Happy summer solstice! This is the day of the year in the Northern Hemisphere with the longest period of continuous sunlight, which increases with latitude.
On the equator, at 0 degrees latitude, day time and nighttime are pretty much equal all year long. The farther north you go, the longer the period of sunlight. At my latitude, which is about 34 degrees north, we’ll get 14 hours and 26 minutes of sunlight on that day.
When you get to Reykjavik, Iceland, you’ll experience 21 hours and 8 minutes of sunlight plus 2 hours and 52 minutes of twilight, which adds up to 24 hours during which the sun never really sets, and if you make it to the North Pole, you’ll experience a full 24 hours of sunlight.
It’s exactly the opposite in the Southern Hemisphere, where this is the winter solstice for them, and the period of the longest nighttime, which works in a similar fashion to the north but in the opposite direction, with the night getting longer the farther south you go.
Don’t worry, though. In December, the tables are turned, and then it’s the summer solstice in the south and the winter solstice in the north.
The word “solstice” is derived from the Latin word for stopped or stationary, and this comes from what the Sun apparently does. From the time of the winter solstice, the Sun appears to climb higher and higher into the sky and it continues to do so for six months.
Then, on the summer solstice, it reaches its highest point and then appears to pause for about three days before turning around and heading back down toward the equator.
The mid points are the equinoxes, which is when the Sun’s apparent position is crossing the equator, and this is when all points on the planet have day and night time of equal length regardless of hemisphere.
We have two equinoxes, vernal and autumnal, more commonly known as the spring and fall equinox, which mark the beginning of these seasons.
Put all four together in order, and there you have humankind’s original and most basic calendar. Once the pattern had been recorded and recognized, annual events became much more predictable — the change of seasons, the likelihood of seasonal flooding, and so on.
The Moon became a secondary counter and, in fact, we landed on the idea of a lunar calendar long before the solar one. The word “month” is derived from “moon,” and one month is approximately the length of one complete cycle of the Moon’s phases, from one full Moon to another — or one new Moon to another, depending on your culture’s preference.
So why do equinoxes and solstices and seasons happen in the first place? By the very fortunate circumstance that the Earth’s axis is tilted at about 23 degrees relative to the Sun when it’s at either solstice, and the tilt remains constant relative to the Earth.
It’s probably best demonstrated with an animation to make the concept clear, so here you go, courtesy of YouTuber Brad Freese:
From the Sun’s point of view, the Earth keeps tilting back and forth as it goes around. Meanwhile, to the Earth, it looks like the Sun is bobbing up and down. The end result is the same either way: changing period of daylight that are more extreme the closer you get to the poles, seasons, and the origin of the first human calendar.
Now, a year generally has 12 full moons in it but the number of days in a lunar year, 354, is a bit short of the solar year of 365.25 days, so about every two and a half years, we get that elusive (or not so much) Blue Moon, which is just the 13th full moon of a calendar year.
But when you take 12 and divide it by 4 — the number of solstices and equinoxes — you get a nice even number, 3. So divide each solstice or equinox into three “moons,” or months, and all you have to do then is watch the phases of the Moon to know when the seasons will change.
You might have noticed that neither the lunar days nor the solar days quite add up to the number of degrees in a circle, which is 360. But add them together, rounded — 354 + 365 — and you get 719. Divide that by 2 and round it again, and you get 360.
This is actually a really interesting number, because it has so many factors, which are numbers it can be evenly divided by. If we include 1 and 360, there are 24 of them (24 being a factor of 360), and in fact every single digit number except 7 is a factor, which is very useful.
Beyond that, a lot of the numbers relate to units of time we still use now, particularly 12, 15, 24 and 60, and it’s divisible by 10, which makes it very compatible with our common base 10 system.
By the way, the Babylonians, who were big astronomers themselves, used a Base 60 system, in which the number 360 would have been expressed as their equivalent of 60.
It’s interesting to think that the larger parts of our timekeeping system are not as arbitrary as they might seem and have a strong basis in reality. As for how we chose to count weeks and the hours in a day, that’s part is totally arbitrary.
We could have just as easily divided each day into 36 hours of 40 minutes each or 18 hours of 80 minutes each, but we didn’t. Why? Who knows, but the most likely explanation is that 12 hours of “day” and 12 hours of “night” just echoed the annual pattern nicely.
Hours, by the way, were the very last thing to be measured and determined, since if you just divided amount of daylight by 12, the length of the hour itself would change throughout the year The more important markers were sunrise and sunset.
By the way, a word about the image up top. This image is what’s called an “analemma,” and it represents the position of the Sun in the sky over the course of the year, in this case, in the Northern Hemisphere. The points where they cross represent the equinoxes.
Also note that the top half forms a smaller loop than the bottom half. That’s because of a nice quirk of orbital mechanics. The Earth is at its farthest point from the Sun around the beginning of July, right after the northern summer solstice, and at its closest around the beginning of January, right after the northern winter solstice.
So… in July, the planet is actually moving at its slowest because it basically reaching the “top” of the orbit. That is, the Sun’s gravity has flung it as far away as it’s going to get, so it’s now going to slow down and come plunging back. Think of it like throwing a baseball in the air.
Once the Earth has passed through the equinox again, it’s now being pulled in by the Sun, so moves a lot faster in the same period of time.
But what I really wanted to point out was this. Although astronomers insist that “Uranus” is actually pronounced “oo-ra-NOOS” in order to avoid immature jokes, what did they name this image?
Anal Emma. Yeah, those mofos knew exactly what they were doing.
Image source: Giuseppe Donatiello, (CC0), via Wikimedia Commons