How to use a star chart
Senior Editor Rich Talcott helps you navigate the night sky using Astronomy magazine's pullout star chart — located in the center of each monthly issue.
The first question most beginners ask is: "How do I find my way around the sky?" The easiest place to start is with an all-sky map, like the one you'll find in the center of every issue of Astronomy magazine. I happen to have the April 2009 issue with me. Open it to the center, and you'll see the fold-out map we call the StarDome. I've mounted this map on cardboard to make it easier for you to see how to use it.
Because the sky lies overhead, we designed the map to be held above your head. Let's start our tour in the western sky. Find the big yellow "W" on the map's right side, and orient it so the "W" lies at the bottom as you look due west. Late on April evenings, you'll see the stars of winter descending toward the horizon. Only part of Orion remains visible, while the bright star Procyon and Gemini's twin stars, Castor and Pollux, appear prominent higher in the west.
Next, shift your gaze to the southern sky. Turn the map 90° counterclockwise and place the yellow "S" at the bottom. More than halfway up in the sky, you'll see two of spring's bright constellations: Leo and Virgo. The planet Saturn currently lies in southern Leo, below it as you look up.
Note that as you face south with the map overhead, east lies to your left and west to your right. This arrangement looks odd when you look down on the map, but it's perfectly logical as you look up.
Go back to Leo and Virgo, then crane your neck and look straight overhead. Here you'll see the large constellation Ursa Major, with its group of seven bright stars known as the Big Dipper. To find the other bright stars and constellations scattered across April's evening sky, simply turn the map to match the direction you're facing.
We plot the stars on the StarDome as they appear from 35° north latitude, so the map works well if you live between 25° and 45°. This covers most of the United States. And the map gives a good approximation of the sky from as far north as 50° latitude and as far south as 20°. Each month's map depicts the sky in late evening, after twilight has faded completely. The exact times appear at the top left of the map, but the sky doesn't change a lot within an hour or two of these times.
The brownish line you see curving from southeast to northwest represents the ecliptic — the apparent path of the Sun across our sky. Of course, the Sun is not out at night, but the ecliptic still serves a useful function. All the solar system's planets lie near the ecliptic, so this is the area to look for our celestial neighbors. In April, only Saturn appears in the late-evening sky.
All stars appear as points of light to the naked eye. But a map with nothing but small points on it wouldn't help much. On our map, we show brighter stars as bigger circles. Astronomers measure a star's brightness on what's called the magnitude scale. We show the scale for our map at the lower left corner. In keeping with ancient tradition, the fainter a star, the higher its magnitude. A star of magnitude 0.0 appears 100 times brighter than a star of magnitude 5.0. Next to the magnitude scale are the symbols we use for deep-sky objects. Many of these are too dim to see with the naked eye, but the ones on our map do appear nice with binoculars or a small telescope.
Under a dark sky, the brightest stars show hints of color. We show this on the StarDome as well. The hottest stars glow with a bluish tinge and can be up to five times the Sun's surface temperature. Slightly cooler stars appear white, followed by yellow stars (like the Sun). Still cooler are orange and, finally, red stars. These may be only half the Sun's surface temperature. Fainter stars don't shine bright enough to excite the color receptors in our eyes, and will appear white unless you magnify them with binoculars or a telescope.
Remember that Astronomy.com offers a host of tools to help you make the most of your time under the stars: