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…