Wednesday, February 26, 2014

Pisces for Skeptics

Pisces, from Atlas Coelestis, John Flamsteed
I was born on the Ides of March, the infamous date of the stabbing of Caesar. Call me morbid, but I've always considered it a rather stylish birthdate. Of course, the older I get the more I beware the Ides of March myself. Et tu, birthday?

In any case, my birthdate makes me a Pisces, and that does not strike me as very stylish. Some people have Scorpio, or Aries the war god, or Taurus the bull for a sign, and what do I get? Two fish bound together by a cord. Bor-ing. And Pisces is an unimpressive constellation to look at, too, full of faint, faraway stars.

Still, I've heard about Pisces all my life, and since the Ides of March are drawing nigh once again, I thought I would finally learn something about it. And it turns out that when you take a closer look, Pisces is pretty interesting. Not because it has any influence on my fate or destiny, as the astrologers say. How could it? Most of the stars in the constellation are so far away that the light that left them on the day I was born won't reach Earth until decades after I'm gone. Centuries, in some cases. 

No, what's fascinating about Pisces has nothing to do with me. But does have to do, at least partly, with how people through history have seen it. After all, a constellation is as much as human construction as an astronomical one. If aliens on a distant planet gazed up at the same set of stars, they would see an entirely different pattern, and interpret it differently still. Even other cultures on Earth see it differently. The Chinese, for example, didn't group the stars of Pisces together, except a few of them, which they saw as a fence next to a celestial pigsty. Maybe I should count my blessings that I wasn't born under the sign of the pig fence.

People in the west have seen fish in Pisces at least since the ancient Greeks, and possibly since the Babylonians, who originated many of the constellations we still recognize. In Greek myth, Pisces is associated with Aphrodite and her son Eros. When the awful monster Typhon chased the two of them, they jumped into a river to escape, and were towed to safety by a pair of fish. In other versions of the story, they briefly turned into a pair of fish themselves. Either way, the fish were immortalized by being set amongst the stars.

Pisces is one the twelve constellations of the western zodiac. While most people associate the zodiac with astrology, it has a real basis in astronomy. The solar system is a flat disc, which means the sun, moon, and all the planets seem to follow a single circular path around the sky; a great circle called the ecliptic. The constellations of the zodiac all lie on the ecliptic, so from earth all the bodies of the solar system seem to pass in front of them. People are said to have a certain sign if the sun is passing through that constellation when they're born (well, that's only sort of true, as we'll see). When I was born, for example, the sun was in Pisces.

This convention is a little surprising, if you think about it, because when the sun is in a certain constellation, that constellation appears in the sky in the daytime--which means you can't see it then. The ancients could be pretty sophisticated stargazers. They knew the constellations were still there in the daytime, and that the sun could be passing through one of them even if it's invisible.

Image by Tau Ľolunga
Of course, the sun isn't really going around the earth as it passes through the zodiac. We're going around it. To see why the sun seems to move through the zodiac, its useful to imagine the stars as fixed to the inner surface of an immense sphere surrounding the sun and earth, as in the picture to the right. The sun is in the center, and earth circles it. As we go around the sun, the sun seems to move across the sphere, following the ecliptic (the red line). Imagine grabbing a lamppost and swinging around it. If you look past the lamppost as you swing, it will seem to be moving past all the scenery behind it. The sun seems to move past the constellations for the same reason. The constellation it's "passing through" is the one that's currently on the other side of it from us.

But here's a wrinkle in this neat image: compared to the flat disc of the solar system, the earth is tilted on its axis. That's why we have seasons, as most school kids, and a few adults, know. This tilt means that if you projected the equator onto the celestial sphere (as in the white line above), it would be at an angle to the ecliptic. As the earth circles the sun, the sun seems to be south of the equator for half the year, and north of the equator for the other half. The points where it crosses over are the equinoxes. In the image above, the sun is right at the vernal equinox--which is in Pisces. That's the spot it passes on March 21, the first day of spring.

The funny thing is, the vernal equinox used to be in Aries. Back 2600 years ago or so, the vernal equinox marked the beginning of the zodiac--it was called the First Point of Aries (Aries is the first constellation in the zodiac, and Pisces the last). The reason the equinox has moved in the time since is that the earth wobbles. As it spins on its tilted axis, that axis slowly moves in a circle--just as the axis of a top moves in a slow circle as the top spins. This wobble is called axial precession. Right now, the earth's northern axis points toward the north star, Polaris, which is why all the stars seems to spin around Polaris. But this handy navigational aid is a lucky, and temporary, coincidence. As the axis wobbles through its circle, over a period of about 26,000 years, it points to different spots in the sky. That means it will slowly drift away from Polaris. We will have no north star until it happens to align with a different one.

Precession is the reason the equinoxes slowly move through the zodiac, also over a period of 26,000 years (called the Great Year).  Since ancient times, the vernal equinox has moved from Aries into Pisces, and is making its way toward Aquarius (which is why people talk about the coming "Age of Aquarius"). Precession is also the reason lots of people are wrong about what sign they are. The timetable of the sun's path through the constellations has changed. Back in the day it passed through Pisces between February 19 and March 20, so people born in that period are said to be Pisces. But now the sun passes through Pisces between March 11 and April 18. So, a lot of people who think they're Aries are really Pisces, and a lot of people who think they're Pisces are really Aquarius. I'm a Pisces either way, for whatever that's worth.

Position of the vernal equinox. Image by Kevin Heagen
What about the actual stars that compose Pisces? Many of them were named by Arab astronomers, which is why they have names like Al Rischa ("the cord") and Fum al Samakah ("the mouth of the fish"). The ones with Arabic names are visible to the naked eye (the medieval Arab astronomers who named them didn't have telescopes) but they're mostly faint, because they're far away. Very far away. Al Rischa is actually a binary star--two stars orbiting each other. They're 139 light years away, which means the light I see when I look at them now left the stars themselves 98 years before I was born. If I ever want to see them as they were during my lifetime, I'll need to live to be at least 139. I'm not getting my hopes up. As for Fum al Samakah, it's 492 light years away. Fat chance.

However, there are a few stars in Pisces close enough for their light to have reached Earth in my lifetime. But they're small, dim stars that are hard to see. The closest, Van Maanen's Star, is a mere 14 light years away. It's a tiny little thing, only a little bigger than the earth. But it's impressive in its own way. That little star is more than half as massive as the sun. It's a white dwarf--the unbelievably dense, cooling remnant of an older, much bigger star that grew unstable and cast its outer layers into space. Before it did, it would have spent a few million years as an enormous red giant, hundreds of times as wide as the sun. That would have been an extremely bright star in earthly skies, though no human was around at the time to gaze upon it.

As far away as they are, the stars of Pisces are practically in our laps compared what we find deeper in the constellation, when we look out beyond the Milky Way to other galaxies. The gorgeous spiral galaxy known as Messier 74 is 30 million light years away. Even from that distance, we can see bright red clouds of glowing gas, blue clouds reflecting the light of young giant stars forming in the spirals, and dark dust lanes like black coffee in a swirl of cream. In the bottom left in the image below, we can see a star in Messier 74 dying. It's exploding as a supernova, giving off more energy in a few months than our sun ever will in ten billion years.

Messier 74. ESO/PESSTO/S. Smartt
Far beyond the range of Messier 74 we find stranger things. Over 200 million light years away there's a galaxy known as NGC 383. It's not a spiral, but an elliptical galaxy shaped more like an egg. In visible light it looks like a run-of-the-mill member of a group of other elliptical galaxies (the blue patches in the image to the lef), but when astronomers look at it with radio telescopes, they see hidden fireworks. The galaxy is blasting enormous twin jets of charged particles from a massive black hole at its center. Those particles are moving at nearly the speed of light, and each jet is almost a million light years long--dwarfing the galaxy that produced it. It's quite a show.

But by cosmic standards we really haven't gone that far. If we look back billions of light years, we find a stunning cluster of galaxies with the prosaic name of CL 0024+1654. The yellow galaxies in the center make up the cluster itself. This image contains trillions of stars and probably untold numbers of planets--more than you could count in thousands of lifetimes. And we are seeing it as it was when the earth was young. What's even more remarkable, though, are the warped blue galaxies that surround the cluster. Those aren't part of the cluster--they're actually far behind it. The gravity of the galaxies, and the mysterious dark matter that helps hold them together, is bending spacetime so much that it bends light, acting like an enormous lens. That's what creates the twisted images of the blue galaxies--ghostly images from a much younger universe.

Credit: NASA, ESA,H. Lee & H. Ford (Johns Hopkins U.)
As it turns out, Pisces has a lot more going on than I ever realized. Those two boring fish held some secrets that I find pretty stunning. It's not that they have anything to do with my personality or destiny--I just happened to be born with the sun was passing in front of them. Why should they influence my life, except by expanding my mind when I learn about them? And why should I expect them to? Who do I think I am, anyway? It's true that Pisces contains some of the wonders of the cosmos, but what's great is that they're real wonders, not imaginary ones.