by a massive object colliding with the Earth billions of years ago — means that for half the year, the North Pole is pointed toward the sun (as in the picture below). For the other half of the year, the South Pole gets more light. It’s why we have seasons — thereby having solstices.
In the Northern Hemisphere, peak sunlight usually occurs on June 20, 21, or 22 of any given year — that’s the summer solstice. By contrast, the Southern Hemisphere reaches peak sunlight on December 21, 22, or 23 and the north hits peak darkness — that’s the winter solstice.
Longer Or Shorter; It’s Day And Night
Ever since the Earth had the moon and the liquid oceans, it’s rotation is gradually slowing down over time due to tidal effects. What this means is that, over the eons, the days have been getting steadily longer. Roughly 4.5 billion years ago, it took the Earth just six hours to complete one rotation; and about 350 million years ago, it took 23 hours. Today, of course, it takes 24 hours as you know. And the days will gradually get longer still.
This isn’t the result of tidal effects alone that’s affecting the Earth’s rotation, there are a few countervailing factors. Global warming — rising climate temperatures melting the glaciers — is actually speeding up Earth’s rotation very slightly, shortening the days by a few fractions of a millisecond each year. Likewise, geologic activity in the Earth’s core — earthquakes, ocean currents, and seasonal wind changes — can also speedup or slowdown the planet’s rotation.
Nowadays, the winter solstice has less of an impact on our lives; but historically, it’s been a big deal of religious practices around the world. Stonehenge is a great example of how ancient people had their lives around the longest night of the year — each colossal stone weighed about 22.5 metric tons (25 tons) apiece. Chances are, they were there to catch a glimpse of the sun before the long night. Every solstice, the sun sets directly between two of the stones — that’s almost certainly not a coincidence. Fortunately, most of our modern solstice traditions involve more hot cocoa than 10-meter slabs of stone.
Happy Solstice!
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Written by: Nana Kwadwo, Tue, Jan 29, 2019.
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