Thursday, June 26, 2008

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

Thursday, June 19, 2008

The Big Dipper Revisited

A couple weeks ago, we located that big drinking vessel in the sky, the Big Dipper. Then we used it to find Polaris, the North Star. This week, let’s take a closer look at the stars of the Big Dipper, both as a group and as individuals.

Most skywatchers know that the famous Big Dipper asterism (recognizable star pattern) is composed of seven stars. What you may not know is that the central five stars are related--what we could call a family group. Collectively they’re known as the Ursa Major Cluster: “Ursa Major” after the constellation that contains the Dipper, Ursa Major the Great Bear, and “Cluster” because they constitute a star cluster, a group of stars that formed around the same time in the same nebula or gas cloud. Astronomers have been able to determine that these five stars are a cluster because of their similar distance from us and the similar direction of their motion through space. All five are around 80 light years from Earth. A light year is the distance light travels in one Earth year, nearly six trillion miles.

Ursa Major Cluster
Chart created with Your Sky

If the Ursa Major Cluster is a family group, then the Big Dipper is an extended family. Let’s get to know each member a little better.

1) If you’re unfamiliar with the location and circular motion of the Big Dipper, read my earlier post.

2) The Summer Solstice on Friday, June 20, is the longest day of the year and, consequently, the shortest night. But even the shortest night gives us plenty of quality time with the Dipper. The bright Moon won’t rise until around two hours after sunset on Friday night and a bit later each subsequent night of our observing week.

Stars of the Big Dipper
Chart created with Your Sky

3) Moving from left to right along the right-side-up Dipper, we’ll start with the three stars of the handle and finish with the four stars of the bowl. The end star of the handle is Alkaid (pronounced AL-kayd). Alkaid, from the Arabic for leader, is a hot, blue-white star six times the size of our Sun.

4) The middle ‘star’ of the handle is probably the best known, because it is a naked-eye double star referred to as the Horse and Rider. The brighter star of the pair is Mizar, and its dimmer companion is Alcor. Mizar and Alcor are reported to have been used by the Romans and the Arabs as a vision test, in the days before they had eye charts with the big “E” on top. Now test your vision: can you see both stars? They are quite close together, and the light from dim Alcor blends together with Mizar’s when you look quickly. If you’re having trouble seeing the double, try using averted vision by looking at a point in space right next to Mizar. Sometimes not looking directly at the main star will help the companion star pop into view. This occurs because our peripheral vision is better than our straight-ahead vision.

Mizar (pronounced MY zahr) is from the Arabic for groin (of the bear). With a telescope, you can split bright Mizar, that is, observe that it is two stars close together. But you won’t have enough magnification to see that each of those two stars is also a double star. That’s right, Mizar is really a system of four stars--two orbiting pairs! Alcor (pronounced AL core) is from the Arabic for black horse. Although Alcor and Mizar both belong to the Ursa Major Cluster and Alcor appears to be incredibly close to Mizar, astronomers believe that Alcor is a bit too distant from Mizar to be in orbit around it.

So, now you can amaze your friends and family when you tell them that what looks like one star in the middle of the Dipper handle is really five stars.

5) The third star in the handle is Alioth (pronounced AL ee ahth), also from the Arabic for black horse. The luminous, white Alioth is the brightest star in the Big Dipper, as well as the entire Ursa Major constellation.

6) Megrez, the top left star of the bowl, is from the Arabic for base of the tail (of the bear). Megrez (pronounced MEG rezz) is the faintest star in the Big Dipper. This white star is about twice the size of our Sun.

7) Phecda (pronounced FEK duh) is the bottom left star of the bowl. Phecda, from the Arabic for thigh (of the bear), is spinning very fast--over 80 times faster than our Sun. It is believed to have a spinning disk of gas surrounding it.

8) Merak, the bottom right star of the bowl, is from the Arabic for flank (of the bear). Merak (pronounced MERR ahk) is one of the two pointer stars that point the way to the North Star. This white star is surrounded by a disk of heated dust, and it may be the most likely candidate of the Dipper stars to have a planetary system.

9) Our final bowl star, Dubhe (pronounced DOO bee), is from the Arabic for--amazingly enough--bear. Dubhe is the second brightest star in the Dipper, just a tad dimmer than Alioth. At around 124 light years from Earth, it’s the most distant Dipper star. In marked contrast to the white spectrum of the other Dipper stars, Dubhe is an orange giant. Can you see the difference in color between the two pointer stars, orange Dubhe and white Merak?

Dubhe is a rebel like Alkaid, moving in a different direction from the five stars of the Ursa Major Cluster. The renegade movements of Dubhe and Alkaid will ultimately distort the Big Dipper’s familiar shape beyond recognition, perhaps in 200,000 years or so. In the way that we now wonder, scratching our heads, how the ancients ever conjured a bear from the stars of Ursa Major, future generations may wonder what possessed us to see a water dipper emerge from a ragtag string of stars.

Thursday, June 12, 2008

Strawberry Moon

This week, the waxing (growing) Moon inches nightly toward total illumination--Full Moon on Wednesday the 18th. The Full Moon in June, according to the Algonquin Indians, is the Strawberry Moon. They named it so because it marks the time of the year to begin gathering ripening strawberries.

If the June Moon of my imagination is now a big, juicy strawberry, then the Moon’s myriad craters are the seeds that speckle the plump fruit’s surface. There are over 800 named craters on the near side of the Moon--the hemisphere that always faces Earth. Many are observable by amateurs like you and me, through binoculars or telescopes.

The word crater is from the Latin word for cup. The Moon’s craters are bowl-like depressions caused by bombardment from rocky space debris such as asteroids and meteorites. The Earth’s atmosphere protects our planet from most hurtling hunks of debris; the friction created on entry vaporizes them. However, the Moon has no atmosphere, so it has taken quite a pummeling.

The trick to successful crater observation is to view them when they’re near the terminator. The terminator is the boundary between the illuminated and the dark portions of the Moon’s face. At the terminator, light from the Sun is striking the Moon’s surface at an angle. This fills the craters with long shadows and gives them dimension.

Break out your optical aid of choice, because we’re going crater hopping!

1) This weekend will be the best time to observe the first seven craters on my list. The final one, Grimaldi, will come into view toward the end of our observing week, Tuesday the 17th or Wednesday the 18th. They are all visible with binoculars, although good-sized binoculars like 10x50s or a modest telescope will make spotting them an easier task.

Keep in mind that the map below is a true image of the Moon. If you are using a telescope that produces a mirror-image view, remove the star diagonal in order to match the map.

2) Lunar craters bear the names of deceased scientists, scholars, artists, explorers, and astronauts who’ve made significant contributions in their fields. Some are household names, while others are a bit more obscure.

3) Find Plato. This crater is 68 miles in diameter. It is known for its dark floor, a layer of solidified lava. Plato was the ancient Greek philosopher and student of Socrates who wrote a number of enduring dialogues--conversations involving Socrates.

4) Find Copernicus. This crater is 66 miles in diameter. The feature at Copernicus’s center is uplift caused by rebound of the crater material after impact. In this case, the uplift formed three peaks rising about three-quarters of a mile above the crater floor. Nicolaus Copernicus was the Polish astronomer who first developed the scientific theory of heliocentrism, which held that the Sun was the center of the solar system and that the Earth and other planets revolved around it.

5) Find Bullialdus. This crater is 37 miles in diameter. It is circular, with a high outer rim. Ismael Bullialdus was a French astronomer who supported the works of both Galileo and Copernicus. His work on gravity laid the foundation for Isaac Newton‘s groundbreaking science.

6) Find Tycho. This distinctive, much-photographed crater is 63 miles in diameter. The eye-catching rays that fan out from Tycho like the spokes of a wheel were formed by material ejected during impact. Many astronomers believe that the presence of rays indicates a relatively young crater, by lunar standards. Ray systems around older craters may have been obliterated over time. Tycho was named for the Danish nobleman and astronomer Tycho Brahe. He is renowned for the accurate observations he made from his island observatories, as well as for the metal nose he wore to replace the piece lost in a sword duel.

7) Find Gassendi. This shallow crater is 68 miles in diameter. A much smaller crater called Gassendi A overlaps it on the northern border. Gassendi was one of the landing sites considered for Apollo 17, the final manned Moon landing. Ultimately, however, a site near Sinus Iridum (Bay of Rainbows) was selected. Pierre Gassendi was a French astronomer and priest. He was the first person to observe a planet crossing the face of the Sun (the Mercury transit of 1631), and he coined the name “Aurora Borealis” for the phenomenon of the Northern Lights.

8) Find Aristarchus. This crater is 25 miles in diameter. It is a bright crater with a prominent central peak, and it is believed to be young. Aristarchus was an ancient Greek astronomer and the first person to propose that the Sun was the center of the solar system. However, he was way ahead of his time, and his views were rejected.

9) Find Kepler. This, the smallest crater on my list, is only 19 miles in diameter. Like Tycho, it has a dramatic ray system surrounding it. It was named for Johannes Kepler, a German astronomer and imperial mathematician to Emperor Rudolph II. He is best known for his three laws of planetary motion.

10) Find Grimaldi. This, the largest crater on my list, is 107 miles in diameter. Grimaldi is a heavily eroded crater with a smooth, dark floor that makes it stand out from the surrounding, light-colored material. It sits close to the limb, the outer edge of the Moon’s disc. Francesco Grimaldi was an Italian physicist and lunar mapmaker who did early work in gravity by observing objects in free fall.

Thursday, June 5, 2008

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