Bad Astronomy: Misconceptions and Misuses Revealed, from Astrology to the Moon Landing "Hoax" - Philip Plait (2002)

Part III. Skies at Night Are Big and Bright

Chapter 12. The Brightest Star: Polaris Just Another Face in the Crowd

 few years ago I was chatting with a friend of mine. The L J night before, he claimed to have seen a bright, slowly moving object in the sky. I realized immediately that he had seen a man-made satellite, but his description confused me. The problem was the way he described where it was in the sky. He said the object was in the west, near the horizon, but he also said it was near Polaris.

"But Polaris isn't in the west," I told him. "It's in the north. And it's well above the horizon."

"Oh, well, the thing I saw was near this really bright star just after sunset," he replied.

Aha! I thought. The bright "star" must have been the planet Venus, which was low in the western sky at dusk at that time of year. Venus was almost painfully bright, far brighter than any other star in the sky, brighter even than most airplanes. He thought it was Polaris; and when I finally figured all this out I realized I had stumbled onto some more bad astronomy.

A lot of people think Polaris is the brightest star in the sky. Let's get this right off the bat: it isn't. Polaris just barely makes it onto the list of the top 50 brightest stars, and, as a matter of fact, it is hard to see if you live in even moderately light-polluted skies. Growing up in suburban Washington, D.C., I could barely see it. If the sky was even a little hazy, which it often is on the east coast of the United States, I couldn't see it at all.

Okay, so Polaris is a bit of a dim bulb. Why, then, is it often mistaken for a powerhouse? I have a theory: people confuse brightness with importance.

Polaris isn't a bright star, but it is an important one. The reason it's important is that it sits very close to the sky's north pole. And to see just why the sky has a north pole at all, we need to do something we've already done a few times in this book: start with the Earth beneath our feet.

The Earth is basically a giant ball. It's also a spinning ball. A sphere sitting all by itself has no real up or down. Nothing on its surface is any different than any other part. But when you spin it, it automatically gets two points that are easily defined: the points where the spin axis intersects the surface. On the Earth, we call these the north and south poles. By definition, the north pole is the point at which, if you are above it looking down, the planet appears to spin counterclockwise. Another interesting place is the line that goes around the Earth halfway between the poles; this is the equator.

Of course you've heard this before, but now comes the fun part. We observe the sky from the Earth, and even though the sky itself isn't spinning, to us it looks like it does because we are spinning. We think of the Sun and the stars as rising and setting during the day and night, but really we are the ones turning around on our giant spinning ball, not the sky. Still, it's convenient to think of the sky as spinning. Ancient astronomers thought the stars were holes in a giant sphere through which shone the light of heaven. Nowadays we know better, but it's still a useful model.

Imagine the sky really is a ball spinning around us. Just like the Earth, then, it has a north pole and a south pole, which we call the north celestial pole, or NCP for short, and the south celestial pole (SCP), to distinguish them from the ones on the Earth. They are reflections of the Earth's own features on the sky. If you were to stand on the Earth's north pole, the north celestial pole would appear to be straight up, directly over your head. The south celestial pole would be straight down, beneath your feet, where you can't see it-there's 13,000 kilometers of spinning planet in the way.

Let's stay at the north pole for awhile (I hope you're dressed warmly). It's nighttime, and you watch the stars. As the Earth turns under your feet, you'll see the sky turn above you. All the stars will appear to make circles over the course of a 24-hour day. Stars near the NCP will make little circles, and stars near the horizon make big ones. All these circles will be centered on the point straight over your head: the NCP.

Can't picture it? Then stand up! Really. Find a room with an overhead lamp, or something in the ceiling you can stand under and use as a reference point. Once there, start spinning, slowlyif you get dizzy you won't be able to read the rest of this chapter. See how the point over your head stays put while you spin? That's because it's your own private NCP. Look at the windows: they appear to make big circles around you as you spin, but that dead spider near the light that you've been meaning to vacuum out for a month appears to make only a little circle.

So it is with the sky. Stars near the NCP make little circles, and stars far from it makes big ones. The NCP takes on a special importance, because all the stars in the sky look like they circle around it. This is true for anywhere on the Earth from which the NCP is visible; that is, anywhere north of the equator. These same arguments are true as well for the SCP. An important thing to know is that since the Earth is spinning, and not just yourself, no matter where you are, the stars go around the NCP while the NCP always hangs in the same spot in the sky. It's like the Earth's axis is a giant arrow, and at the north pole it sticks out of the Earth and always points to the same position in the sky. It's always in the north because no matter where you are on the Earth, the north pole is to the north.

Remember, these places on the sky are just like places on the Earth, but projected into the sky. For me, it's behind an ancient maple tree when I look at the sky from my backyard. For you, it might be next to a building, or over a mountain, or beneath the ledge of the apartment above yours; but it's always there. It never moves.

A long exposure of the night sky reveals the elegant motion of the stars. From our vantage point on the spinning Earth, the stars appear to make circles in the sky. In this picture, taken in Colorado and facing north, the stars in the northern hemisphere arc around Polaris. Note that Polaris is not exactly on the pole, so it too makes a short arc. (Photograph courtesy of Jon Kolb, Adventures in Astrophotography, http://home.datawest.net/ikolb/.)

Now, as it happens, there is a middling bright star near the NCP. You wouldn't give it a second glance if it were anywhere else on the sky, but since this one is near the NCP it never rises and it never sets. All night long this star sits there while other stars get higher or lower in the sky. Wouldn't you think it's important? Think of it this way: before people had satellites, or airplane reconnaissance, or handheld Global Positioning System (GPS) devices, they had to know north from south and east from west. This star took on great importance to them because it showed them which way was north, all night long. Even today, if you get lost in the woods without a compass you'll be glad to see it.

This star has the somewhat unremarkable name of Alpha Ursa Minoris, but due to its proximity to the NCP it has taken on the popular name of Polaris. The star itself is actually rather interesting; it's really a multiple star consisting of at least six stars in orbit around each other. They appear to be one star to us because they are so far away-430 light-years-that all the stars merge into one point of light, the same way that a pair of headlights on an automobile might look like one light from far away.

Polaris is hundreds of light-years away, so the fact that it's near our NCP is simply a coincidence. Just to prove that point, the nearest star to the south celestial pole is the barely visible star Sigma Octans, which is something like the three-thousandth brightest star in the sky. And note that these stars only work for the Earth; from another planet, like Jupiter, Polaris is nowhere near its NCP.

Actually, it's not even precisely on the NCP as seen here on Earth. Currently, Polaris sits about a degree away from the NCP, the equivalent to twice the diameter of the full Moon as seen from the Earth. Still, compared to the whole sky, that's pretty close.

But it's more than just a coincidence in space; it's a coincidence in time as well.

Remember, Polaris is what it is because the Earth's axis points more or less toward it. However, the Earth's axis isn't perfectly fixed in space. As we saw in chapter 5, "A Dash of Seasons," the Earth's axis drifts slowly in space, making a circle roughly a quarter of the sky across every 26,000 years or so. This precession of the axis means that the Earth's north pole changes its position relative to the sky over time. So the fact that it's near Polaris right now is simply a coincidence. Over the years the Earth's pole will move slowly away from Polaris, leaving behind the relatively faint star, demoting it to its proper place among the second-tier stars in the sky.

Worse, in 14,000 years or so, the star Vega will be near the NCP. Vega is the fourth-brightest star in the sky, a shining, brilliantblue gem in the northern summer sky, and very obvious even in light-polluted skies. If people mistake the brightness of a star with its importance now, with the dim Polaris sitting on the throne, then the situation will be far worse when Vega occupies that position.

Until that time off in the distant future, we'll still need Polaris to tell us which way is north, and that's enough to make Polaris important. But it's still not bright, which is why I think people confuse its brilliance-or lack thereof-with its stellar status. Just like people, stars can be important without being terribly bright.