You may not have noticed dear readers, but it happened just a few hours ago.
At 9:57AM to be exact, my time. Or, if you prefer, three minutes to ten in the morning.
12:57PM in New York.
What happened? Funny you should ask. As the title of this post strongly suggests, at 9:57AM the Earth experienced the Vernal Equinox (Vernal; adjective: of, in, or appropriate to spring. Equinox; noun: "equal night") as the Sun crossed directly over the Earth's equator, thus signaling the end of winter up here in the Northern Hemisphere, and the beginning of spring!
Isn’t it wonderful!
Due to the 23.45 degree tilt of the Earth's axis in relation to the Sun’s ecliptic plane, if you lived in the Southern Hemisphere just the opposite would be true. You would be experiencing your Autumnal Equinox, thus signaling the end of summer, and the beginning of the fall, or autumn. We’ll have our very own Autumnal Equinox in September, September 22nd to be exact, at 7:29 PM... my time.
10:29PM in New York.
2:29AM September 23rd in Timbuktu.
What’s up with that equal night business? Well when the Sun is positioned above the equator, day and night are about equal in length all over the world during the the two equinoxes. This statement is truer the closer you are to the equator, as latitude will contribute to the actual time that the disk of the Sun will appear in your horizon, thus marking the beginning of the day. But for Meivelyn Noemi Cacao Mendoza, the little 7 year old (soon to be a wise 8 year old in exactly 18 days) girl I sponsor through Children’s International, and who lives in Guayaquil, Ecuador, this statement is very true.
As we examined during the Winter Solstice, our planet’s seasons manifest themselves due to that 23.45 tilt, and not the Earth’s distance from the Sun. If the Earth rotated on an axis perpendicular to the plane of the Earth's orbit around the Sun, there would be no variation in day lengths or temperatures throughout the year, and we would not have seasons, which would be sad.
The Earth’s distance from the Sun does indeed vary because the Earth’s orbit around the Sun is not exactly circular, but elliptical, as Johannes Kepler discovered way back in 1605. Our average distance from it is approximately 93 million miles, way too far to walk, even if you wanted to... which you don’t, trust me (A complete orbit of the Earth around the Sun occurs every 365.2563666 mean solar days (1 sidereal year). The orbital speed of the Earth around the Sun averages about 67,108.0888 miles per hour, which is fast enough to cover the 238,900 miles to the Moon in less than 4 hours. And you thought you were sitting still while reading this. Viewed from directly above the north poles of both the Sun and the Earth, the Earth would appear to revolve in a counterclockwise direction about the Sun. From the same vantage point both the Earth and the Sun would appear to rotate in a counterclockwise direction about their respective axes).
Now back to that ellipse. It does have an effect on the severity of the seasons, but over a long time frame. It has absolutely nothing to do with the equinox, yet it’s pretty cool, and does have a perceptible effect on each and every one of us, as it will our children, and our grandchildren, and so on, who will have to contend with a world polluted beyond belief and various degrees of global warming, because we worked so hard in their past to keep making money for the Charles and David Koch and Exxon.
The degree of a planet's orbital ellipse is referred to as its eccentricity. What that means is that there are times of the year when the planet is closer to the sun than at other times. When the planet is closer to the sun, it receives more solar radiation. This theory (some say it’s not a “theory” like that nasty old evolution, but a fact that we can test... scientifically. Just turn on the stove in your kitchen and pretend it’s the Sun. Now pretend your hand is the Earth and move it closer to the hot stove. Pretty soon no doubt you’ll come to the realization that the closer you get to it the hotter your hand will be. See, isn’t science great! (Warning! Don’t do this at home)) is fairly fundamental.
The point at which the Earth passes closest to the sun is called perihelion (mid 17th cent.: alteration of modern Latin perihelium (by substitution of the Greek ending -on ), from Greek peri- ‘around’ + hēlios ‘sun.’), and the point furthest from the sun is called aphelion (mid 17th cent.: alteration of modern Latin aphelium (by substitution of the Greek inflection -on ), from Greek aph' hēlion ‘from the sun.’).
As it happens (“As it is supposed to happen,” Bokonon would say) the shape of the Earth's orbital eccentricity varies over time from being nearly circular (low eccentricity of 0.0034) and mildly elliptical (high eccentricity of 0.058). It takes just about 100,000 years for the Earth to undergo a full cycle. In times of high eccentricity, radiation exposure on the Earth can accordingly fluctuate more wildly between periods of perihelion and aphelion. Those fluctuations are likewise far milder in times of low eccentricity. Currently, the Earth's orbital eccentricity is at about 0.0167, which means its orbit is closer to being at its most circular, which has been advantageous for us humans during our 12,000 years, or so, on this planet as humans.
What many people don't realize, and I certainly didn’t, is that the angle at which the Earth tilts varies according to a 40,000 year cycle. These axial variations are referred to as a planet's obliquity.
For the Earth, the tilt of the axis varies between 22.1 and 24.5 degrees during this 40,000 year cycle. When the tilt is at a higher degree, the seasons can likewise be more severe. Currently the Earth's axial obliquity is at about 23.45 degrees — roughly in the middle of the cycle — and is in a decreasing phase.
If all of this weren’t enough we’ve come to discover that the Earth “wobbles” on it’s axis. This is called the Earth’s precession. This can create a huge difference in the severity of the seasons, depending on whether you live in the Northern or Southern Hemisphere. For instance, if it is summer in the Northern Hemisphere when Earth is in perihelion, then that summer is likely to be more extreme. By comparison, when the Northern Hemisphere instead experiences summer in aphelion, the seasonal contrast will be less severe.
This “wobble” fluctuates on roughly a 21- to 26,000-year cycle. Currently, summer solstice in the Northern Hemisphere happens near aphelion, so the Southern Hemisphere should experience more extreme seasonal contrasts than the Northern Hemisphere, all other factors being equal.
Personally I find it comforting to realize I live on a spinning, tilted, wobbling ball of rock and molten iron, protected by an atmosphere that is relatively as thin as the skin of an apple, circling a massive nuclear fusion reactor, which itself is spinning around a giant black hole in the center of the Milky Way galaxy at 492,125.984 miles per hour, which itself is moving at 1,342,161.78 mph with respect to cosmological frames of reference, toward the great Andromeda Galaxy, which we’ll probably bump into in 3 or 4 billion years or so.
It makes me feel special.
Talking about special, what makes the Vernal Equinox so?
Well it’s the beginning of springtime, so our ancestors could stop all of that partying they’d been doing during the winter, and look forward to growing some more crops, or hunting again to get fresh food, and start planning for the next winter. It’s a pretty important time of the year and has been celebrated throughout the world.
For instance witches (real witches, not those fake ones on “Bewitched,” and ”Charmed”) and other neopagans observe the day as Ostara, a festival that celebrates the season's change from dark winter to brightening spring.
The first day of spring marks the beginning of Nowruz, the Persian New Year. The celebration lasts 13 days, which would be a golden window of opportunity for war freaks like John McCain and Bill Kristol to attack Iran... to stop them from attacking someone with that nuclear bomb they don’t actually have. Get’em while they’re all whooped up on Baklava and yogurt juice.
It is also probably no coincidence that the early Egyptians built the Great Sphinx so that it points directly toward the rising Sun on the day of the Vernal Equinox. A kind of seasonal alarm clock you might say.
The coming of spring is important to those people who believe that Jesus Christ was resurrected 3 days after he was crucified, which a lot do (and we’ll be discussing this at some length very soon), because the celebration of this pivotal event for Christians, called Easter, always falls on the first Sunday after the first full moon after the Vernal Equinox, which will be April 20th this year.
So happy spring, dear readers! We made it! Next up the mighty Summer Solstice, on June 21st!
3:51AM my time.
10:51 in Timbuktu.