I was going to write a one-sentence, spring-themed message, wishing everyone a happy equinox and pointing out that Melbourne is probably experiencing roughly what we’re feeling in Northern California. Then, it turned out much of what I knew about the equinoxes was wrong.
The earth is not round.
It doesn’t move in a circle around the sun.
Day and Night are not equal to each other.
The North is not treated the same as the South.
Since I was This Old when I learned The Truth, I will share it with you, appropriately, on the northern vernal equinox, i.e. today, March 20th, 2025. There will be science, although I will not discuss the ecliptic because I have hard time visualizing it. I promise there will be no arithmetic. There will be geometry. And you might find yourself wobbling a little, which will be in keeping with the situation.
The Classic Equinox
As you may have learned in grade school, the earth circles the sun, and one full orbit makes a year. The earth is a bit tilted on an axis, about 23 degrees. The reason we have summer in the north is because the tilt part is closer to the sun. We have winter when that tilt is away from the sun.
Yeah, yeah, you know all this. Do you know WHY the earth is tilted, that is, why its axis of spin is at an angle to its orbit of spin around the sun? I never thought about that until today, and until five minutes ago, did not know that a monster planet Theia crashed into the earth, blasting off the Moon, and causing us to start our wobble in that acutely odd direction. This is not the plot of a season “The Umbrella Academy”; there really is a scientifically valid Theia Impact Hypothesis.
You may also have noticed that when you look at any of these graphic pictures of the earth, you never see Australia. Ninety-nine percent of all the pictures I sifted through for this post showed me the Western Hemisphere–even when I asked for Australia’s point of view. So if I were a first-grader, I’d assume that Australia never faced the sun. Or was always hiding, maybe a little ticked off. Today, I learned why.
By the way, when I was looking for a graphic to illustrate the equinox, this cute and colorful one popped up quite often. The circling part, as I will explain in a minute, is really incorrect. But before I get there, notice anything weird about this one? It doesn’t show you the western hemisphere. Instead, you can clearly see the top of Russia and the Arctic circle. This view looks directly at the North Pole, and it’s fine to do that. North is not up, and the United States is not the only thing on the earth. However, in this graphic, the metaphorical axis is still stuck in the earth, but in the wrong place, piercing the equators instead of the poles. Imagine if the Sahara were the North Pole… If you want to get first graders really confused, use this graphic!
But I digress. The other standard thing taught about the equinoxes is that it’s the occurrence when day and night are equal. Turns out that ALSO is not true.
Thursday, March 20:Vernal Equinox
from astrology site Star World News
Day and night of equal length today. More light falling on the Northern Hemisphere now, increasing day light time. In this celestial schema, the North’s gain is the Southern Hemisphere’s loss.
The Force Is Not in Balance
You see it repeated all over that the equi- part of the -nox means day and night are balanced. However, that isn’t the case. *GASP*
This is because the sun isn’t a single point but a disk, and because sunlight is refracted through the atmosphere when it appears. In combination, it means that the amount of sunlight that seems to occur is distorted a little. This site explains the gory details.
If the equinox today is not when day and night are equal, does that still happen? Why, yes! It’s called the equilux, and it happens a few days either before or after the equinox. The equilux is the day when day and night are roughly equal, and because of the sun-disk-refraction stuff that I mentioned above, the day of equilux is different at different latitudes/hemispheres. Today in Northern California, there will be 12 hours and 10 minutes of daylight, whereas three days ago, there was 12 hours of daylight. In Australia, that will be in a few days.
Hey, I didn’t make the rules, that was Theia.
Word for the Day: Perihelion
Maybe you’re thinking the above was hairsplitting. So what if we have an extra 10 minutes of daylight on the equinox, the day when the axis is neither tilted toward or away from the sun? The other thing I learned today was more startling. I had heard that the earth’s rotation around the sun is not a circle but an ellipse. And yet, this view of the explanation of the equinoxes seemed kind of nutso when I looked at the diagram.
The earth’s axis is still tilted, and that DOES account for the difference between summer and winter. But there is also this other thing caused by the earth going around in an ellipse. When the earth is closest to the sun–called the perihelion–the sun seems a bit larger (3%) in the sky. That happens in January, dead winter for the north, less sunlight but bigger-appearing sun. Cray cray!
And not just cray cray to look at because here’s what happens, and I’m going to let the jolly folks at thesuntoday.org explain it:
During Northern Hemisphere summer, Earth is at aphelion and moves more slowly in its orbit. This results in a longer summer and shorter winter.
During Southern Hemisphere summer, Earth is at perihelion and moves more quickly in its orbit, leading to a shorter summer and longer winter.
Due to these orbital dynamics, the Northern Hemisphere summer is approximately 4 days longer than the Southern Hemisphere summer.
Since the earth moves in an ellipse AND is tilted (Theia + boom), San Francisco gets four more days of summer than Melbourne. Why am I comparative San Francisco to Melbourne? Because they’re both at roughly 37.8 latitude, equidistant from the equator. However, northern California gets more days of summer. They used to say that the sun never set on the British Empire, but it set a little sooner in some parts of the empire than others. Too bad for you, India and Australia, you get less sunlight!
Man, if I was the Southern Hemisphere, I’d be kind p’oed. I mean, no wonder they don’t want to show their faces on the maps!
Plus the Wobble, which Leads to the Age of Aquarius
If you got this far, I might as well add a little more complexity to this increasingly weird view of the equinox. I wrote about this before, but there’s this thing called the precession of the equinoxes. The earth is not only not going in a circle and not treating the hemispheres equally, but it’s also not round, i.e. not spherical. It bulges.
The earth started spinning because Theia/BOOM, but although billions of years have made it pretty spherical, it’s still not perfectly so AND it has things on it. As it spins, those things and its slight asymmetricality (that is TOO a word!) make it wobble. What things you ask? Oh, continents, water, molten lava, fracking, that kind of stuff.
To quote everybody else on precession of the equinoxes, think of a spinning top that wobbles. Only the wobble is not because it’s slowing down, just the bulge. That “wobble” means the imaginary axis will itself shift over time. It points in a different direction, so that our north star Polaris right now, was not the north star 2000 or 4000 years ago.
The way the astrologers explain this is that the Sun at the time of the vernal equinox is in a zodiac sign, i.e. one of the 12 artificial divisions of the sky. The spring sun has been rising in Pisces since about the time Jesus was born and at some point (or maybe it’s already happened, scientists don’t agree) will move into Aquarius. Astrologers have long explanations as to why this caused bull jumping in Crete (Age of Taurus) and created the dawn of philosophy in Greece (Age of Aries). The Greeks, by the way, did figure this out, so the Age of Aries seemed to make them pretty smart.
What I really can’t figure out is why, on the meticulously created graphic for the precession above, they’ve put the sign for Aries stuck on Aquarius and the sign for Libra stuck on Leo. This graphic, too, is repeated everywhere, so I could not find any explanation for this.
In other words, every time I try to look at one of these damn universally distributed graphics, there’s something wrong with it.
So happy equinox, even though it’s not the equilux, we might be moving into Aquarius or Aries, who knows, we’re doing the wobble, and I get more sunlight here than the opposite side of the earth. Sorry, Australia. Send your complaints to Theia.
Thanks for making learning fun!