I was thinking this morning that a year is real; it’s the period of revolution of the earth around the sun. But New Year’s Eve is an imaginary and arbitrary boundary point in that revolution. That got me wondering if there are any more natural and observable year boundaries.
The solstices and equinoxes are good candidates, but they are kind of secondary to the cycle of the year. They happen because of the tilt of the axis of rotation of the earth. Solstices happen when the tilt of the rotational axis lines up with the radius of the sun to the earth.
If you were watching the earth go around the sun from some place outside the solar system, the solstice would be hard to detect.
Two other important parts of the year cycle are the perihelion (furthest point from earth to sun) and aphelion (nearest point). Because the earth’s orbit is an ellipse, not a circle, it gets closer and farther from the sun during the year. It’s a difference of about 5 million kilometers, or a few percentage points, but it’s noticable. About 6% less solar energy reaches earth at aphelion as at perihelion.
There’s a common misunderstanding that perihelion and the winter solstice are at the same time. It’s not true! They’re independent phenomena.
As far as I can tell, no current or historical calendar uses the perihelion or aphelion as the year boundary. I thought this might be because they’re hard to detect from earth with the naked eye. We didn’t even figure out that the earth’s orbit was elliptical until Kepler’s time in the 1600s.
It might also be because the perihelion and aphelion are totally undependable. They apparently vary by a day or two every year, and not just because our calendar has leap years.
Irritatingly, the perihelion in 2020 will be on January 5; very close to the arbitrary year boundary. This is a real but coincidental hazard to my thesis that our year boundary is completely made up.
In addition to bopping back and forth a day or so, the perihelion gradually moves through the year; in the 1200s it lined up with the December solstice, and in the 6000s it will line up with the March equinox. It’s apparently on a roughly 25,000 year cycle, moving through the calendar year.
This happens because the other planets, moons, and other chunks of rock in the Solar System influence the earth’s orbit. Honestly, I’m not sure I understand how they can wing the earth’s orbit this way and that without changing the period of revolution. I also don’t get why the equinox and solstice cycle isn’t influenced the same way.
But suffice it to say that aphelion/perihelion are bad year boundaries. I think the best we get are the solstices and equinoxes, which although not strictly related to the shape of our elliptical path are at least pretty reliable and observable.
It’s also worth noting that imaginary things can have real meaning and effect. The December 31/January 1 boundary is made up and arbitrary (and there are about 24 of them), but they are powerful enough to make us all chant the numbers from 1 to 10 backwards, and wear funny hats and oversize numerical sunglasses, and kiss and drink champagne.
Happy imaginary boundary to all my family and friends. I hope the fictitious point introduces real and positive change in your real and wonderful lives.