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