The Star-Formerly-Known-as-Pole

Snaking along the long body of Draco the Dragon from his head, which we examined in last week’s post, we eventually come to a fairly faint naked-eye star opposite the bowl of the Little Dipper. This is the notable star Thuban.

The name Thuban (THOO-bahn) comes from the Arabic for serpent. Because of the star’s importance to ancient cultures, however, it has born many other names: “Judge of Heaven,” “Proclaimer of Light,” “Crown of Heaven,” and “High One of the Enclosure of Life,” to name a few.

Looking north to Draco and the Dippers
Star maps created with Your Sky

Thuban’s ancient significance stemmed from the fact that it was once the North Star, marking the position of the North Celestial Pole. The North Celestial Pole is the imaginary fixed point in the sky that the Earth's axis would intersect, were it extended from the North Pole into space. The North Star, because it marks geographic north, has been important throughout the ages for navigation.

We are accustomed to the star Polaris, which marks the end of the handle of the Little Dipper, being our North Star. It seems odd at first to think that another star might have been the North Star or Pole Star long ago. In fact, Polaris won’t continue to be our North Star forever. This phenomenon is caused by precession, the change in the alignment of Earth’s axis.

Because the spinning Earth wobbles slowly over time, our axis doesn’t always point at the same spot in the sky. This wobble is caused by gravitational tugging by both the Sun and the Moon. The result is that the position of the North Celestial Pole--where Earth’s axis points--moves over time against the backdrop of the stars, completing a circle in about 26,000 years. The North Celestial Pole will pass closest to Polaris around the year 2100, after which it will begin to move away from it.

The good news is, if we wait just 26,000 years, Polaris will be our North Star once more. What goes around, comes around.

Astronomy Essential: Space is extremely cold.

Intergalactic space, the regions of space that lie between galaxies, is an inhospitable minus 455 degrees Fahrenheit.

Compare that to absolute zero, the coldest theoretical temperature, which is minus 459.67 degrees Fahrenheit, not much colder.

Kids, More Kids, and Something Fishy

Last week we located the constellation of Auriga the Charioteer and explored its central asterism (recognizable star pattern), the Pentagon.

Now let’s take a closer look and see if we can spot three smaller, quirkier asterisms.

Auriga the Charioteer in Johann Bode’s 1801 star atlas
Courtesy of Linda Hall Library of Science, Engineering and Technology

1) About an hour after sunset, locate the Pentagon. Consult last week’s post if you need help finding it.

The Pentagon of Auriga
Star maps created with Your Sky

2) Just south of bright Capella is a small triangle of stars. This is the asterism known as the Kids, a reference to the two baby goats being cradled by the ancient figure of the Charioteer. The three stars are all dimmer than any of the five stars of the Pentagon.

The brightest of the three is Almaaz, which is Arabic for he-goat. (Do you recall which star in Auriga has a name that means she-goat?) Almaaz is a yellow-white supergiant that undoubtedly will expire someday in a cataclysmic explosion called a supernova.

The other two dimmer stars of the Kids asterism are called the Haedi, Latin for the kids.

Asterisms in Auriga

3) Hoofing it across the Pentagon, south of Menkalinan, look for another small triangle of stars that resembles the Kids, if the Kids were dimmer and upside down. This is the asterism called the Little Kids.

4) Finally, look inside the Pentagon, down toward its “base.” If you’re in a dark location, free of light pollution, you’ll see an elongated patch of light. This is the combined light from a string of four naked-eye stars plus a couple dimmer ones, and this is the asterism known as the Minnow. Can you see the silvery flash of its scales as it navigates the currents of the Milky Way?






Astronomy Essential: Polaris, the North Star, is NOT the brightest star in the night sky.

Many beginning stargazers have trouble picking out Polaris (poe-LAIR-uss) because they figure the North Star must be really bright and flashy. But on the scale of cosmic bling, it’s not much of a sparkler. In fact, it’s number 48 on the brightest-stars hit parade.

The importance of Polaris lies in its location, not its magnitude of brightness. It lies very close to the North Celestial Pole, the imaginary fixed point in the sky that the Earth’s axis would intersect, were it extended from the North Pole into space. Therefore, if you locate Polaris and face it, you’ll be facing geographic north, also known as true north.

To learn how to locate Polaris using the Big Dipper, visit this post.

By the way, the brightest star in the night sky is Sirius, aka the Dog Star, found in the winter constellation of Canis Major, the Big Dog.

The Little Dipper

Now that you’re well acquainted with the Big Dipper and can use it to find the North Star, let’s visit its circumpolar companion, the Little Dipper. The Little Dipper is the defining asterism (recognizable star pattern) of the constellation Ursa Minor, the Little Bear. As discussed in an earlier post, circumpolar refers to stars that appear to circle the North Celestial Pole.

In the Northern Hemisphere, we’re fortunate to have a naked-eye star marking the position of our Celestial Pole. That all-important star is Polaris, the North Star. Also known as the Pole Star, the Lodestar, and the Steering Star, Polaris has been used from antiquity through modern times to point the way north. The seafaring Phoenicians may have been the first culture to rely heavily on its nearly fixed position in the sky to aid them in navigation.




I say “nearly fixed” because if you were to observe Polaris over a 24-hour period, you would discover that it appears to move, inscribing a very small circle in the sky. This is because Polaris and the North Celestial Pole are not one and the same. Polaris simply happens to be the star that’s currently closest to the North Celestial Pole, that spot in the sky where the Earth’s axis points. But Polaris does not line up precisely with the Pole. Will this small inconsistency matter when you’re lost in the woods and you’re using that old reliable, the North Star, to find your way out? Of course not!

Now that you’re feeling confident again that Polaris is as solid as the Rock of Gibraltar, I’ve got to pull the celestial rug out from under you. Polaris wasn’t always the North Star, and it won’t continue to be the North Star! You see, our planet may spin reliably on its axis 24/7, like a top, but that top has developed a bit of a wobble.

Precession of Earth’s axis
Image by Robert Simmon, NASA GSFC

The gravitational pull of both the Sun and Moon on planet Earth has caused a phenomenon called precession. Precession is the change in the alignment of Earth’s axis. Because our spinning top is wobbling, our axis doesn’t always point in the same direction. The result is that the position of the North Celestial Pole--where Earth’s axis points--moves over time against the backdrop of the stars, completing a circle in about 26,000 years. The North Celestial Pole will pass closest to Polaris around the year 2100, after which it will begin to move away from it, toward the stars of the constellation Cepheus the King. The good news is, if we wait just 26,000 years, Polaris will be our North Star once more. What goes around, comes around.

Our current Steering Star marks the end of the handle of the Little Dipper. Like the Big Dipper, the Little Dipper has seven stars. But with four of its stars dimmer than the dimmest star in the Big Dipper, it is more challenging to see. Let‘s try to pick an observing site without a lot of light pollution, OK?

1) You’ll need to face north, so if you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your left shoulder to the west, and you’ll be facing approximately north.

2) Wait at least one hour after sunset to begin observing, so that twilight’s over and your sky’s good and dark. During this observing week, the Moon won’t rise to interfere with your stargazing until well after midnight. New Moon, when the Moon doesn‘t appear at all in the night sky, occurs on Wednesday, July 2.

3) Locate the Big Dipper and use the Pointer Stars to locate Polaris. If you need a refresher, review the earlier post.

4) The Little Dipper will be oriented as if it’s balancing on its handle, with Polaris at its southernmost point. Notice the Little Dipper’s size relative to the Big Dipper. Notice also the orientation of the handle and the bowl; they’re reversed from the Big Dipper. Finally, notice the curve of the Little Dipper’s handle relative to its bowl; it is the opposite of the Big Dipper’s handle curve. Noting these differences now may aid you in tracing the Little Dipper asterism when sky conditions aren’t so good.

The Little Dipper
Chart created with Your Sky

5) After Polaris, the next star in the handle is Yildun (pronounced yill DUNN). Yildun is from the Turkish for star. It’s a fast-spinning but otherwise ordinary white star. The third star in the handle has no traditional name, so we simply call it Epsilon, its star catalog designation. Epsilon (pronounced EPP sill ahn) is a dying star, preparing to swell into a red giant.

6) With the Little Dipper oriented right side up, the top left bowl star is Alifa al Farkadain, a white dwarf star. Alifa al Farkadain is Arabic for the dim one of the two calves. The bottom left bowl star is Anwar al Farkadain, a yellowish-white dwarf. Anwar al Farkadain is Arabic for the bright one of the two calves. The names are a bit confusing, as Anwar is actually the dimmer of the two stars, as well as the dimmest of the Little Dipper’s seven stars.

7) The bottom right and top right bowl stars are Pherkad and Kochab, collectively known as the Guardians of the Pole, because they endlessly circle Polaris counterclockwise. Pherkad (pronounced FIRK uhd) is from the Arabic for two calves. Pherkad is a white giant, the hotter and more luminous of the two Guardians. Kochab (pronounced KOH kabb) is from the Arabic for star. This orange giant star was probably considered the North Star around 1100 BCE, when the North Celestial Pole drew close to it.

It’s comforting to know that, while Polaris shines on as our beloved guiding star, it has two beefy bodyguards who are always on duty, relentlessly sweeping the perimeter in a tight, protective circle.

On thy unaltering blaze
The half-wrecked mariner, his compass lost,
Fixes his steady gaze,
And steers, undoubting, to the friendly coast;
And they who stray in perilous wastes, by night,
Are glad when thou dost shine to guide their footsteps right.

~ from “Hymn to the North Star” by William Cullen Bryant

Follow the Drinking Gourd

Two weeks ago, we used the most famous star pattern in the night sky to locate an exceptional deep-sky object that lies outside our Milky Way galaxy. This week, let’s return to our galactic ‘hood and take a closer look at the notable star pattern we call the Big Dipper.

Known to every child in the Northern Hemisphere, the Big Dipper may be the most ancient of all star patterns recognized and recorded by humans. The Big Dipper is not a constellation, as many believe. The Big Dipper is the central asterism (recognizable star pattern) in the constellation Ursa Major, the Great Bear.

Ursa Major is a circumpolar constellation. In the Northern Hemisphere, a circumpolar constellation is one that circles the North Celestial Pole, the imaginary fixed point in the sky that the Earth‘s axis would intersect, were it extended from the North Pole into space. That imaginary point just happens to be extremely close to the star Polaris, which is why we call Polaris the North Star. We can use the Big Dipper to locate the North Star and geographic north; more on that in a minute.

Diagram of celestial poles by Dr. Guy Worthey

For skywatchers in latitudes above 35 degrees north, circumpolar constellations never set; they are always above the horizon as they endlessly circle the North Star counterclockwise, making one revolution per day. But although we commonly say they are "circling," constellations like Ursa Major are not really moving around the Pole. Their apparent motion--or the way they appear to move in our sky--is actually caused by our rotation, as planet Earth reliably spins 24/7 on its axis.

The Big Dipper’s proximity to the North Star has made it a favorite of travelers and navigators throughout history--a reliable guide in the night sky when plotting one’s course across land, water, or air. Let’s get oriented, shall we?

1) You’ll need to face north, so if you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your left shoulder to the west, and you’ll be facing approximately north.

2) Wait at least one hour after sunset to begin observing, so that twilight’s over and your sky’s good and dark. A waxing (growing) crescent moon will be in the western sky after sunset through the weekend. First Quarter Moon, when the Moon’s face is half illuminated, occurs on Tuesday, June 10. The earlier in this observing week you look at the Dipper, the less interference you’ll have from moonlight. Or, if you’re a night owl, you can simply wait until the Moon sets.

3) Facing north, look for a distinctive seven-star asterism that looks like a giant, long-handled saucepan. It will be due north to a little west of north and somewhere between the zenith, the point directly above your head, and the northern horizon. The saucepan will be oriented upside-down. Found it? Great. This is the famous Big Dipper (water dipper), or as the French call it, “La Casserole” (saucepan).

Chart created with Your Sky

4) All seven stars of the Dipper are bright enough to be seen even in urban areas that are somewhat light polluted. In a future post, we’ll take a closer look at the individual stars.

5) Make a point of looking at the Big Dipper early in the evening and noting its orientation. Then look at it again before you turn in for the evening, or if you get up before sunrise, look at it again before dawn lights the sky. Do you see how its orientation has changed, how it has rotated counterclockwise and is approaching right-side-up? It’s exciting when you first realize how the stars above the North Pole will 'move' in a predictable, circular motion every night of the year, simply because we live on a spinning globe.

They are all ‘moving’ around the North Star. You can locate Polaris using the two stars on the right side of the right-side-up Dipper’s bowl. These two stars are called the pointer stars, because they point the way to Polaris.

Chart created with Your Sky

6) Draw an imaginary line connecting the two pointer stars. Then extend that line about five times its length, above the right-side-up Dipper. The first fairly bright star you come to is Polaris; it’s slightly dimmer than the top pointer star. Beginning stargazers are often surprised to learn that the North Star isn’t one of the brightest stars in the sky. Its importance to us is its location, not its magnitude of brightness. Now that you’ve found Polaris, face it square and you will be facing geographic north, also known as true north.

Our ability to recognize the Big Dipper enables us to reliably find north at night--without a compass, a map, location familiarity, or knowledge of where the sun set earlier. This is a celestial landmark that might even spell the difference between life and death, or captivity and freedom. According to American folklore, runaway slaves traveling under cover of night oriented to the Big Dipper and followed it north to freedom. They called it the “Drinking Gourd,” a reference to the hollowed-out gourd used in rural areas as a water dipper.

A hauntingly beautiful folk song of uncertain origin called “Follow the Drinking Gourd” may have been used by the Underground Railroad to provide coded directions in its lyrics, guiding the slaves to freedom. Click here to play an MP3 of “Follow the Drinking Gourd,” courtesy of Roger McGuinn’s Folk Den.

When the sun comes back and the first quail calls,
Follow the Drinking Gourd.
For the old man's waiting for to carry you to freedom,
If you follow the Drinking Gourd.

Reward notice for runaway slaves Harriet Tubman and her brothers