Some Light Science Reading. The Constellations: Ursa Minor

As first appeared in the January/February/March 2012 edition (yeah, I know) of the Syracuse Astronomical Society newsletter The Astronomical Chronicle (PDF) and, I am proud to say, soon to be included in an edition of the Mohawk Valley Astronomical Society (MVAS) newsletter, Telescopic Topics.

Image generated with Starry Night Pro 6.

[Author's Note: A tradition owing to Dr. Stu Forster during his many years as President and Editor, the Syracuse Astronomical Society (www.syracuse-astro.org) features (at least) one Constellation in each edition of its near-monthly newsletter, the Astronomical Chronicle.]

The Constellation discussion for this year is going to take a bit of a turn.

As part of the 2011 Syracuse Astronomical Society (SAS) lectures presented at Liverpool Public Library and Beaver Lake Nature Center, I spent a few minutes covering (briefly) how to navigate the Night Sky. By way of introduction, I described how one of my graduate advisors, Dr. Bruce Hudson, began scribbling furiously a long string of quantum mechanical equations about something-or-other that devoured the lion's share of a whiteboard. Upon mentioning that I had no idea how he kept such information at the ready in his noggin, he replied "Try doing it 50 years."

It is, in my humble opinion, useless to present the 88 Constellations to a general, new-to-observing audience in an hour and expect anyone to remember information that I, as el presidente, am still trying to digest after several years (a problem made all the more infuriating by the fact that this information hasn't changed in several millennia). The problem that I and others at this latitude have is that the vast majority of the Night Sky changes throughout the year and, given that weather conditions often result in short spells of clear sky and long patches of overcast conditions, there is often little opportunity for "mental reinforcement" to help commit the lesser (well, at least smaller or dimmer) Constellations to memory.

The solution I discussed in the lectures was to play the "observability odds" and focus on learning those Constellations that you can, given clear skies, see all year long from Central New York (CNY). This group of Constellations are defined as "circumpolar" and, by their location about the axis of rotation of the Earth, never dip below the West/Northwest Horizon (or, at least, they do not entirely disappear over the course of a long evening of observing unless you're surrounded by considerable foliage).

The set of images at the end of this article will show you how to kill six birds with one long, clear turn of the stone we call Earth. The small family of six Constellations I've included in this discussion are (1) Ursa Major (although, here, I'm only including the Big Dipper asterism for ease of identification. This is obviously a better target for new observers), (2) Draco the Dragon (a long and winding Constellation that is curled around the Little Dipper), (3) Cepheus, the late-late-late King of Ethiopia (as much as I dislike the use of simple geometric objects to identify groups of stars (because, well, they're all points on imaginary polygons), the odd pentagon does stand out at night), (4) Cassiopeia (Jonathan Winters' Big "W" and, thanks to Earth's rotation axis, also sometimes a "3," or an "M," or an "E," but obvious upon first being pointed out), and (5) Camelopardalis the giraffe (one of the last Constellations you might otherwise learn. Also one of the last Northern Constellations marked as such, in this case in 1612 by Petrus Plancius. You might even have a little trouble picking this one out. The Greeks (for instance, and in their infinite wisdom (I note with a 100% Greek heritage)) did not even bother to identify anything in this part of the sky as being of significance given how relatively dim the stars are). This list leaves number six, Ursa Minor, which I denote in the images as "0" as your celestial clock face base of operations.

Ursa Minor, or the Little Dipper (below, shown at its approximate orientation at 10:00 p.m. on March 23rd), is a nondescript Constellation that requires a bit of searching to find in the Night Sky. Polaris, its last handle star (2.0 mag.), is made easier to find by the fact that it is in a very dark, very nondescript piece of sky (it is identifiable simply by being where it is). Its cup-edge stars Pherkad (3.0 mag.) and Kochab (2.1 mag.) are a bit brighter and also in a dull region of the sky. The four remaining stars are the ones that become more visible as you mark their location with your scanning eyes. These four are made a bit more difficult to find from Darling Hill Observatory (home of the SAS) because of the bright light bulb directly at our Northern Horizon that is downtown Syracuse.

A possible trick to finding Polaris for the new-at-observing is to use the two most prominent Constellations in the North, Ursa Major (again, using the Big Dipper asterism here) and Cassiopeia. Finding the bowl of the Big Dipper and imagining a clock face, find Cassiopeia at nearly 7 o'clock to the edge-most bowl stars, then aim for the location where you'd expect those hands to be riveted (as shown below). Again, you'll find a single bright-ish ("eh") star at this location.

Having sufficiently talked down the significance of Polaris as a celestial observable, this otherwise nondescript star has something other nondescript stars have. To quote "Glorious John" Dryden:

Rude as their ships were navigated then;
No useful compass, no meridian known;
Coasting they kept the land within their ken;
And knew no North but when the Pole star shown.

Or, as William Tyler Olcott sums more quickly in his book "Star Lore," Polaris is "the most practically useful star in the heavens." Modern civilizations know Polaris as the star around which the Earth appears to spin, making it the most stably-placed object in the Night Sky over any reasonable span of human existence (a qualification I use in this article to avoid a discussion of the fascinating but "not relevant to learning the Night Sky right now" Precession of the Equinoxes).

The apparent constancy of all of the star positions (and Constellations) in the Night Sky relative to one another is, of course, due to stellar parallax, the celestial equivalent of the more familiar terrestrial parallax. If you've ever been the passenger on a long drive, you've borne witness to the trees along the road moving at a tremendous clip while the distant trees slide far more slowly through your field of view (that is, stay in your field of view while the trees along the road fall far behind you over the same amount of time). Polaris provides an ideal example of this same phenomenon on a celestial scale by its apparent immovability in the Night Sky despite the best efforts of Earth as it reaches nearly 300,000,000 kilometers of physical separation from its starting point every six months. The two images below demonstrate the phenomenon…

Your Green Laser Along Earth's Rotation Axis (Pointing UP From The North Pole), One Beam Every Three Months, Separated By (At Best) 2 Astronomical Units (a.u.), Looking At A "Close Object" With A Large Apparent Motion Against The "Background"

Your Green Laser Along Earth's Rotation Axis (Pointing UP From The North Pole), One Beam Every Three Months, Marking A Position 431 Light Years Away (Looking At A "Distant Object") And A Small Apparent Motion Against The "Background" (All NOT To Scale)

At above-left you see a small slightly-sideways model of Earth's motion around the Sun (at points being marked about every three months), with the left-most and right-most positions separated by two astronomical units, the astronomical unit being the mean distance between the Sun and Earth (bearing in mind Kepler's Elliptical description of our orbits), a value of about 150 million kilometers. To objects in our own Solar System or even a few nearby stars, this large change in position is enough to clearly see those objects that are nearby move more than the "background" of more distant objects (you could do this at home with a decent scope and excellent note-taking skills, possibly reproducing the 1838 work of Friedrich Bessel in his measurement of the parallax of 61 Cygni). In our case, the more distant objects are the stars far from our vantage point (think of "stars" as "trees" and the same driving analogy works, although now you're driving around a circular track and paying your passenger to always look North). Polaris, as measured by the Hipparcos satellite (using parallax to exacting detail), determined that Polaris is 431 light years away, a distance of 27.5 million a.u.! And this is a CLOSE star considering the 100,000 light year diameter of the Milky Way. At this distance, if the four green laser pointer beams were a meter long, their separation in Earth's orbit would be a small enough measuring distance to map out the contents of a single-celled organism in exacting detail. My ability to draw a proper parallax-like image to show this is limited by the pixels on my screen being gigantic compared to the apparent change in position in this crude image (so the above image is decidedly NOT to scale).

All of this discussion above is basically to convince you that, when you look up in the Night Sky, Polaris will effectively NOT move to the best of your ability to observe it, making it a best starting point for your Constellation memorization adventure.

Well, Polaris will NOT move provided you always observe from the same latitude on the Earth's Surface. The last piece of the puzzle to put ourselves into proper perspective comes from a zoom-in of our Earth, shown below. You'll see that our North Pole, appropriately placed at 90o North Latitude, is aligned nearly exactly with Polaris (again, for our purposes, this approximation is fine). What does that mean? It means that, with the right low Horizon (or high hill), nearly ALL of the Northern Constellations are circumpolar at the North Pole! Think of the memorization mess! Alternatively, at the equator (0o), the Night Sky is, effectively, constantly in motion (this should make you truly appreciate the navigational and astronomical skills of the Polynesians in their spread across the South Pacific islands).

As you walk from the Equator to the North Pole, moving from 0o to 90o North Latitude, the North Star appears to get higher and higher in the Night Sky. By this, the angle of Polaris above the Horizon (its altitude) is equal to our latitude (so when you know one (say, by getting your latitude and longitude from google maps or the like), you know the other. This is one of the great "then explain this, dummy!" rhetorical smack-downs to members of the Flat Earth Society). In our case, Polaris is about 40o above our horizon. Personally, I think 40o North Latitude is a perfectly reasonable place to begin Constellation memorization. Not too many, not to few. And, as is the common theme we'll explore this year, once you have a reliable base of celestial operations, learning the remaining Constellations becomes a significantly easier (but still Herculean) task.

The Counterclockwise Circumpolar Map

Your Northern Horizon from CNY will, clear skies permitting, ALWAYS look something like the following, with the Constellation closest to the N/NW Horizon labeled as follows (0 = Ursa Minor, the Little Dipper. * = Polaris, which appears to not move (to a coarse approximation)):


A. Big Dipper (1, technically, Ursa Major, but the Big Dipper is smaller and more obvious)


B. Draco (2, aim for the dragon's head. If the Big Dipper is N/NE, an easy find)


C. Cepheus – 3, a crazy house standing upright, just right of a bright "E"


D. Cassiopeia – 4, the big "W," at the horizon an "E" (or its canonical chair)


E. Camelopardalis (?!) – 5, the back-end of a giraffe(with Cassiopeia as a "Big W," the giraffe is drinking from the tipped bowl of the Big Dipper).

NOTE: The Earth's rotation makes 1-to-5 move counterclockwise! Fresh Constellations over your Eastern Horizon, stale ones disappear at your West.

Happy Hunting – Damian

Some Light Science Reading. The Constellations: Pegasus

As first appeared in the September 2010 edition of the Syracuse Astronomical Society newsletter The Astronomical Chronicle (PDF).

Constellation Map generated with Starry Night Pro 6.

The Constellation this month is one light on interesting binocular and telescope objects but heavy in mythology and Naked Eye observing. To the Babylonians, the stars in this region also (or first) took on the shape of a horse known as MUL.ANSHE.KUR.RA. To the Greeks, sometimes a horse is not (just) a horse (of course, of course). The Greek mythology surrounding the winged horse Pegasus is, to say the least, involved and undecided. There are several pages discussing the mythology of Pegasus, which I refer you to in the interest of local brevity.

The torso of Pegasus is composed of a "Great Square" of stars that is very easy to see and is very often pointed out to visitors at this time of year at Darling Hill. This asterism (simply any grouping of stars that are not officially constellations) lies to the right (or south) of the Andromeda Galaxy (M31), one of the great views in the Autumn skies. As the scope is pointed in this direction anyway for a good block of time during Public Viewing sessions, the walk through some of the nearby Constellations (Cassiopeia, Perseus, Andromeda, Pegasus, Cepheus) reads like a Cliff Notes version of both Clash Of The Titans movies (unless John McMahon is running the presentation, in which case you're guaranteed a much better show). In the modern definition of the Constellations, the south-most (or upper left corner) star belongs instead to the Constellation Andromeda (but anyone staring at this part of the sky would be hard pressed to be struck more by the "Great Triangle" of Pegasus than the "Great Square" of Pegasus).

There are only two significant (and visually accessible) objects within Pegasus (the Constellation, that is) for the binocular and telescope viewer at Darling Hill. The first of these is the appropriately named Pegasus Cluster (M15), an ancient globular cluster clocked at 13.2 billion years of age. This cluster appears as a smaller version of M13 in Hercules, as captured by our own Stu Forster in the September 2008 Member Gallery and shown below. The second object is a far more difficult find, the very unique spiral galaxy NGC 7742 (below). The presence of a prominent ring in this galaxy (or, more specifically, the absence of pronounced spiraling from the center of the galaxy out to the edges) is a point of unexplained inquiry in modern astrophysics.

M15, photo taken by SAS member Stu Forster.
NGC 7742, image from the Hubble Heritage Team (AURA/STScI/NASA/ESA).

The most curious content on the wikipedia page for Pegasus involves the nontrivial amount of discussion about the reinterpretation of its connectivity by one H. A. Rey in, specifically, his book The Stars — A New Way To See Them. Rey's goal in this book is to redefine connectivity of some of the Constellations to make them a bit easier to see as the mythical beasts they are known for. For Pegasus (see below), Rey has eliminated any mention of Sirrah (or Alpheratz, as it's known within Andromeda), using the Great Square as a Great Triangle that marks the above-the-shoulder wings over the trapezoid torso (with the rest of the limbs along the southern edge of the Constellation). Upon inspection, his reinterpretation looked more to me like one of the drawings of The Man In The Yellow Hat who, along with Curious George, is perhaps the more famous of the illustrated characters created by H. A. Rey.

A new view of an old constellation, or The Batman In The Yellow Hat.

I'll admit I'm mildly ambivalent about the redefinition of Constellation connectivity. On the one hand, the Constellations are one of the oldest memes in human history among all societies (extant or extinct) and, to that end, connectivity has meaning as a way of marking out specific arrangements that have largely stood the test of time. The consistency of connectivity also provides a way to reduce the memorization fatigue that comes from having to see groups of stars in slightly different ways (clearly, one arrangement is easier to know and explain than several). This is of further significance when one uses Constellations as a specific guide to locating Messier (or other) objects. If I tell you that "M15 is on an almost straight line about 1/2 the distance of the two stars that make up the snout," you really have to trust that we're seeing the same horse!

On the other hand, there are many amateur astronomers who use Constellations largely as tools for finding smaller objects (with or without a knowledge of their history) and, as we are a species that excels at pattern recognition (how many flying faces and hippopotami can you see on a partly cloudy afternoon?), anything that makes life easier for you the observer (especially on cold nights when observing time is at a premium) should be added to your observing arsenal. H. A. Rey's interpretation of Pegasus connectivity might cloud just how pronounced the Great Square is (so you have to then present this Constellation with an addendum!), but it certainly does look more like a complete flying horse than the common artistic rendering of only the front half (clearly the side you'd want to have over you anyway given both choices).

Any way you look at it, it's still safe to assume that the winged horse must have been the most efficient way to travel in the ancient world. It certainly speeds up a good plot.

Clear skies, Damian