Merry-Go-Round of the Sky

All it takes is to look at the night sky more than once over the course of the evening to see the stars sweeping westward. But Heraklitos could not have known the half of it.

You see, the Big Dipper has not always looked like it does now, nor will it retain its present shape for too many more millennia. Here's a picture showing the once and future Dipper.

Or, watch the past 100,000 years and the next 100,000 years in eight seconds!

Knowing this little tidbit isn't essential to naked astronomy – though it is a cool fact to amaze your friends with – but knowing that the stars move is. Or, perhaps it's better to say, it's essential to know that the stars move and that they also only appear to move. Real movement and apparent movement. Knowing the difference between the two is key to understanding why we need so many star charts, or why we need to keep adjusting one of those star wheels. Why can't we just have one sky chart and use it all year 'round? Why does the sky change? Why do we see Orion in the evening sky in the winter, but not in the summer?

We can't observe, in our puny lifespans, the real movement of the individual stars in the Big Dipper. Real movement of the stars requires years of observance to see. Mostly much longer than any of us live. But apparent movement takes only a few minutes. Site a star or constellation against some fixed object — such as a tree or pole or building, any thing that isn't going to move — and come back in ten or fifteen minutes and you'll see the star or constellation has moved a little in relation to your stationary object. You can also do this on two successive nights: look at the star or constellation at exactly the same time on two successive nights and you'll see it's moved a little from where it was at that exact time last night. What's going on?

Everybody knows the Earth rotates on its axis once a day. So far, so good. And eveybody knows that it takes precisely 24 hours to make a complete rotation. And everybody is wrong about that.

It actually takes 23 hours and 56 minutes for the earth to make a complete roatation. That means that a star or constellation appears to rise four minutes earlier every succeeding night. So, if Arcturus rises at 8 o'clock on Sunday night, it will rise at 7:56 on Monday night, and at 7:52 on Tuesday night. So, if you have a relatively flat horizon, and you look due east at 9 pm on January 1st, you'll see the front half of Leo rising just over the horizon. If you come out again on February 1 at 9 pm, you'll see all of Leo and some space between his back haunches and the horizon. If you come out again on March 1, Bootes will be where Leo was on New Year's Night, and Leo will be halfway up the eastern sky. Four minutes a day surely adds up after a couple of months! If the daily rotation of the Earth were exactly 24 hours, much of the fun of gazing at the stars would be gone, because, though the stars would still move westward across the sky every night, it would be the same show every night: the same stars would rise at the same time and in the same part of the sky. You'd get to see changes only if you lived for hundreds or thousands of years. Boring.

The reason our clocks and watches and smart phones don't have this problem is because when people first built time pieces, they based them on the sun, not the stars. And this is one of the reasons why sundials are actually more truthful about what time it is at your house than your clocks are. The two different ways of reckoning time are known as Solar Time and Sidereal Time, and most books on astronomy will devote at least a paragraph or two to the subject. For that reason, and also because I'm a bit on the lazy side, I'm going to recommend that if you want to know more on the topic, consult a work where the ink has already been spilt. For the purposes of naked astronomy, it's quite enough to know that because of this, it's handy to have about twelve star maps per year, one for each month, and most astronomy books will do precisely that.

It's also enough to know that because Sidereal Time and Solar Time are not the same thing, this is why we see Orion on winter evenings but not on summer evenings, and why we see Scorpius in the summer but not in the winter. The ancient Greeks thought it was because Orion and Scorpius fought so much that the gods needed to keep them separated. But they also though that bears have long tails. Which proves that the Greeks, like all of us, weren't quite as smart as they thought they were.

All this seems rather abstract until you see it in action by observing the Big Dipper swinging around a star named Polaris. The best way to do this is to go out every clear evening for the next year and look. Just so we don't have to wait until next year before continuing this conversation, here's a sneak preview:

The star map shows the approximate positions of the Dipper at about 10 pm on the equinoxes and solstices. But, get this: you don't have to wait until December 21st to see the Dipper standing on its handle. You could see it in that position on September 21st, but you'd have to be looking at it around 4 o'clock in the morning. But the even better news is that you can get a taste for this by looking at the Big Dipper every couple of hours on a single night!

This is because the Dipper also makes a complete revolution around Polaris every 24 hours. We just don't get to see the whole circle every day, because the sun comes up and obliterates all the stars. Consider it the price we pay for life on this planet: no sun, no life – at least, not life as we know it. But we do get to see a pretty good swath of the Dipper's circuit every night, and more of that swath in the winter when it's dark most of the time. And, of course, if you go out at the same time every night, or even once every week, you'll notice the Dipper's position has changed a little bit. In fact, if you are the sort who has a good memory for things, you can use the Dipper as a clock. The instructions are here. Or, if you're good with scissors and paper fasteners, you can make a little gadget like this. Having such a gadget might not make you throw your smartphone away, but it will make your friends think differently about you. Not better or worse, necessarily, but differently.

Now, you can make these year-round observations with the Dipper, but not with Orion, nor with Scorpius, nor with Leo the Lion, nor with Taurus the Bull. That's because the Dipper belongs to a very exclusive club known as the Northern Circumpolar Constellations. Only five constellations have membership: Ursa Major, Ursa Minor, Draco, Cassiopeia, and Cepheus (some people also count Camelopardalis, but since it's not an "ancient" constellation, it sometimes gets left off the list). There are eleven members of the Southern Circumpolar Constellations Club. Must be that "southern hospitality" thing. (Here's a chart with membership lists of both clubs.) Being circumpolar (circum="around" + polar="the pole"), they never rise nor set and they stay in the sky all year. All the constellations that are not in this exclusive club rise and set every day, but not all of them are visible in the sky every night of the year. Like birds and tourists, they're seasonal.

That exclusivity is a good thing for us naked astronomers, for it means we get a changing sky throughout the year – not the same ol' boring sky night after night. But it also means there are enough old friends there every night that we have some help finding our way around!

So, the Circumpolars are always above the horizon, night and day. But every night, they're in a different position. However, the constellations closer to the horizon are not above the horizon all day and all night, and some nights they're not there at all. You can get a hint of why this is so by standing under an overhead light and turning around in a circle. Keep the light directly overhead and watch it as you turn. The light is always in your view, though its position relative to the top of your head changes as you rotate in place. Your face is the Pole (North or South, depending on your hemisphere) and your navel is your horizon. As you turn, notice that your navel faces one wall for part of the turn, then another wall, and another and another until you return to the same position as when you began. If you were looking out of your navel, instead of your eyes, then the view would change dramatically. Each turn is like a year, and each wall is a season. Just as you would see the different walls out of your navel each quarter turn, we see a different part of the sky in each season. Don't turn too many years in a row or you'll get dizzy.

And here's where the analogy breaks down. It was really meant to see the difference between the Circumpolar constellations and the other 80-some. But the real reason the night sky changes with the seasons has to do with the orbit of the Earth around the sun, as well as its rotation on its axis. This gets a little more involved than we need for present purposes. But now that I've gone and brought up the topic, you probably won't get to sleep tonight until you get an answer. So, for the sake of your health and your performance at your duties tomorrow, take one minute and 51 seconds for a quick and dirty layperson's explanation in this video by a real scientist. Just don't take her examples of which constellations you'd see each season too literally. She's being a little loosey-goosey, because it's really beside the point. My kinda gal!

Whew! All that, from one constellation. And there's more that the Big Dipper has to tell us yet! But it's all academic until you start to make your own observations. That's the difference between a naked academic and a naked astronomer. So go out and find the Big Dipper, get really familiar with its mysterious ways. Train yourself to find the North Star, Polaris, and to trace the fainter stars in the Little Dipper.

Now we know the Pointer Sisters, Dubhe and Merak, who show us the way to the Little Dipper. And the Little Dipper points right back to the Big Dipper. But if that's all we know about pointer stars, we're done with the sky, because we're caught in a loop between two constellations. But we're in luck. Just as there are three members of the singing group called the Pointer Sisters, there is a third Pointer Sister in The Big Dipper that will have us off and running across the vault of heaven. Her name is Alkaid.