That exclamation comes from astronaut David Bowman in Arthur C. Clarke's book 2001: A Space Odyssey. Without realizing it, Bowman may have been quoting Galileo. I can imagine the great Italian scientist uttering those same words 400 years ago when he aimed his little telescope at the Milky Way, the diffuse white ribbon that arches across the night sky and that too many of us never see anymore because of light pollution.
Early cultures knew nothing of the Milky Way's true nature. They called it the Galaxy because, in Greek, galaxias kuklos means "circle of milk," which is what it looks like under a dark, moonless sky. According to Sky & Telescope columnist Fred Schaaf, "For some it was the bridge of the gods, or the path that recently departed souls traveled to heaven. For others it was 'the backbone of night.'" Writing in Sidereus Nuncius (The Starry Messenger) in 1610, Galileo finally settled the matter: "The Milky Way..., with the aid of the spyglass, may be observed so well that all the disputes that for so many generations have vexed philosophers are destroyed by visible certainty, and we are liberated from wordy arguments. For the Galaxy is nothing else than a congeries of innumerable stars distributed in clusters. To whatever region of it you direct your spyglass, an immense number of stars immediately offer themselves to view."
Astronomers now understand that what we see as the Milky Way is the combined light of billions of stars in our galaxy's flattened disk. Our solar system is embedded within that disk, about halfway out from the center to the edge. In northern summer, when we look toward Sagittarius, we're looking in the plane of the disk, toward the densely populated galactic center, so we see the Milky Way at its brightest. In northern winter — which we're just coming out of — we're looking the other way, toward Orion, where the Milky Way is much fainter. I rarely get to see the winter Milky Way, but I was at Kitt Peak National Observatory in Arizona a week ago, and from there — far from the lights of Tucson — the celestial ribbon was truly sublime.
Most participants in 100 Hours of Astronomy events won't get to see the Milky Way because the sky will be too bright, but no matter where you are at night in early April, you can see "stars distributed in clusters," just as Galileo did. Two of the finest star clusters are well placed in the evening sky this time of year. One is making its seasonal exit to the west: the Pleiades, or Seven Sisters, in Taurus. The other is high in the south: Praesepe, or the Beehive, in Cancer.
Both clusters are visible to the unaided eye. Some neophyte observers mistake the Pleiades, upper right of Orion (as seen from the Northern Hemisphere), for the Little Dipper, because its naked-eye stars form a tiny dipper shape. But you can blot the whole cluster out with your little finger held at arm's length, whereas the real Little Dipper, part of the constellation Ursa Minor (the Smaller Bear), spans a full hand-width in the north. Some people see five or six stars in the Pleiades without optical aid, whereas others see as many as eight or nine. Whatever your visual acuity, if you look with binoculars or a small telescope, and you'll see dozens of stars in the cluster. Here's a modern photo of the Pleiades along with Galileo's drawing from Siderius Nuncius.
If you look closely, you shouldn't have too much trouble seeing the correpondence between the photo and the drawing. But why isn't it more obvious? Because Galileo's telescopes had very narrow fields of view, too narrow to encompass the entirety of the nearly 2-degree-wide Pleiades cluster. Thus Galileo had to piece his drawing together one field at a time, so the relative positions of stars aren't recorded as faithfully as in a single wide-field photo.
Now here's the Beehive, also accompanied by one of Galileo's drawings. No matter which way I turn the drawing, I'm never able to see much similarity with photos of the cluster. It's probably a matter of not knowing the scale of Galileo's sketch and not knowing exactly which area of the photo is supposed to match the drawing.
Galileo's discovery of stars invisible to the unaided eye was, like many of his other discoveries, a source of controversy. Why, wondered many of his contemporaries, would the Good Lord create stars that His pinnacle of Creation, Man, couldn't see without having to invent the telescope? What was the point of having stars shine in the sky if nobody before Galileo's time could see them? This made so little sense to so many people that they refused to believe that Galileo's telescopes were revealing new truths about the universe. Instead, these doubters maintained, the telescope was somehow deceiving us.
Four hundred years later, we know better — though there's still plenty about the universe that we don't understand. Keep this in mind as you observe the Pleiades and Praesepe during 100 Hours of Astronomy. Both clusters are splendid sights in binoculars and small telescopes. Both are, after all, full of stars!
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