Those who know me know that I’m not big on calendar holidays. Even my birthday tends to pass without fanfare. That comes from being single, the island that supposedly no one is. After a lifetime of Christmas and New Years’ being ordinary days, you get used to it.
But I do honor the Solar Event Days because (as I’ve mentioned many times) light is so important to me (and because I’m a geek). Christmas may not mean much to me, but the Winter Solstice does! The days finally start getting longer! Summer Solstice is a day of mourning for the opposite reason.
Today — the Autumnal Equinox — marks the halfway point.
For the past half-year, the days have been longer than the nights. For the next half-year, the nights will be longer. But today they are the same length.At 20:44 UTC, the Earth swings through one of its equinoctial points — specifically through the autumnal point. In a half-year, it will swing through the vernal (spring) point.
As we do, some interesting things are true:
The day-night terminator passes through the poles and is perpendicular to the equator (as shown to the right).
Also, the sun is directly above the Earth’s equator. In the image, I’ve included three “rays” of sunlight. Because the Sun is 93 million miles away, it’s basically a point source, and its light reaches us in parallel rays. Anyone on the surface of the Earth looks back along the rays.
The part I’ve always found interesting is that the terminator is also perpendicular to the sun’s rays. Since the terminator runs exactly north-south, this means the sun’s rays run exactly east-west.
Specifically, if you sight on the rising morning sun during the Equinox, you are looking dead east. If you sight on the setting evening sun, you are looking dead west!Compare this with the situation during the Summer Solstice (image to right) in late June.
Now the northern hemisphere of the Earth is angled towards the sun, which — at noontime — is directly above the Tropic of Cancer.
In fact, that’s what the Tropics of Cancer and Capricorn are: they are the line marking the furthest points north and south that the overhead sun reaches throughout the year. On my Earth images, I’ve included the equator, both Tropics and both Arctic Circles (I’ll return to the Circles below).
The Equinox and the Summer Solstice images provide some insight regarding the long days of summer. Notice how, in the summer, the west coast of North Africa and much of western Europe still receive the sun’s light. It is a late summer evening for them, and there’s still some light for Cricket or Soccer.
But in the Equinox picture above, night has set for all of Africa and Europe!Compare this to the Winter Solstice, and you’ll see that the situation is even worse (for Cricket and Soccer). Now Europe and Northern Africa are well into the night.
(All images use the same time — roughly noontime in the USA. I realize that’s a bit biased. I can make images for whatever part of the world you favor upon request.)
In the winter, the sun (at noon) is directly over the Tropic of Capricorn, and of course our northern winter is summer for our friends below the equator. The situation is completely symmetrical. Everything that is true in the northern hemisphere is true for the southern hemisphere, it’s just a half-year out of phase.
I mentioned the Tropics of Cancer and Capricorn, which mark the furthest points north and south of the overhead sun. The Arctic Circles (there’s an Antarctic Circle around the South Pole) mark the “horizon” of the sun.During the summer, they indicate the line of “midnight sun” — above the line, the sun never sets. During the winter, they are the line of permanent darkness — above the line, the sun never rises!
The image to the right shows the Earth from directly above the North Pole during the Summer Solstice. We’re looking at the entire northern hemisphere, and it’s pretty clear its more in the sunshine than not. This shows again why the days are longer during the summer.
You can’t see it well in the small image, but if you click the image you’ll get a larger version that shows the Arctic Circle more clearly. (The white half-circle arrow indicates the direction of the Earth’s spin — counter-clockwise when viewed from “above” (i.e. north). The Moon and the Earth both orbit their primaries counter-clockwise as well. We live in a decidedly counter-clockwise Solar system, and that may, or may not, explain things.)
I created all these images using POV-Ray, which I’ve mentioned several times previously (here, here and here). The day and night Earth just use cylindrical projections freely available on the ‘web (likewise the Moon). POV-Ray can wrap these images around a sphere resulting in a nice “planet.” (Since you can get cylindrical projects of most Solar system planets, I can also make Jupiter and Mars!)
For my Equinox Project, I even whipped up some “satellites” that I could hang in geosynchronous orbit. I tried to get the relative sizes of the Earth, Moon, and their orbits correct (and I believe I’m very close). My satellites are a bit big (larger than Rhodes Island), but that’s required to see them at all. (They’re big and a bit crude. One of these days I’ll add some solar panels and other goodies.)
I’ll leave you on this fine Autumnal Equinox day (we’ve got blue skies today after several cloudy ones) with an exit shot. Here’s a view you might see on your way out to the stars. One last look back at Home.
Of course,… it’s also what alien invaders might see on their way in!
P.S. You know how people who haven’t been around say, “Well, I haven’t been idle in all this time…” Well, I have been idle —very, very idle — and that’s been great. I keep waiting to get bored, but so far, not so much. But I haven’t been completely unproductive.
I’m building a ballpark!