Wednesday Wonders: Equinox

Welcome to September 22, the date in 2021 when the Earth stands up straight and faces the Sun head on. Well, side on. The point is that on this date the Earth’s poles point at exact right angles to the Earth’s orbit around the Sun, so it marks a change of season. If you’re in the northern hemisphere, this is the first day of fall, and days will begin to grow shorter as the north pole starts to tilt away from the Sun. Meanwhile, if you’re in the southern hemisphere, today is the first day of spring, as the south pole starts to tilt toward the Sun. Oh, don’t worry, though. It’s not like the Earth is literally rolling over. The actual angle of its axis is always the same — it’s just its position relative to the Sun that changes, depending upon which quarter of its orbit it’s in. Today, though, magic happens, as the length of day and night everywhere on the planet is the same — but it’s not 12 hours, because a day is not quite 12 hours. Rather, one day is 23 hours and 56 minutes, so on this day each side of the globe receives 11 hours and 58 minutes of daylight. Close to 24, but no cigar. Each day is missing four minutes, and yes, over the year, the time on your clock technically gains because of this, but that is part of why we have leap years and all that shit in the first place. And the fact that the Earth’s axis is tilted and the apparent height of the Sun in the sky plus the length of day changing regularly is probably what led to human civilization in the first place. As soon as people noticed that the spots on the horizon where the Sun rose and set day-to-day changed, and then started to notice how high it did or did not make it into the sky by noon got folk to taking notes. Next up would have been timing the periods between its rise and set over the course of… well, it’s not defined yet. Figuring out the timing might have been tricky when there was no way to actually tell time, so maybe those first experiments just meant tracking the length of an object’s shadow right as the Sun rose and when it set, them recording the length of each one and comparing it to each subsequent day. This wouldn’t give you a length in terms of hours per se, but it could tell you, for example, how long the shadow on a particular day was. Since the Sun doesn’t cast shadows at night, this gives you length of daylight, and you can then compare that length to the length on other days. This will tell you the rate of change per day, as well as give you the relative lengths of longest and shortest days. Repeat the experiment until the pattern starts to repeat — i.e., you hit the same shadow length you started with and see the same lengths appear over the next, say, dozen measurements, and now you suddenly know how many day/night periods there are before the Sun returns to where it started. People may not have understood the concept of orbits yet and probably thought the Earth was at the center of everything, but only because that’s exactly what it does look like from down here, but it did give them a starting point from which to be able to predict the regular course of the Sun. Finally, key these measurements into the seasons, as in when is it cold, when is it time to plant, when does it flood, and when can we harvest? Eventually, over time, ta-da: You’ve created the calendar, and the basic parameters of Earth-Sun dynamics, plus axial tilt dictate the creation of four annual divisions; call them seasons. Of course, some cultures, who tended to not be agricultural, watched the Moon instead, and also divided the year into months (literally named for moons in many different languages) although not seasons. However, since the lunar month wound up much shorter than the solar month after one Earth orbit was divided appropriately, lunar calendars would always lag behind. This is why, for example, the Hebrew calendar has to add in an entire month every so often instead of just a day, and why Jewish and Islamic holidays seem to slip around the Gregorian calendar. Here’s a nice irony, though. The ultimate holiday for once-a-year Catholics, Easter, is itself set based upon a lunar calendar. The date of Easter is set as the first Sunday after the full Moon that occurs on or after the spring equinox. If that equinox is on a Sunday itself, then Easter is the next Sunday. Why this is the case, I have no idea — but events in the Islamic Calendar, as well as things like the Chinese New Year, are also set based on a particular New Moon, although they are all at different times of the year. If you’re part of the early agricultural movement, though — which led to permanent settlements and cities and irrigation, and all kinds of fancy shiz — then your city is probably going to have a nice observatory, but it wouldn’t be bristling with telescopes, which would not be invented for thousands of years. Rather, it would be a prominent building or even a temple-like structure — think Stonehenge — with very specific architectural features designed to align with the movement of the Sun, Moon, and possibly certain stars to become one giant indicator of when those significant dates passed. For example, a slit in one wall might direct sunrise light on the first day of summer onto a specific plinth or marker, or maybe even a mural or statue depicting the god or goddess of the season. Likewise, the same slit would hit a different marker for the first day of winter. Those two are easy because the sun will be at its northernmost point when summer starts and its southernmost when winter starts, which is why you can use the same slit. Spring and fall, being equinoxes, both come in at the same angle, so the light would hit the same place. However, the good news is that if you know which one was the last season, then you know what’s coming, so spring always follows winter and fall comes after summer. Somewhere around these times, these early astronomers may have even figured out the concept of the analemma and begun tracking it. This is the huge figure-8 pattern that the Sun, when recorded at the same time each day (usually noon) makes over the course of a year. Tracking the Moon would probably allow for the ability to predict solar and lunar eclipses — at the time, probably more useful as a political/religious function over anything else. “The Moon is going to die on Tuesday, and it will be your fault unless you pay us tribute and fealty, peons!” Ewww. Out front would be a huge sundial to impress and mystify the populace who, of course, would never be allowed inside such a sacred space. Same as it ever was. Except… We do have access to this knowledge now, and the short version of it is this. The Sun is the center of our Solar System, mostly by virtue of having 99% of its mass, and holding everything else in its thrall via gravity because of that. There are two planets closer to the Sun than Earth — Mercury and Venus — and both of those are barely titled. Mercury is off-axis by barely 3 hundredths of a degree, while Venus is inclined at 2.6 degrees. However, since Mercury is tidally locked with one side boiling and the other frozen, it’s not likely to benefit at all if it suddenly tilted. As for Venus — this is our solar system’s true shit-hole planet, with the hottest temperature and thickest atmosphere, a hellscape where it rains sulfuric acid constantly. If we look at the rest of the solar system, the only planets that come close to having the same axial tilt are Mars (25.19º), Saturn (26.73º) and Neptune (28.32º). Meanwhile, the planet Uranus has the most extreme tilt in the solar system, with its axle rolled over a full 97.77 º, which indicates that, at some point in the past, Uranus must have gotten rammed pretty hard by some asteroid or even a small planet. The end result is that Uranus always keeps its pole pointed at the Sun, alternating between north and south so that its seasons basically travel sideways — although those seasons aren’t much to speak of there, because the surface is pretty much a nearly featureless ball of methane with some lighter clouds in high latitudes (or is that high longitudes?) and, as recent studies have determined, there is also a big, dark spot on Uranus. Of course, we wouldn’t even know about the other planets and stars and so in if we hadn’t started looking up to figure out what was going on with our own Sun and Moon in the first place, and Uranus, Neptune, and Pluto (not a planet) were not even discovered until modern times. Uranus wasn’t even named until the late 18th Century, but the convention of naming planets over Roman gods carried on, and the place got its name at the suggestion of German astronomer Johann Elert Bode. Whether or not he knew about the comedy potential in English of that name choice is anyone’s guess, but officials agreed. And although it’s properly pronounced “OO-ra-noos,” almost no one says it that way, and so stories about the sixth planet are always unintentionally hilarious. And it all started in ancient days, when the first farmers started to pay attention to the change of seasons and tried to learn whether the gods had left them in clues in the heavens above, eventually leading to an understanding of the cosmos that required no gods at all but adhered to its own inexorable set of laws that were an intrinsic property of reality itself. Thanks, farmers! Images: (CC BY-SA 2.0)

Sunday Nibble #66: Putting the Sun on pause

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?

Analemma.

Anal Emma. Yeah, those mofos knew exactly what they were doing.

Image source: Giuseppe Donatiello, (CC0), via Wikimedia Commons

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