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How Do I Read a Star Chart?
How Do I Read a Star Chart?

With so many modern stargazers relying on computerized telescopes to locate objects, learning to read a star chart might seem like a lost art form. However, using a star chart is very easy, and it helps you really understand what you're looking at and the way the sky works. Additionally, not all telescopes have on-board computers and not all star gazers want to use them. There's a lot of satisfaction to be gained by learning to use a star chart and manually locating objects. By knowing a few simple instructions, you'll find using a star chart is like a recipe. All you need to know are some common measurements and how to find the key ingredients! Let's start with the basics.

Planispheres and All Sky Chart
The first type of star chart you'll probably encounter is a planisphere or an All Sky Chart. These are normally presented in a round format which depicts the brightest stars and constellations as they are seen during specific times of the year. A planisphere is printed on a wheel with dates and times to help you select a key "window" showing the general night sky for the selected time. Like the planisphere, the seasonal All Sky Chart is designed to be held over your head and aligned with the cardinal directions. Unlike a terrestrial map, all star charts have east to the left and west to the right - the correct orientation to match Earth's movements and the apparent movements of the celestial sphere. To help you understand the concept, hold an All Sky Chart over your head and face south. You'll notice east, the direction of sunrise, is to the left, north is behind you and west, the direction of the setting Sun, is to the right. Now all you have to do match up bright star patterns with what you see and identify the primary constellations. These types of star charts are like gathering together the ingredients.

Formal Star Chart
Once you have familiarized yourself with the constellations and key stars that you see, it is time to get more specific. First, choose an area of the sky in which you'd like to work. You'll notice two sets of numbers along the margin of the formal star chart. Like on a terrestrial map, which uses the alphabet in one direction and numerals in the other, these sets of numbers divide the map into sections. In reading from left to right - which (unlike a terrestrial map) represents east to west - you'll see hours, numbers and seconds. These numbers along the top margin are called Right Ascension and it is abbreviated in celestial coordinates as RA - the equivalent of terrestrial longitude. Each celestial "day" is divided into 24 hours (of rotation of the Earth) and begins on the point of vernal equinox. This "zero hour" is the place in the sky where the Sun crosses the celestial equator during the March equinox (equal times of day and night). The second set of numbers reads from top to bottom - north to south - and they are either positive or negative. This set of directions is the Declination and is abbreviated as Dec. It is the celestial equivalent of latitude. Positive numbers are located above a line on the charts, and an imaginary line in the sky, called the celestial equator. This is the dividing line between the northern half and southern half of the night sky. Negative numbers lay to the south.

Why are these numbers important? All astronomical objects, even stars, are given a set of celestial coordinates which use right ascension and declination. These coordinates remain constant. If you are looking for a specific object, you can use its assigned directional value as a recipe to help you find it! For example, if you wanted to view the Crab Nebula you might ascertain its coordinates from a resource such as an astronomy magazine, an astronomical catalog or an observing book. In this listing you would see the directions: RA 05 34 31 - Dec +22 00 52. This means you can locate it both on your star chart and in the sky at 5 hours, 34 minutes and 31 seconds in right ascension and north of the meridian at +22 degrees, 00 arc minutes, 52 arc seconds. Begin reading your star chart at the zero hour and count the hours across to the west (right) until you reach the fifth hour. Use the declination scale along the side margin and locate positive twenty-two degrees. By looking at the larger scale, you can determine the constellation in which it is located and the general area in which it can be found. Until you have become familiar with the night sky at all seasons, you may need to use a planisphere or All Sky Map to make sure the constellation where your object is located is visible. Once you've determined this, it's time to go on to the next step.

Take a look at the large, printed stars around your object on the map. These primary stars are the ones you'll be looking for when translating map to sky. Note they also have a designation. If you're thinking "That's all Greek to me!" then you'd be correct. Very bright stars are given a Greek letter designation such as Alpha, Beta or Gamma. These designations are not only stellar names, but indications of the star's brightness - starting with Alpha as the most luminous and descending in order. Most good star charts will have a key to these letters as well as a magnitude key where the printed star size is also given a brightness value. Common numerical designations on stars are called either Bayer or Flamsteed numbers. These are stellar catalog numbers assigned to bright stars and were originally created by historical astronomers, Johann Bayer and John Flamsteed. Most ordinary star charts use Bayer numbers - along with Greek letters - but Flamsteed numbers are used where no Bayer designation is given.

From Star Chart To Sky
The next step in using a star chart is to match what you see on the map with what you see in the sky. Begin with the constellation, and then identify the very brightest of the stars you see around your designated target and locate that pattern in the sky. For example, you know from looking at the map that your object is located about ten degrees west of Alpha X. The key is to start big and get smaller, but how do you translate degrees from a book to degrees in the sky?!

Don't panic. A simple way of measuring the sky is to use your hands. Hold your hand outstretched at arm's length. From the tip of your little finger to the tip of your thumb is approximately 20 degrees. If you make a fist, that is about 10 degrees. The width of your thumb is somewhere around 2 to 3 degrees. This easy way of measuring will assist you in finding the general location of what you're looking for. Take an even closer look at your star chart and you'll notice it is also divided into other, smaller sections - usually 10 degrees. While this might seem a little confusing at first, you can learn if you practice!

Now, look at your star chart again. You have found the primary stars around your target and your telescope is aimed in the general direction. Use your optical finderscope to assist you in locating and identifying even fainter stars which match the pattern on the star chart. By knowing how many degrees your finderscope reveals, you can even further refine your hunt.

If you use an equatorial mount, you're also in for a big surprise. Once you have reasonably polar-aligned your telescope and set the axis at least close to your latitude, take a close look at those numerical dials on the mount. Do those numbers look familiar? Darn right, they do! One set is Right Ascension and the other is Declination. It might be like comparing an abacus to a calculator, but these handy tools can also get you very close to the perfect recipe for a starry night!

Date Taken: 11/15/2012
Author: Orion Staff
Category: Astronomy

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