Download FINDING YOUR WAY AROUND THE CELESTIAL

FINDING YOUR WAY AROUND THE CELESTIAL FOREST
by John C. Miller
Learning to understand the use of a roadmap can be an important part of driving your car into unknown territory. The
same can certainly be said for exploring the sky.
One of the easiest and fun methods of getting to know your way around the sky is to become acquainted with a few of
the brighter constellations. Most people growing up in the United States have heard of the “Big Dipper.” Many of us
were introduced to this star pattern during childhood. Luckily, this easily recognized pattern also serves as an
efficient guidepost to other features in the night sky.
“Pattern” is highlighted above, because that is what constellations are. Our brains are programmed at recognizing
patterns. The ancient Greeks and Romans were particularly keen to visualize animals, gods and inanimate objects in
groups of stars. Many of these mythologies have carried through to today.
Using patterns of stars as roadmaps and pointers is the key to a very useful skill called star hopping. No – this has
nothing at all to do with Hollywood. By learning to find a few key constellations (with the help of your trusty
planisphere, of course), you will be well on your way to identifying all the brighter stars (by name), positions of any
naked-eye planet visible in the sky, as well as some key “deep sky” objects.
Let’s start with the “Big Dipper.”
This well-known pattern is actually part of a larger pattern – the larger pattern comprising the actual constellation,
called “Ursa Major,” or “The Great Bear.” A pattern such as the Big Dipper is known as an asterism. Now, let’s find
out how this pattern of stars can act as a very efficient guide to other objects in the sky
STAR HOPPING
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As seen on the previous page, the “Big Dipper” can act as a multipurpose signpost. One of the most popular summer
bromides in observational astronomy is: “Arc To Arcturus.” Here’s where it comes from..
Take a look at the diagram on the previous page – or your planisphere. Find the handle of the Big Dipper. Now
follow the ARC of the handle around (south) to the first bright star. That bright star is Arcturus (a red giant 37 light
years distant) and is part of the constellation Bootes, The Herdsman. This works like a charm in the field. You’ll see!
Once you’ve got a bead on Arcturus (yes, Virginia, it really does look red!), have your mind’s eye “draw” a straight line
south to the next bright star. That white-blue blazer is named Spica (“spike ah”) and is in the constellation Virgo.
You’ve made a straight run (south again), or SPIKE to Spica.
So, starting with the easy asterism of the Big Dipper handle, you’ve made an ARC to Arcturus and then SPIKED to
Spica. You are on your way to understanding your way around the night sky.
Now..I’ve mentioned looking south. And, rightly, you may ask “how do I know which way is south?” Fair enough.
South is the opposite of north. So, let’s find North. North is a good place to know anyway, whether you are a scout or
astronomer.
Go back to the Big Dipper diagram on page 1. Find the stars Merak and Dubhe in the cup portion of the asterism.
Got ‘em? Good. Starting with Merak, draw a straight line through Dubhe and continue that straight line about 2 ½
fist-widths at arm’s length. You will run smack into a not particularly bright star, standing pretty much by itself. That is
Polaris, the “North Star.” Again, this trick works like a charm in the field. Run your gaze between Polaris and the two
“pointer” stars in the Big Dipper to get your brain used to the pattern.
Now you know where Polaris is, thus you know within about 1 ½ degrees where the true north celestial pole (NCP) is.
Incidentally, the word “degrees” here doesn’t refer to temperature, it refers to a visual distance. For example, your
fist, held at arm’s length against the sky, portends a distance of about 10 degrees. So, if you’re facing the North Star,
your back is toward due south.
OK. Let’s call on the Big Dipper one more time. Find Merak and Dubhe again. This time, draw a straight line starting
with Dubhe, going through Merak (you’re heading south) and “down” to the next bright blue white star. That’s
Regulus, in the constellation Leo, the Lion. This might be called RUNNING to Regulus or LEAPING to Leo, it’s your
choice. Leo is one of the few constellations that has some rough resemblance to its namesake. And Leo is in a great
position for Spring observers in New Jersey. Pretty much right overhead at 10 PM in April.
Let’s recap. Clear night. Open field. Find the Big Dipper and:
1.
ARC to Arcturus
2.
SPIKE to Spica
3.
RUN to Regulus
4.
POINT to Polaris
→
STAR HOPPING
Page -4These quick exercises are more important than you might initially realize. Review your planisphere, by all means, but
you’ll see the impact of these little practices only when you are under the night sky. When you find Polaris and
Arcturus using the Dipper, you’ll smile and raise your eyebrows. I guarantee it. And, as the charts indicate, Polaris is
itself part of an asterism of the constellation Ursa Minor, or, the “Little Bear.” The asterism is, coincidentally, the Little
Dipper, and Polaris is the last star in the Little Dipper’s handle. Can you pick out the Little Dipper? It’s a dim
constellation difficult to see in its entirely in ground light-saturated New Jersey. The two stars on the end of the
dipper’s bowl, however, are fairly easy to see. Their names are Kochab and Pherkad.
Viewed from latitudes north of the equator, the entire sky appears to revolve (counterclockwise) around a point (called
the North Celestial Pole). This process is very apparent in time lapse photography, as seen below. The short arc
very close to the center is Polaris. As seen from NJ, does the North Star ever set??
In a relatively short time, when you’ve become adept at star hopping, you will be able to use this technique to find
galaxies, star clusters and nebulae otherwise invisible to the naked eye. But for now, it is very important to learn
some of the brighter constellations and their component stars.
OK. Find the Big Dipper again. Locate, with eyes only, the second star in from the end of the handle. This is Mizar.
Now, find Mizar in your binoculars. There! Mizar is actually a gorgeous pair of blue white diamonds. Mizar’s partner
is named Alcor. This is an actual (gravitationally bound) double star. As a matter of fact each these stars hosts other
stellar bretheren..a total of six stars in the system, as far as we currently know. Mizar and Alcor lie about 70 light
years distant. Yes, by all means, invest in a good pair of binoculars if your budget allows.
STAR HOPPING
Page -5Let’s see if we can find a “deep sky” object with binoculars.
Time: 9:30 or 10:00 PM DST (daylight savings time). Find Arcturus. Find the last star in the end of the handle of the
Big Dipper (it’s named Alkaid). Now, locate the star located in a straight line about 1 ½ fist-widths (15 degrees) below
Alkaid. That star is named Cor Caroli in the small constellation Canes Venatici (The Hunting Dogs). Cor Caroli
stands pretty much alone.
Incidentally, note the “DST” above. Please remember your planisphere is always thinking Eastern Standard Time (as
long as you are observing from an EST zone). Thus, if it’s 10 PM DST, set your planisphere at 9 PM.
Take your binoculars and slowly scan a small area of sky on a line equidistant between Cor Caroli and Arcturus. You
will come across an object that looks rather like a dim, “fuzzy” star. That is the globular cluster Messier 3, or more
commonly, M 3. Find this object, and you’ll be looking at an enormous, tightly packed sphere of stars situated just
outside the plane of our galaxy. We estimate it contains well over 100,000 stars and lies approximately 45,000 light
years distant.
MESSIER 3
Caveat!! The image above is not what you’ll see in your binoculars. This picture was created with sophisticated
equipment and a long time exposure. However, remember this photo while looking at M 3 and you’ll have a much
better sense of the “fuzzy dot” in the binoculars. Oh, incidentally, you are looking back in time 45,000 years. Cool.
As you introduce yourself to the night sky, and to appropriate guide tools (planispheres, star charts and web sites),
invariably you will read many terms unknown to you. Get to know these words at your own pace, but do familiarize
yourself with them.
Some of these sky navigation terms are:
declination
right ascension
seconds (of arc)
minutes (of arc)
hours (of arc)
altitude
zenith
azimuth
meridian
arc (not Noah’s)
ecliptic
celestial equator
nadir
Discover their meaning at:
http://www.enchantedlearning.com/subjects/astronomy/glossary/ (great site !)
------------------------------------------------John Miller
Amateur Astronomers
Association of Princeton
[email protected]
© 2007