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, slowly — if 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
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.
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
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
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
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.
13.
Shadows in the Sky: Eclipses and Sun-Watching
We humans have spent a long, hard time learning that the Earth is not a special place. It’s not at the center of the universe, there are probably millions of planets like it in the Galaxy, and we may not even be the only place with life.
But there is one thing special about our blue home. It’s a coincidence of place as well as time, and it is unique among all the moons and planets in the solar system. The Sun is much bigger than the Moon — about 400 times as big — but it’s also 400 times farther away from us. These two effects cancel each other out, so, from our perspective down here on Earth, the Moon and the Sun appear to be the same size in the sky.
Normally, you’d hardly notice this. For one thing, the Sun is so bright it’s hard to look at, making its size difficult to judge. For another, when the Moon and Sun are near each other in the sky, the Moon is a thin crescent and difficult to see. (Check out chapter 6, “Phase the Nation,” for more about the Moon’s position and shape relative to the Sun.) But there is one time when it’s pretty obvious that they’re the same size, and that’s when the Moon passes directly in front of the Sun. When that happens, the Moon blocks the Sun and we get what’s called a solar eclipse. The eclipse starts small, when only a bit of the Sun gets blocked by the edge of the Moon. But as the Moon’s orbital motion sweeps it around the Earth, more and more of the Sun disappears behind the Moon’s limb. We see the Moon in silhouette, a dark circle slowly covering the Sun. Eventually, the entire disk of the Sun is blocked. When this happens, the sky grows deep blue, almost purple, like at sunset. The temperature drops, birds stop singing, crickets will chirp, and it’s like having a little night in the middle of the day.
This would be odd enough, but at the moment of totality, when the Sun’s disk is completely covered by the Moon, the Sun’s outer atmosphere, called the corona, leaps into view. Normally invisible because the Sun’s surface is vastly brighter, the corona is wispy, ethereal, and surrounds the Sun like a halo or aura. When the corona becomes visible, viewers almost universally gasp in awe and delight, and some have been brought to tears by the sheer beauty of it.
