Almanac: Time Travel

The Andromeda Galaxy in its full glory. It is not quite as impressive with the naked eye, but well worth searching out. Photo: creative commons

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I had the rare luxury last week of watching the stars come out. I walked from the bike path up to a wide-open, east-facing hill off of South East Street near the top of which is a collection of Adirondack chairs that somebody (probably Amherst College) has thoughtfully set out for anyone to enjoy. The sun had set, the sky was clear, and the air was cooling. Dew was already beginning to form on the chair arms as I settled in.

At first, only three things were visible in the part of the sky I could see: brilliant Jupiter in the southeast, fainter Saturn nearby, and low in the northeast the bright yellowish star Capella. Because it was so low, Capella was twinkling most beautifully. Since the light from a star near the horizon passes through more of our atmosphere than when directly overhead, the twinkling effect is more pronounced, often producing flashes of color from every part of the spectrum. I had a pair of binoculars with me, so I pulled them out and intentionally de-focused the image of Capella so that it was a blob rather than a sharp point. This little trick allows you to more easily see the color changes resulting from atmospheric refraction. The blob shifted colors like a celestial disco ball.  

As a kid I learned that stars twinkle but planets do not. Like a lot of what I “learned” this turns out to be only partially true. Stars twinkle because they are so far away they are simply points of light, easily distorted by the atmosphere. Planets, however, being nearer, are actually tiny discs or crescents, which you can only see with telescopes but which, nonetheless, causes their images to resist the distortions of the atmosphere when they’re well above the horizon. When down low, however, planets will twinkle as readily as stars—and can be especially beautiful because some, such as Jupiter and Venus, are so exceptionally bright.

After enjoying the Capella light show for a bit, I swing my binocs over to Jupiter and was able to see three tiny pinpricks in a line on one side of the bright yellow dot of the planet. These were three of the four big Galilean moons, so-called because Galileo was the first person to see them. Galileo’s telescope was roughly as powerful as my binoculars, so I was seeing things roughly as he did, more than 400 years ago. In his notebook Galileo drew pictures of the little pinpricks of light he saw and found that they moved quite rapidly in the course of a single night and from day to day. He realized these must be moons orbiting Jupiter, which violated the reigning, earth-centered picture of the cosmos. This was just one more piece of evidence Galileo used to defend the alternative, sun-centered model of the solar system—for which he was famously placed under house arrest for the last years of his life. 

As the sky darkened, more and more stars appeared allowing me to see the constellation Cassiopeia, which looks like a big “W” that wheels around the North Pole every night. Cassiopeia was an ancient, and very vain, queen. In the sky around Cassiopeia are constellations depicting her husband, King Cepheus, her daughter, Andromeda, and the hero Perseus, who rescued Andromeda from a sea monster, Cetus. The only one of these constellations that I find easy to see is Perseus—the others require both a dark sky and a vivid imagination to see.

Learning to read the night sky is a little like learning a language—it takes time, study, and practice. I’ve been at it for years and still sometimes I look up and can’t quickly get my bearings. That’s because everything in the night sky is moving, not just hour by hour as the earth rotates, but from season to season as the earth orbits the sun. Plus the danged planets follow loopy, complicated paths in the sky due to their own motions and that of the earth. But the job of learning the night sky is easier these days with handy phone apps that allow you to easily identify what you’re seeing in the sky. I use the free version of SkyView, but there are many others.

As darkness deepened I looked for the sight that never fails to set my mind boggling: the Andromeda galaxy. I knew where to look, but could not see it with my naked eyes because the sky was not yet dark enough and, in our area, there is a fair amount of light pollution. I have seen it naked-eye in the pitch black of an Adirondack sky, however…it’s a faint little fuzzy blur that is thoroughly unimpressive except for the fact that it is the most distant thing one can see with one’s eyes alone. 

On a very dark night at this latitude you can see roughly 4000 stars on any given night. All of these are, essentially, right next door in our little corner of the Milky Way galaxy. Some, like the brightest star in the sky, Sirius, are quite close—only about 8.5 light years away. That means it takes light, moving at 186,000 miles per second, about 8.5 years to get from Sirius to your eyeball. Capella is about 43 light years distant. And the farthest, dimmest star you can see on the darkest night would be about 7500 light years away. You can see the smear of our galaxy itself, of course, which contains stars tens of thousands of light years away, but our eyes can’t resolve the smear into individual stars.  

Last week I needed binoculars to see Andromeda…and even then, it was still just a nondescript little fuzz ball. But the photons of light streaming in through the binoculars and into my eyes that night had come to the end of a very long journey. The Andromeda galaxy lies about 2.5 million light years away. That means the photons tickling my retina had left that galaxy 2.5 million years ago. And in all that time nothing had blocked their path, neither other stars, nor cosmic dust and gas, nor black holes—and, finally, they were stopped by some neurons in my retina that got jangled enough to send signals to my brain that I perceived as a fuzzy ball of light.

When we look up at the night sky we are literally looking back in time. We see Jupiter as it was about 12 minutes ago, Sirius as it was 8.5 years ago, and so on. There is no universal “now” in other words. And this applies on Earth as it does in space, but the speed of light is so fast we can ignore the fact that when we look at a distant mountain we are actually seeing it a very tiny fraction of a second ago. Such mind-warping thoughts arise naturally when star gazing, and I enjoy that slightly vertiginous feeling of wonder. But a starry sky is also just deeply beautiful. 

Life is so full and so many things demand our attention that savoring the sky on a clear, moonless night is an uncommon activity. But it’s an elemental pleasure well worth making an effort to experience—especially now that the biting hoards from hell have mostly abated. As a nudge to head out and look up on the next clear night (preferably when the moon is absent) consider this perspective from the wily transcendentalist Ralph Waldo Emerson: 

“If the stars should appear one night in a thousand years, how would humans believe and adore; and preserve for many generations the remembrance of the city of God which had been shown! But every night come out these envoys of beauty, and light the universe with their admonishing smile.” 

Almanac is a regular Indy column of observations, musings, and occasional harangues related to the woods, waters, mountains, and skies of the Pioneer Valley. Please feel free to comment on posts and add your own experiences or observations. 

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3 thoughts on “Almanac: Time Travel

  1. Once again, Steve poetically merges “rhyme” and “reason”!

    For a bit more on the role fine-scale atmospheric turbulence plays, and how averaging over angularly-extended light sources accounts for the differences, in the twinkling of stars and planets, here’s a succinct – and remarkably poetic for Wikipedia – account:

    https://en.wikipedia.org/wiki/Twinkling

    Curiously, that page has a link to the song “Twinkle, Twinkle…” whose octave structure has been used as mathematical mnemonic for one of the deepest mathematical theorems of the past century (the Bott Periodicity* Theorem), whose conclusions could even be learned by young children if they were to be taught this new verse:

    Z2, Z2, 0, Z, 0, 0, 0, Z

    (where Z2 is sung as “zee-two,” 0 as “zero,” and Z as “zee”).

    *For the very, very curious:

    https://en.wikipedia.org/wiki/Bott_periodicity_theorem

  2. Coda: Thus, when Bott penned his Periodicity Theorem, the starlight light Steve was seeing had just passed through an imaginary celestial sphere stretching about 19 light years beyond Capella.

    So sing it again: Z2, Z2, 0, Z, 0, 0, 0, Z [a capella ;-]!

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