Wednesday, September 21, 2011

The Equinox And The Tropics And Me

The whole constellation of Cancer was invisible to my plain eyes in the light polluted skies here south of Chicago. With binoculars I was able to see Delta and Gamma Cancri easily enough. Through my binoculars the Beehive was a just barely visible smudge between the two stars. But through my telescope the Beehive was fantastic. That whole experience made me very thoughtful for some reason.

Just after sundown, Capricornus is visible in the southeast sky. One of my favorite celestial sights is Beta Capricorni. This is a wide binary star that displays remarkable colors. The brighter component is a golden-orange tint, and the dimmer companion is cerulean blue.

Today’s post is a kind of confession, but it’s such a dim-witted, pointless confession I’m almost ashamed to post it.


And I really should save this post for Friday, but I’m thinking about it now so I’m going to do it now.

Finally, this is going to be a strange post in that it is not really empty in terms of content. However, the content is so trivial and meaningless outside of my own subjective world that, again, I’m almost ashamed to post it.


But this is something I think about a lot, so I want to post it. It’s an example of knowing something—for years!—without even knowing that you know it. It’s an example of the difference between intellectual brain knowledge and experiential body-knowledge.

It’s all about the tropics. Here goes.


So this Friday is the autumnal equinox.

It’s the start of fall.

In astronomical terms, Friday is one of the two times during the year when the Sun will be almost directly between the tropic of Cancer and the tropic of Capricorn.

Now here’s the deal: Henry Miller wrote a couple of books, “The Tropic of Cancer” and “The Tropic of Capricorn.” I’ve never read either of those books, but I know a lot of cool people have read those books and liked them a lot. The hipness of the books even made it into pop culture when Rosanna Arquette played a cool, beautiful girl who talks about them in the Martin Scorsese film, “After Hours.”

But that’s not what the deal is with me exactly. My issue is: All my life I’ve been interested in astronomy, but for some reason that eludes me, I never could remember which tropic marked summer and which tropic marked winter.

I knew there were two tropics, of course. And I knew that, in fact, we get the phrase “the tropics” in geography because it refers to the areas north and south of the equator where the Sun appears directly overhead, the north line of latitude and the south line of latitude which mark the extremes of the Sun’s apparent movement (which, of course, is really caused by the axial tilt of the Earth and the Earth’s yearly motion around the Sun).

This might sound crazy, but I knew all this stuff as a kid. I used to hang out at the Adler Planetarium and Northwestern University the way other kids hung out at the street corner.

But I always confused the two labels on the tropics and I always had to look at a map to see which tropic was which. And I knew Henry Miller had written the two books that cool people liked so I figured everyone knew about this tropic stuff but me.

I’m grimacing as I type this because it’s so dumb.

Anyway. At some point, as a movie buff, I learned that Alfred Hitchcock made a film called “Under Capricorn” and I knew that the story takes place in Australia and I knew that Australia was south of the equator so I was able to make this link from an entirely different area of thought to associate the tropic of Capricorn with the Sun moving south during the Northern hemisphere’s winter.

That kind of worked as a mental trick to remember which tropic was which, but it always felt like what programmers call a “kludge”—something that works but something that is inelegant and not really the way something should be implemented. I mean, I was a science buff. I should be able to remember something scientific without having to think about pop films.

Then something wildly unexpected happened.

Over the years of doing this blog, now and then I’ve done astronomy posts. I’ve mentioned checking out Mars passing in front of the Beehive Cluster in Cancer. And I’ve talked about star-hoping through Capricorn and how one of my favorite double stars is in Capricorn.

Here’s what happened.

One night I was standing in my back yard looking at the beautiful, colorful Beta Capricorni. And, just absently, I thought to myself, “Isn’t it strange that Capricorn and Cancer are both Zodiac constellations but Cancer, north of the celestial equator, appears so high in the sky and Capricorn, south of the celestial equator, appears so low in the sky.”

And then it was like one of those moments when someone throws a switch and a light comes on or a Zen monk hits another Zen monk in the head with a bamboo stick.

Of course, I realized, that’s where the tropics come from!

Ancient sailors watched the Sun appear to move north, appear to rise in the sky and then stop around the level of the constellation Cancer. Then sailors watched the Sun appear to go low in the sky and then stop around the level of the constellation Capricorn.

The tropic of Cancer and the tropic of Capricorn. They’re named after the constellations, of course, because one is north of the celestial equator and one is south. And the celestial equator is more or less the projection of the Earth’s equator onto the celestial sphere.

Oh my.

I knew all that stuff when I was a kid. But it wasn’t until I was actually standing in my backyard physically looking at Capricorn (and having recently also physically looked at Cancer) that the knowledge in my head took on real meaning for me and I understood the reality behind the names of those lines of latitude.

This still almost gives me goose-bumps, remembering how the realization touched me so deeply. It was almost a sensation of magic, a connection to the ancient past, to those ancient sailors for whom the sky was so real, for whom the sky provided actual real landmarks that divided and bounded their day-to-day (or night-to-night) world.

And, of course, it became impossible for me to ever forget which tropic was which. It’s not even something anyone has to remember. On any clear night you can go outside and see which tropic is which! (Well, around here you need binoculars to see Cancer, but you know what I mean.)

So this Friday is the autumnal equinox.

The Sun is more or less right above the Earth’s equator. The Sun is in the process of moving south, moving from the tropic of Cancer to the tropic of Capricorn.

It took me almost fifty years to figure it out but: The Sun is moving from the tropic of Cancer to the tropic of Capricorn — Anyone can see that!

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