Sunday, March 13, 2005

Ezekiel's vision

As I ease into retirement, I have taken to spending part of each year on a quiet little island in the Caribbean. I came here looking for winter warmth, of course, but also for dark skies. I am a stargazer by lifelong habit, and my primary home near a major American city is awash in artificial light.

For the time being at least, the island is satisfyingly free of light pollution. Comet Machholz, which passed by recently, was an easy naked-eye object. Back in the States it would have been lost in the pervasive orange glow which passes as a night sky.

Moonless midwinter evenings on the island are ideal for skywatching. Stars in their thousands wheel overhead, from east to west. Or so it seems. It is sometimes said that the art of stargazing is 10 percent perception and 90 percent imagination. I try to imagine myself on the spinning Earth, whirling eastward at 1000 miles per hour, under the fixed stars.

In the west a band of faint light reaches up almost vertically from the place on the horizon where the Sun has set, as if from some enchanted city, just there, over the horizon. This is the time of year when the light is best seen, but only from the darkest places. It is called the zodiacal (zoh-DYE-a-cal) light, and lies along the constellations of the zodiac.

The glow is caused by sunlight reflected from meteoric dust lying in the plane of the solar system, leftover debris from the great flat pancake of whirling dust and gas that gave birth to the solar system 5 billion years ago. We ride on the planet Earth with that cloud of primeval dust, around the Sun once each year, flying at 67,000 miles per hour through the space of the galaxy.

Another band of pale light, the winter Milky Way, arches overhead from north to south, streaming down at Orion's back, the light of billions of stars individually too faint to be visible to the unaided eye, stars that with our Sun are part of the spiraling disk of hundreds of billions of stars we call the Milky Way Galaxy.

The Milky Way Galaxy turns on its ponderous axis once every 200 million years. We reside about two-thirds of the way out from the center of the disk, just on the inside of one of the spiral arms. As the galaxy spins, our Sun is carried with it, at 630,000 miles per hour, in a great circle with a radius of 30,000 light-years.

At this point my head spinning. Wheels within wheels within wheels, like the vision of the prophet Ezekiel.

But my flight isn't over yet. In the northwestern sky, in the constellation Andromeda, I can just make out a blur of light, too fuzzy to be a star, like a smudge on the dark windowpane of night. This is the central part of the Great Andromeda Galaxy, another giant spiral, a near twin of the Milky Way Galaxy and the closest spiral galaxy to our own.

The Andromeda Galaxy is 2 million light-years away. The light that enters my eyes left the there at about the time our first hominid ancestors were appearing in Africa. It is the oldest light that we can see without benefit of a telescope.

The Andromeda Galaxy is part of what is called the Local Group of galaxies. Our galaxy and the Andromeda Galaxy are moving towards each other -- adding another 300,000 miles per hour to my cosmic flight.

And that is about as far as I can go with 10 percent perception and 90 percent imagination. For the last step of my flight I need 100 percent imagination. Beyond the Local Group of galaxies there is no object I can see with the unaided eye.

The Local Group is collapsing, pulled together by the mutual gravity of its members. But the universe as a whole is expanding. The galaxies are flying apart from the impetus of the Big Bang, 13 billion years ago, when the universe was born from an infinitely hot, infinitely small seed of energy. The universe is blowing up like the surface of an inflating balloon, and like dots painted on the balloon, the galaxies are racing apart.

It doesn't make much sense to say what speed the inflation adds to my cosmic flight. Speed with respect to what? I can tell you how fast I am moving away from any particular distant galaxy, but what's the point when there are hundreds of billions of galaxies we can see with our most powerful telescopes.

Galileo got himself into a lot of trouble for suggesting that the Earth moves, and he didn't know the half of it. His opponents said that if we were carried along at 1000 miles per hour on a spinning Earth we would be blown over backwards, tumbling head over heels with everything else that wasn't tied down.

Galileo had to invent a new kind of physics, with a new concept of inertia, to explain why we can stand upright on our breathtaking flight through space, but it took a while before the imaginations of his contemporaries caught up with him.

It still takes a practiced imagination to fly unperplexed on the speeding Earth. On these midwinter evenings, on this dark island, when the zodiacal light reaches up from the western horizon, the Milky Way streams overhead, and the faint smudge in Andromeda is just visible in the northwest, I close my eyes and fly through space in my mind's eye, as Galileo asked us to do all those many years ago.

And while I'm at it, I take time to remember that blind old man kneeling on the floor of the Office of the Inquisition in Rome, having renounced his belief in the double motion of the Earth, whispering -- as legend has it -- under his breath, "And yet it moves."

Further Reading

For more on the universe of both vision and imagination, try my most recent book on the subject, An Intimate Look at the Night Sky. For more "spiritual" mediatations on the dark, see The Soul of the Night: An Astronomical Pilgrimage.

Student Activities

  1. For a really good understanding of our place in the universe, check my calculations for the speeds of our various motions. If you find that I've made a mistake, let me know. Remember, I did some rounding off.

Discuss this essay and more over on the Science Musings Blog.