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.

The Reclusive Ram

Aries the Ram is a bit of a recluse, hiding in plain sight in the late autumn sky. Although not terribly impressive to behold, Aries is historically an important constellation. In the astrological zodiacs of numerous ancient civilizations, it was considered the first zodiacal constellation, or first sign. This designation may have been made when it was noted by the ancients that the Sun was positioned in Aries at the spring equinox.

The spring equinox is the moment when the center of the Sun is directly above the Earth’s equator. This occurs only twice a year; the other occurrence is known as the fall equinox. Another name sometimes given for the spring equinox is first point of Aries, which, of course, derives from old astrological traditions.

Aries in Bayer's 1603 star atlas
Courtesy of Linda Hall Library of Science, Engineering and Technology

The “first sign” tradition continues today; Aries is usually listed first in horoscope columns found in newspapers and magazines. You’ll also note that the conventional date range given by astrologers for people “born in the sign of Aries” begins on March 21; spring equinox typically occurs on or near March 20.

By the way, the Sun is no longer in Aries at the spring equinox. Due to precession— sometimes called precession of the equinoxes— which is the wobbling-top motion of Earth’s axis, the equinoxes occur a few minutes earlier each year. As a result, at each spring equinox, the Sun is not in the same position it was the previous year against the background stars, as seen from Earth. If we track the Sun’s position over the years at a fixed point in time (such as the spring equinox), from our perspective on Earth the Sun would appear to move through all 12 zodiacal constellations in reverse. As a result of precession, the Sun is now in Pisces, the twelfth zodiacal constellation, at the spring equinox.

Aries in Middleton's 1842 star atlas
Courtesy of Linda Hall Library of Science, Engineering and Technology

In mythology, Aries represents the ram whose golden fleece was famously sought by Jason and the Argonauts. Let’s conduct our own search for the storied sheep.

1) Wait about two hours after sunset to begin observing, so that Aries is high enough in the sky.

2) Face east. 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 back to the west, and you’ll be facing approximately east.

Star maps created with Your Sky

3) Find the Great Square of Pegasus, which is nearly overhead, and the Pleiades star cluster, which is low in the east. These are both easy to spot using the star map above. Midway between them is Aries. Look for a small, curved line of three stars. This is the most notable asterism (recognizable star pattern) in Aries. It always remind me of a comma, so I call this asterism the Comma. Unfortunately, there’s nothing in the scattering of stars in Aries that particularly resembles a ram, unless you want to think of the Comma as a curving ram’s horn.

4) The brightest star in Aries is Hamal (hah-MAHL), which comes from the Arabic for lamb. Other ancient names for this star had meanings like ram’s eye, horn star, and leading one (the ram that led the celestial flock). Hamal is an orange giant star, 66 light years away. One light year is the distance light travels in one Earth year, nearly six trillion miles.

5) The second brightest star in Aries is Sheratan (SHER-uh-tunn), from the Arabic for the pair, a reference to Sheratan and the final star in the Comma, which we’ll look at next. Sheratan is a white star. When you compare it to Hamal, can you see that orange giant Hamal has a more golden hue?

6) Mesarthim (mess-ahr-TEEM) is from the Hebrew for servants. Mesarthim, a white star, may only be Aries’ fourth brightest but it’s known as the first star of Aries. This is because in antiquity, when the spring equinox was in Aries, Mesarthim was the closest Ram star to the equinox. Physically, the equinox can be described as the intersection of the Sun’s path with the celestial equator, the imaginary line that represents the Earth’s equator extended out into space.

7) Although the third brightest star in Aries has no traditional name and is instead saddled with the uninspired moniker Flamsteed 41, it does manage to be part of a funky little asterism. The blue-white dwarf star is the brightest member of a four-star asterism known as the Northern Fly, riding on the ram’s back. To find Flamsteed 41 and the Northern Fly, look northeast of Hamal and east of the constellation Triangulum.

In its glory days, this asterism was part of a now-defunct constellation by the same name, which was introduced in the 17th century. Now it’s merely a curiosity for naked-eye stargazers like us and, I imagine, a constant irritant for our celestial ram.

The Ram and the Fly in Bode's 1801 star atlas
Courtesy of Linda Hall Library of Science, Engineering and Technology

Moon in the Balance

I am torn.

As an amateur astronomer, I must arrange my observing life around avoidance of the Moon; wait impatiently for it to set; nervously watch the eastern horizon for its arrival; hide from it behind trees, rock outcroppings, vehicles, and outhouses; wail and curse it when it appears. Let’s face it, that old incandescent Moon is the bane of every hunter of "faint fuzzies": deep-sky objects pursued with telescopes, binoculars, and much lurching about in pitch-dark places.

Faint Fuzzies: Hickson 44 galaxy group

As a human being, though, I must marvel at the Moon and think how lucky we are. We have a natural satellite that shows us, through its phases, that it revolves around us. When it covers the Sun during a solar eclipse or moves into the Earth’s shadow during a lunar eclipse, we have a glimpse into the clockwork of the universe. We have a naked-eye view of the surface of another solar system body. We even have a cosmic timepiece that ticks along reliably enough to give us the basis for our system of calendar months. (Did you know the word month is derived from the Old English word for moon, mona?)

The approach of Full Moon on Friday the 18th has me pondering what I’d do if given the power to choose between keeping the Moon or banishing it forever. A giddy free-for-all of deep-sky diving, unfettered by the lunar cycle, versus a glowing orb that has inspired humans throughout history to pen poems, create art, write songs, do science, and pitch woo. The everlasting happiness and gratitude of my stargazing brethren versus the artistic, scientific, and romantic outpourings of the human race. It would be a tough call.

There are some practical reasons to keep the Moon. In a previous post, I discussed precession, the slight wobble of Earth’s axis. This wobble is caused by gravitational tugging by both the Sun and the Moon. If the Moon were removed from the picture, Earth’s wobble would destabilize, becoming more erratic and pronounced. This would most likely result in dramatic climate changes.

Image courtesy of PDPhoto.org

Another reason to hang on to our satellite has to do with tides. The combined gravitational pull of Sun and Moon causes the cyclical rise and fall of Earth’s oceans, the tides. Although the removal of just the Moon would not eliminate the tides, it would certainly disrupt them. The feeding and breeding activities of various marine and shore creatures could be severely impacted.

Even if we subtract these practical considerations from the equation, ultimately I think I’d miss the Moon if it were gone. I’d miss how the waxing crescent looks like a big, happy tangerine slice before it sets, and a crescent in the clouds like a shark fin slicing through seafoam. I’d miss that loopy Moon illusion that makes the rising Full Moon look unnaturally big. I’d recall the lunar eclipses I’ve watched and how the Moon in Earth’s shadow always turned red--sometimes russet, sometimes scarlet. I’d miss the gasps and whoops that a telescopic view of the Moon’s crater-pocked surface elicits at public astronomy events. I’d miss my own telescopic tramps across the Moon and moments of discovery. Strangely, I think I’d even miss the collective groan that rumbles across a star party’s observing field when the Moon bobs up in the east.

Observing field at a star party

So, at the end of the day, with all that fateful power at my fingertips, I’d have to give La Luna the thumbs-up. Of course, I’d also have to change my name, move out of state, and never show my face at another star party. Anywhere. Perhaps we should keep this just between us.

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