Due north, halfway up the dome of the clear night sky, glimmers Polaris, the North Star. It’s not very bright; its fame comes instead from its odd coincidence of position. It stands almost exactly above Earth’s north pole. This means that Earth’s axis points almost straight at it, so as Earth turns, it looks to us as if the whole sky is turning around Polaris while we’re standing still. Don’t get dizzy.
That’s what it looks like not just at the North Pole, but anywhere you live in the Northern Hemisphere. In the Boston area, 48 degrees of latitude away from the North Pole, we see Polaris 48 degrees north of our zenith, and the whole sky seems to rotate around that off-kilter point.
Two bright, well-known star patterns decorate the northern sky on either side of Polaris: the Big Dipper and Cassiopeia. As the world turns, they wheel around Polaris as if painted on the inside of an umbrella, if you aimed the shaft of the umbrella at Polaris and turned it once a day. I’ve seen umbrellas decorated with the Dipper and Cassiopeia for just this purpose.
In spring and summer, the Dipper is on the high side of the umbrella during evening and Cassiopeia is low. Now that summer has turned to fall, Cassiopeia is becoming ascendant and the Dipper is heading down toward its low evening haunts for the cold season.
Face northeast after nightfall and look about halfway from horizontal to straight up. Cassiopeia is a zigzag of five stars like a broad W. Its right-hand side is currently tipped up. It’s a little wider than your fist held at arm’s length.
The Big Dipper, meanwhile, has sunk to a lower altitude in the northwest, resting with its bowl to the right and its handle to the left.
Cassiopeia, aside from being a celestial landmark in its own right, guides the way to two astronomical showpieces of the autumn sky: The Great Andromeda Galaxy and the Double Cluster in Perseus.
Both are plain to the naked eye in a dark, moonless sky such as we never experience near the city. But you can log both of these astronomical trophies with binoculars even through a fair amount of urban light pollution.
Look back to Cassiopeia. The W is made of four line segments between stars. Counting down from the top, the third segment currently points almost straight down. Follow it farther down for nearly twice its length. Just a little bit to the right of where you land is the Double Cluster in Perseus.
Seen from my wife’s family’s old fishing camp in the wilds of Maine, the Double Cluster looks like an elongated, extra-bright patch in the Milky Way arching over West Grand Lake. Seen through binoculars just outside Route 128, it’s a pair of glow-puffs with starry speckling. In a 6-inch telescope from a shady spot even in the middle of Newton, it’s a pair of diamond-on-velvet star swarms amazing to behold.
The Andromeda Galaxy is farther off to Cassiopeia’s right. Here’s how to find it:
Follow the top line segment of Cassiopeia downward for nearly six times its length. You’ll hit a star as bright as those of Cassiopeia. Through binoculars, it looks golden yellow.
Sweep upper right from there to another similarly bright star, also golden yellow. From there, follow the smaller line of fainter stars to the upper left, as drawn here.
The galaxy is a dim, woolly gray glow just above the last little star in that final line. The line just fits into the view of an 8-power binocular.
Even if you live under a lot of skyglow, give your eyes a good 15 minutes to adapt to what darkness there is. Or longer if you’ve just been looking into a screen. Computer monitors are notorious among astronomers for killing night vision more effectively than any other kind of light. When you first go out, don’t be surprised if there’s a big rectangle of blankness on the sky where a screen has recently imprinted itself on your eyeballs.
The Double Cluster and the Andromeda Galaxy are examples of what astronomers call “deep-sky objects,” things beyond our solar system that show size and shape, not just unresolvable pinpoints.
In addition to star clusters and galaxies in all their varieties, the third big category of deep-sky objects are the nebulae: luminous clouds of hydrogen, oxygen, nitrogen, and other fluorescent gases. No really bright examples show in the October evening sky, but wait a few months. One of the best is in an easy-to-spot location in Orion, the most dramatic constellation of winter. Watch this space.
Easy-to-use maps of stars and constellations across the entire evening sky are available at SkyandTelescope.com/