How to Read Star Charts

At first glance star charts may appear to be filled with a confusing web of thousands of dots and a criss-cross of lines. All this and a maze of mysterious acronyms and foreign sounding jargon make celestial maps appear to the novice like an encoded treasure map.  Star charts however can be just that, a guide to hidden riches of the starry skies – if you know how to decipher them. Filled with a bounty of maps of the entire sky, star atlases can easily point out hundreds if not thousands of celestial jewels visible from the naked eye, through binoculars to telescopes. There are a number of good atlases in print today, and below is a quick run down on how to plunder this essential tool of the amateur astronomer.


Marks the Spot – Dialing up the Heavens

Before using even the most basic of sky maps it’s important to understand how we pinpoint the positions of stars in the sky.  As the Earth spins on it’s axis, all stars appear fixed in place in relation to each other and move together across the night sky from east to west.  Picture the Earth at the center of a huge imaginary celestial sphere. With all the stars visible on its inside surface, this sphere’s Celestial North is marked by the North Star, Polaris. Therefore, when reading star charts always remember that Celestial North is not up but towards Polaris.

If we then transfer the familiar coordinate system of latitude and longitude used on the Earth’s surface onto this star globe, we have a celestial grid system that mimics its terrestrial counterpart.  In the sky,  “latitude” is called <em>declination</em> while the “longitude” equivalent is <em>right ascension</em>, so that every star, nebulae, and galaxy has a fixed address in the sky – just as every house, town or city has its precise location back on Earth. Declination (abbr. DEC) is measured in degrees, minutes and seconds starting from 0 degrees at the “celestial equator” to 90 degrees at the poles. Declinations north of this equator are listed as positive while south is negative. Right ascension (abbr. RA) is measured in hours, minutes and seconds where a complete 24 hours marks a full rotation equal to 360 degrees. The baseline of 0 hour RA (equivalent of Earth’s Greenwich meridian of longitude) is marked by the point where the Sun’s path passes through the celestial equator.

Universally all star charts use this grid system. Celestial coordinates do change over time however, as the Earth slowly shifts its orientation slightly over the years. You might notice that in many atlases the coordinates are accompanied by a year date. This means that X marks the position precisely for that particular year.


Choosing the Right Map

Maps of the night sky may have lesser or greater details. At the lower end, maps of the entire seasonal sky may only show the brighter stars and constellations suitable for the casual or backyard stargazer. More advanced skywatchers with binoculars and telescopes will need higher resolutions to unveil fainter stars and many deep sky objects like star clusters, nebulae and galaxies. These magnified, more detailed charts may be a bit confusing at first, but by taking note of only the brightest of stars, familiar constellations can still be recognized. As a rule of thumb the fainter the stars plotted on a chart, the more crowded it becomes.  Wil Tirion’s Sky Atlas 2000, for instance, has over 40 000 stars while Uranometria 2000 plots an astounding 280,000! The main reason for such a difference is magnitude.


Star Light, Star Bright

Size is important when it comes to reading star atlases, especially when comparing the brightness of celestial objects.  It’s easy to notice on star charts the graded series of dot sizes- some are large circles while others are mere specks. Based on a scale first proposed in the beginning of the second century B.C. , the brilliance of stars is measured in terms of magnitude (abbr. Mag.). The fainter a star in the sky the tinier it’s counterpart “dot” on a star chart, with each size representing a whole magnitude. Also the higher the magnitude numbers the fainter the star. As mentioned earlier, atlases vary greatly in the amount of plotted stars. For example the Cambridge Star Atlas plots objects as faint as the unaided eye can see from super dark skies (approx. 6.0 Mag.). While Uranometria 2000 displays objects down to the limit of today’s popular 7x 50 binoculars (9.0 Mag.).


What’s in a Name ?

All 88 official constellations boundaries are clearly outlined in major atlases, while their constituent stars can be identified in a number of different ways. The brightest may be accompanied by their proper names derived from ancient Greek, Roman or Arabic languages, others simply by Greek letters only, as started in 1603 called the Bayer system (i.e. brightest star in constellation is Alpha). Another is the Flamsteed system, which numbers the stars of a constellation from west to east (i.e. Deneb is 50 Cygni). Other star catalogs that are in use include Henry Draper (HD), Smithsonian Astrophysical Observatory (SAO) and Aitken’s Double Stars (ADS).  Brighter variable stars, usually marked by empty circles, are identified by a letter designation.

Star clusters, nebulae and galaxies are mainly identified by either their number in the Messier catalog (M) or their designation in the more extensive New General Catalog (NGC) and its extension Index Catalog (IC). Certain individual objects may be accompanied by their popular names such as the Orion nebula. Major atlases may mark these deep-sky objects with various symbols like ovals, circles with cross-hairs, and squares, all graded according to their size.


Once you’ve explored the standard naked-eye objects found on most all-sky maps, it’s time to turn to a star atlas that can offer greater detail and higher accuracy.  With a comfortable limiting magnitude of 6.5 and a manageable 9,500 stars, the Cambridge Star Atlas is great for showing beginners the pathways between constellations, finding well known double stars and clusters.  With more experience and higher magnification, hopping from one star to the next using binoculars or a finder’s scope, Star Atlas 2000 offers more of a challenge with an 8.1 magnitude limit and more than a four time increase in stars with 2,500 deep sky objects.  Of course neither of the above are able to show all that would be actually visible through a small or medium sized scope. For that, Uranometria 2000 with its fainter 9.5 magnitude limit, hundreds of thousands of stars and over 10 000 deep-sky objects, make it easier to zero-in on your favorite, gem-like planetary nebula or far-flung galaxy. Regardless of experience use an atlas where the labeling and lettering are the most clear and least confusing. As you become more comfortable cruising among the constellations and locating ever-fainter deep-sky challenges, star atlases will remain your key to charting your next course among the starry skies.


– Copyright Andrew Fazekas