Thursday, August 18, approximately 11:00 pm CDT
I suppose I should be thankful that my neighborhood streets and sidewalks are so well lit at night. After all, it’s a very safe residential part of the city, not too dense, and even after 10:30 pm on a weeknight, there are people strolling, walking dogs, and even biking. And I was among them on Thursday night, setting out on my first urban skywatch. The safe, well-lit streets, unfortunately, made it a lot harder to find a dark (and I use that term relatively) place to stop and look at the stars.
There are very few open areas with unobstructed views near our house; there are buildings and trees to contend with and the street lamps seem overly harsh. (At least they are going to be replaced gradually.) A small parking lot up on an embankment by the Metra tracks seemed promising, but unsurprisingly that was where the street lamps shone brightest. A path through a community garden also seemed promising, and indeed it was darker, but only because it wound its way under shade trees, which, naturally, blocked the view.
I could see that the Big Dipper was low in the northwest and that the moon (about 3/4 full and waning) had just risen above the buildings in the east. So I decided my best bet was to return home and sit in our backyard.
In the backyard, I could position a chair in such a way that our garage would shade me from the street light in the alley. Furthermore, I could hold up my sketchbook to block our neighbor’s back porch light when necessary (though why it should still be on escaped me). I faced the chair due north and looked up.
My view of the northern sky was pretty narrow; there was only a little space between the houses on my left and the garages abutting the alley on my right. The Big Dipper, which I had seen while I was walking around the neighborhood, was now completely blocked by the houses on my left. So there really weren’t any obviously bright stars in that direction.
Yet there was Polaris, the North Star, exactly where it should be, and a nice reminder of Chicago’s grid system. People expect it to be brighter: “Hey, it’s the North Star!” It’s really much more modest, though. Still, it is easy to spot because there isn’t anything near it. It looks almost as if there were a nearly circular gray void in the sky with a lone point of light at the center.
Because of its position directly above the North Pole and the Earth’s axis of rotation, Polaris is always as high above the horizon (as measured in degrees) as your latitude, provided you are in the Northern Hemisphere. You cannot see Polaris from anywhere in the Southern Hemisphere, as it would always be below the horizon. In Chicago, we are located at about 42 degrees north latitude; so Polaris is almost halfway between the horizon and a point directly above your head.
Polaris is part of the Little Dipper, or Ursa Minor (the Smaller Bear). The Little Dipper is much fainter than the Big Dipper. In fact, the number of stars that you can see in the Little Dipper is an indication of how dark the sky is. Unsurprisingly, I could only see three of the seven stars in the dipper: Polaris (the North Star, at the very end of the handle) and the two stars marking the edge of the dipper bowl at the opposite end. These two stars are called Kochab (the brighter of the two) and Pherkad.
See the star Log for facts about Polaris, Kochab, and Pherkad.
When I looked up the names of those two stars, I was reminded that so many star names are derived from Arabic. There was wonderful astronomy in the Islamic world during the 11th to 15th centuries AD, at a time when Western science lagged behind that of its Middle Eastern and Far Eastern counterparts. Few Americans today would associate the Middle East with astronomy or scientific contribution, for that matter. Yet the Arabic star names live on. So the stories are there, if we are willing to read them.
Pherkad is derived from the Arabic for “calf,” while Kochab’s meaning is unclear. Wikipedia suggests the name may come from a Hebrew or Arabic phrase calling it “the star” or “the northern star.” That is a reminder to me that Polaris’s status as the North Star is not permanent. Earth’s axis precesses, or wobbles like a top, pointing at different directions relative to the far away stars. This movement happens very, very slowly in human terms; the axis completes one circular loop every 26,000 years. So approximately 3000 years ago, Kochab was much closer to the spot above the North Pole than Polaris was. But I’m skeptical this was part of the word origin of Kochab, because the high season of Islamic astronomy was only about 1000 years ago, 2000 years after Kochab was near the pole.
Also see: Part 2 of Night One, where I talk about Cassiopeia.