Sunday, April 02, 2017
Issue #537: The Novice Files: Star Charts Part I
When we last left off with the Novice
Files, we’d talked about stars, constellations, and catalogs of stars and deep
space objects. This time we’ll be dipping a toe into the somewhat deep water of
star charts, “sky maps.” Like that guy standing on the
corner in Hollywood hawking his wares says, “You can’t find your way to the
stars’ homes without a map.”
Basic Star Charts
You’ve got to have star charts if you’re going to learn
how to use star charts. If you want to begin cheaply, I suggest these maps from Sky & Telescope, which cost
just a couple of dollars, an equatorial star chart (SC001)
and a north circumpolar star chart, SC002 (they have a south circumpolar chart, too). These simple
paper maps have been around for decades, will teach you a lot about both the
sky and star charts, and will remain useful as long as you do astronomy.
Let’s look at the equatorial star chart first. As the name
implies, this chart is centered on the Celestial Equator, the imaginary line in
the sky that divides the sky globe into Northern and Southern Celestial
Hemispheres. The Equator on this map is the horizontal, triple, hash-marked
line that divides the chart in two. Everything above the line is the Northern
Celestial Hemisphere, and everything below is the Southern Celestial Hemisphere.
Intersecting the Celestial Equator at two points is a curving, sine-wave-like
line. That is the Ecliptic, the
apparent path of the Sun through the sky. Why is it curved? As we
learned previously, due to the tilt of Earth’s axis the path of the Sun moves
north and south in the sky over the course of the year. When the Sun’s path is
as far to the North as it goes, we have summer in the Northern Hemisphere. When
it is as far to the south as possible, it’s Winter (and summer in the Southern
Hemisphere).
You’ll further note that the ecliptic is marked with dates.
Those dates represent the position of the Sun at noon on that date with relation
to the background stars. On June 1, for example, you’ll find the Sun in the midst of the
stars of the constellation Taurus. Finally, the places where the Ecliptic
intersects the Equator are the Equinoxes, the Autumnal and Vernal Equinoxes.
 |
| A portion of the equatorial chart... |
What was the first thing that probably caught your attention on the chart? The stars and constellations. The stars are represented by dots of
varying sizes. The bigger the dot, the brighter the star. The range of stars
shown on this simple map goes from -2 at the bright end to 6 on the dim end.
Actually, while there are a few stars down to magnitude 6 shown, most are left
off of this large-scale chart. A magnitude 6 star is the dimmest star most
people can see with their naked eye from a reasonably dark site.
How does stellar magnitude work? It’s a logarithmic scale. A
magnitude 1 star is 2.5 times dimmer than a magnitude 0 star, and a magnitude 2
star is 2.5 times dimmer than a magnitude 1 star. There are objects, like the planet Venus, the Sun, and the Moon that are brighter than
magnitude 0, so there are negative magnitude values as well. Something with a
magnitude of -1 is 2.5 times brighter than something that shines at magnitude 0.
You’ll notice that every star on the chart is identified,
either by its proper name if it has one, a Greek “Bayer” letter, or a Flamsteed
Number, all of which we went over in the last edition of the Files. You’ll also
see there are a few deep sky objects scattered amongst the stars, but just a
few; mostly the brightest Messier objects. There’s a key at the top of the
chart next to the magnitude scale that identifies deep sky object symbols,
allowing you to tell if an object is a nebula, galaxy, or star cluster.
But how do you find
things on the map? The same way you do on a terrestrial map, using latitude and
longitude. As we learned previously, celestial latitude is declination, and
celestial longitude is right ascension. The right ascension scales run across
the top and bottom of the chart, showing distances east and west of the Vernal
Equinox (located at 0h right ascension), while the declination scales are,
naturally, on the right and left, since declination is position north and south
of the Celestial Equator.
There are two ways to use the declination and right
ascension scales. You can, most of all, use them to locate objects. If you have
the right ascension and declination of Sirius, the Dog Star, for example (from
a catalog or from a Google search, perhaps), you can easily find the star on
the chart.
First, locate Sirius’ right ascension, 6h 45m, on the
scale at the top or bottom of the chart (each little tic is 5’). Place an index
finger on that. Now find -16-degrees on the right or left dec scales (each tic
is one degree). As you’ll recall, a minus declination is a south declination,
so you’ll be on the part of the scale below the Celestial Equator. Place your
other index finger on -16-degrees. Now, run your two fingers down and across. Where
they meet will be, approximately anyway, the location of Sirius.
 |
| The circumpolar chart... |
The other way to use the scales is to use them to find the
declination and right ascension of an object. We see where Sirius is, but what
are its coordinates? Place an index finger on Sirius, go straight up or down to
the right ascension scale, and you’ll have its R.A. Move your other finger
straight left or right to the declination numbers, and you’ll have its dec.
In addition to stars and a few deep sky objects, the chart shows the constellations,
the “stick figure” star patterns we introduced a few weeks back. The Sky & Telescope charts use a set of
stick figure designs sometimes referred to as “traditional” that are in my
opinion the clearest and most easily remembered shapes for the star figures. One
question I’m occasionally asked is “What is the right ascension and declination
of a constellation?” Since the star patterns cover a fairly large area of the
sky, the way you do that is either to use a point in the middle of the stick
figure, or to use the constellation’s brightest star as your reference point.
The Circumpolar Chart
While using the declination scale on the equatorial chart, you may have
noticed it stops at 60-degrees and -60-degrees, the chart is cut off to
the north and south. Why is that? Think back to elementary school. Likely there
was a map of the world on the wall, probably a Mercator map, a map using the
Mercator projection system. What else might you remember? What I remember is
that, weirdly, Greenland was bigger than South America on the map,
something I knew wasn’t true.
“Spreading out” the curved surface of the earth onto a flat
plane causes distortion north and south. That makes the smallish
Greenland huge. If the equatorial star chart continued above 60-degrees north
or south, there’d be this same sort of distortion—the far northern and southern
constellations would be badly misshapen. The mapmakers here decided to avoid
that by placing those constellations on a separate chart, which shows the last
30-degrees before you get to the pole.
Everything in the circumpolar (“around the pole”) chart is the same as on the equatorial chart with two exceptions. The right
ascension scale goes around the outer circumference of the chart circle, and the declination scale cuts the map in half. To find right ascension of, say,
the bright star at the end of the Big Dipper’s handle, Alkaid, move straight
down from it to the periphery of the chart circle, landing on 14h 10m
(approximately). Declination is slightly harder to find, but not much. In
addition to the declination scale that cuts the circle in half, there are
additional unnumbered scales with tick marks. By referencing the one closest to
Alkaid, I see it’s one tick down from a major line of declination (each tick is
one degree). Referring the labeled scale, I determine the star is at just a bit
over 49-degrees north.
There’s one other interesting feature on the circumpolar
chart, a big, dashed circle labeled “orbit of the precession of the pole.”
What’s that? Well, have you ever played with a child’s top? What happens when
it begins to run down? It begins to wobble.
The same thing is happening with the Earth. Don’t worry; it isn’t going to fall
over, but it is wobbling. Imagine
placing a laser beam at the north pole. As the Earth wobbles, the laser will
scribe a circle on the sky globe. The point where the laser beam touches the sky globe is, or course, the position of the North Celestial Pole. Precession, the wobble, is slow and it would take
25,765 years for the Celestial Pole to move around the circle one time.
Because of Precession, as the long years roll by, the North Celestial pole moves
among the stars. Looking at the circle on the chart, in the distant past, in
the days of the ancient Egyptians, the pole was closest to the bright star
Thuban in Draco. At that time Thuban was the North Star. In the distant future,
the pole will be nearest Vega, and it will be a brilliant pole star. Naturally, the same thing is happening with the South Celestial Pole, and our colleagues to the south will eventually get a good pole star (their current one is relatively dim). Since the pole, 90-degrees declination, is moving
against the background stars, the coordinate system and the Equinoxes are
being dragged along with it. That’s why star atlases are often identified as
“Epoch 2000” or similar. That means that the coordinates in the charts were in
the places shown with reference to the stars in the year 2000.
There’s another result of this slow movement over the
centuries: it’s put astrology’s Sun
signs off by one constellation. Find your birthday on the ecliptic and you may
be surprised your “sign” is totally different from what’s given in the
newspaper horoscope. According to the astrologers, I am a Cancer, but looking
at the ecliptic on the chart shows that on my birthday, July 17, the Sun is
actually closer to Gemini. The astrologers set up their Sun Signs many a long
year ago and never bothered to change them despite Precession throwing
everything increasingly out of whack. Oh, and as you'll see if you look along the path of the ecliptic on the equatorial chart, the band of constellations that lie along it, the Zodiac, includes our old friend Ophiuchus, which the astrologers somehow overlooked.
So, you can find stuff on the equatorial chart now. But it
would also be nice to know what was where in the sky for any given time. At
first, it’s not immediately obvious how to do that with a chart like this that
shows the whole (equatorial) sky, but it’s really simple.
Want to know what’s overhead? Find today’s date on the
ecliptic. The constellations that lie long the line of right ascension that
passes through that point on the ecliptic are those that are overhead at noon.
Unfortunately, it’s not too helpful to know which constellations are overhead
at noon. Midnight would be better. That’s easy to do, though. Say the right
ascension line overhead at noon is 21h. Count 12 hours of RA to the east
(left). That line, 9h, and the stars and constellations along it will be overhead at
midnight. If you're interested in what's up at 11 p.m. go 13h to the left, and so on.
Monthly Star Charts
Determining what is "culminating" (straight overhead) for a given time is easy enough
with the above system, but it’s not overly convenient. It would be nice to have
a chart that shows how the evening sky looks at a given time of year, maybe for the
current month. You can get that easily. Sky
& Telescope includes an excellent monthly sky chart in each issue. One
won’t lead you to tons of deep sky objects, but the brighter ones are marked,
and the monthly chart is wonderful when you are just learning the
constellations. The Sky & Telescope "annual," Skywatch, features 12-months of these charts under one cover.
Planispheres
What would be better still? Something, some sort of sky map,
that would tell you exactly how the sky looks right now, at the current time and date. Certainly, there are
plenty of computerized star charts, “planetarium programs,” that will do that,
but we’re not quite ready for them. Instead, let’s begin with a simple
analog computer.
You know what an analog computer is, right? Like a slide
rule (blank looks from youngsters). A planisphere is a special sort of analog
computer that can show how the sky looks for any time or date. It’s very
simple—no batteries, no lights, no screen, just a couple of pieces of paper or
plastic—but this device has helped generations of amateur astronomers.
A “sky wheel,” as some people call planispheres, is as
above in two parts: a round wheel on which the sky is printed, and a stationary
piece with a window. Around the sky wheel’s periphery are dates, and around the
stationary piece are times. Line up the current time (or the time you are
interested in) with the date and the planisphere will show the way the sky
is laid out at that time/date.
It gets better. Set the planisphere for the current
date/time, go outside, hold it over your head with the arrow or letter on the
stationary part that indicates “north” pointed north, so that the west side of
the planisphere lines up with actual western horizon and east and south line up
with actual eastern and southern horizons, and you’ll be looking at a chart
that not only shows what’s where in the sky, but which corresponds to actual
directions in the sky.
Using the planisphere to show how the sky looks at a given
date and time is one way to use it. There’s another way, though. You can use
the planisphere to tell you when some event will occur. When will Orion rise on
a given date? Turn the sky wheel so Orion is just above the eastern
horizon. Then find the date you are interested in. The time Orion will be in
that position will be opposite that date.You can also find the date when Orion will be rising at, say, 10 p.m. Locate 10 p.m. and read the date opposite it. Simple, neat, elegant.
A planisphere is very useful no matter what your level
of experience in astronomy, and I always keep one in my accessory box.
Where do you get a star wheel? Sky & Telescope sells a nice one. One of the better planispheres I’ve used over the years is made by David Chandler. You can even find
them in book stores, including a cool oversize model by my friend David Levy (of Shoemaker – Levy fame, natch).
Are there any gotchas to planispheres? Only a few. In the
spring most of us have to move our clocks forward for daylight savings time.
While we can move the hands of the clock to suit ourselves, however, we can’t grab
the sky and move it forward. That means planispheres always work on standard time. If DST is in effect and
you want to know how the sky will look at 9 p.m., you must set the planisphere
for 8 p.m.
Looking at the Sky
& Telescope planispheres on the webpage, you’ll notice star wheels are
sold for specific latitude ranges, 30N, 40N, etc. They are tailored so their
northern and southern horizons are at the proper (approximate) declinations. In truth, even
if all you can find is a 40-degree one and you live at 30-degrees, or vice
versa, that “wrong” planisphere will still be quite usable.
Finally, because the sky on a planisphere is just a map
printed on paper or plastic, it can’t depict the planets, which move among the "fixed" stars. That is where computer planetariums come in. And that subject,
getting started with computerized charts, will be up next for the vaunted Novice
Files. Till then…
Comments:
<< Home
What I'd like to have is a small planisphere with a LOT of star names on it, to keep up with the names of stars we are told to find by alignment algorithms in computerized mounts. "Quick, find Diphda!"
Actually, I wish computerized mounts would use Bayer letters. If it sent me to Beta Ceti, I would at least know to look in Cetus.
Actually, I wish computerized mounts would use Bayer letters. If it sent me to Beta Ceti, I would at least know to look in Cetus.
# posted by 
: 10:08 AM
: 10:08 AM
Your link to Sky Publishig for the S&T equatorial chart seems to go to the one that plots absolute rather than apparent magnitudes.
# posted by : 10:10 AM
: 10:10 AM 
Great post and another alternative for those with $0 (or not!): Toshimi Taki's atlases: http://www.geocities.jp/toshimi_taki/
Yes, they're PDFs, in fact mostly groups of PDFs, but that can be an advantage since if offers the option to go electronic or paper. For those so inclinded, there are lots of free tools to merge and split PDFs, so that's not a problem, either.
I've really come to like his 6.5 mag atlas for quick work, particularly with binoculars and my little 80mm refractor. I printed it at work (don't tell anyone!) on 11x17 paper and put it in an artist's presentation portfolio that has clear 11x17 carrier pages. It's dew-protected and easy to use.
I also printed (at Kinko's this time) his 8.5 mag atlas on 8.5x11 and had them spiral bind it with a clear plastic cover. Not free, but at ~$30 it's at excellent atlas at an excellent price. Being 8.5x11 (with spiral binding) and completely "foldoverable" it's really handy at the eyepiece.
He even has a double-sided planisphere, but it's obviously a bit more work (and less durable) than purchasing a plastic one.
Yes, they're PDFs, in fact mostly groups of PDFs, but that can be an advantage since if offers the option to go electronic or paper. For those so inclinded, there are lots of free tools to merge and split PDFs, so that's not a problem, either.
I've really come to like his 6.5 mag atlas for quick work, particularly with binoculars and my little 80mm refractor. I printed it at work (don't tell anyone!) on 11x17 paper and put it in an artist's presentation portfolio that has clear 11x17 carrier pages. It's dew-protected and easy to use.
I also printed (at Kinko's this time) his 8.5 mag atlas on 8.5x11 and had them spiral bind it with a clear plastic cover. Not free, but at ~$30 it's at excellent atlas at an excellent price. Being 8.5x11 (with spiral binding) and completely "foldoverable" it's really handy at the eyepiece.
He even has a double-sided planisphere, but it's obviously a bit more work (and less durable) than purchasing a plastic one.
Nice post! This is a very nice blog that I will definitively come back to more times this year! Thanks for informative post. Personalized star map
Post a Comment
<< Home
