Star Charts Got You Confused? Worry No More!

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How to Read A Star Chart and Stargaze

Here's a simulation of how the sky looks, using a program called Stellarium in sky observing mode. Carolyn Collins Petersen

Okay, you've got the binoculars, the observing spot, and a star chart. Maybe you and the kids are about to head out for an evening of stargazing.  Let's find out how that star chart will help you the most. 

Here is a typical star chart you'd see in an app, program, or magazine. You might also see them in black and white, with a white background and black stars. Just as an example, here's a chart for the night sky for 17 March, a few hours after sunset.  The design is pretty similar throughout the year, although you'll see different stars at various times of year. The brighter stars are labeled with their names. Notice that some stars seem to be larger than others. This is a subtle way of showing a star's brightness, its visual or apparent magnitude

You can read more about magnitudes here. Magnitude also applies to planets, moons, asteroids, nebulae, and galaxies. The Sun is the brightest at magnitude -27. The brightest star in the night sky is Sirius, at magnitude -1. The dimmest naked-eye objects are around 6th magnitude. You're going to be interested in everything you can see with the naked eye, as well as those things you can see with binoculars and/or a typical backyard-type telescope (which will extend the view to about magnitude 14).  

It seems backwards to have the brightest things be in the minus number category, while the dimmest ones have large positive numbers. Think of it as a way to distinguish bright from dim objects. 

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Finding the Cardinal Points: Directions in the Sky

Cardinal points are the directions north, south, east west. Finding them in the sky requires some knowledge of stars. Carolyn Collins Petersen

Directions in the sky are important. Here's why. Next time you're out, and if you live in the Northern Hemisphere, look for the Big Dipper. It has four stars in its handle and three in the cup.

The two end stars of the cup are important. They are often called the "pointers" because, if you draw a line from one to the other and then extend it down about one dipper length to the north, you run into a star that seems to be by itself—it's called Polaris, the North Star.  

When you find the North Star, you're facing North. It's a very elementary lesson in celestial navigation that every astronomer learns and applies as they progress. Once you find North, you can figure out other directions. To make it easier for you, most star charts show you what are called the "cardinal points": north, south, east, and west in letters along the horizon. 

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Constellations and Asterisms: Star Patterns in the Sky

Constellations, asterisms, and their names. Carolyn Collins Petersen

As you watch the sky for a while, you'll notice that the stars seem to be in patterns. The lines in this star chart mark out (in stick-figure form) the constellations in that part of the sky. Here, we see Ursa Major, Ursa Minor, and Cassiopeia. The Big Dipper is part of Ursa Major. 

The names of the constellations come to us from Greek heroes or legendary figures. Others—particularly in the southern hemisphere—are from 17th- and 18th-century European adventurers who visited lands never before seen. For example, in the southern skies, we get the Octans, the Octant and such mythical creatures as Doradus (the fabulous fish)

The best and easiest-to-learn constellation figures (in my opinion) are the H.A. Rey figures, as laid out in the books "Find the Constellations" and "The Stars: A New Way to See Them".

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Star-hopping Your Way Across the Sky

The blue lines show some typical star-harps in the northern hemisphere sky. Carolyn Collins Petersen

In Cardinal Points, you saw how to "hop" from the two pointer stars in the Big Dipper to the North Star. You can also use the handle of the Big Dipper (which is sort of an arc shape) to star-hop to nearby constellations. You "arc to Arcturus", as shown in the chart. From there, you spike over to Spica, in the constellation Virgo. From Spica, you leap UP to Leo and the bright star Regulus. This is one of the easiest star-hopping trips you can make.  Of course, your chart won't show you the leaps and hops, but you'll figure it out from the patterns of stars (and constellation outlines) that are part of your chart. 

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What About Other Directions in the Sky?

The zenith and meridian of the sky and how they look on a star map. Carolyn Collins Petersen

There are more than four directions in space. "UP" is the zenith point of the sky. That means "straight up, overhead". You might also hear the term "meridian" used. In your night sky, the meridian goes from north to south, passing directly overhead. In this chart, the Big Dipper is on the meridian, almost but not quite directly at the zenith. 

"Down" for a stargazer means "toward the horizon", which is the line between land and sky. It separates Earth from the sky. Your horizon may be flat, it may have landscape features. 

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Angling Across the Sky

Grids help you do angular measurements across the sky. Carolyn Collins Petersen

You probably have noticed that the sky is spherical. We often refer to it as the "celestial sphere", as seen from Earth. To measure distances between two objects in the sky, with respect to our Earthbound view, astronomers divide the sky into degrees, minutes, and seconds. The entire sky is 180 degrees across. The horizon is 360 degrees around. Degrees are divided into "arcminutes" and "arcseconds".

Star charts divide the sky into an "equatorial grid" extended out to space from Earth's equator. The grid squares are ten-degree sections. The horizontal lines are called "declination". These are similar to latitude. The lines from the horizon to the zenith are called "right ascension", similar to longitude. 

Each object and/or point in the sky has coordinates of right ascension (in degrees, hours, and minutes), called R.A., and declination (in degrees, hours, minutes) called DEC. In this system, the star Arcturus (for example) has an R.A. of 14 hours 15 minutes and 39.3 arcseconds, and a DEC of +19 degrees, 6 minutes and 25 seconds. See how this is noted on the chart. Also, angle measure line betweenthe star Capella and the star Arcturus is about 100 degrees. 

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The Ecliptic and its Zodiac Zoo

The ecliptic and the zodiac. Carolyn Collins Petersen

The ecliptic is simply the path the Sun makes across the celestial sphere. It cuts across a set of constellations (we see just a few here) called the Zodiac, a circle of twelve regions of the sky divided up equally into 30-degree parts. The Zodiac constellations correspond to what were once called the "12 Houses" astrologers once used in their hobby.

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Finding and Exploring the Planets

How planets are noted on a star chart, and some of the symbols you'll see. Carolyn Collins Petersen

The planets, since they orbit the Sun, also show up along this path, and our fascinating Moon follows it, too. Most star charts show you the name of the planet and sometimes a symbol, similar to the ones in the inset here. The symbols for Mercury, Venus, the Moon, Mars, Jupiter, Saturn, Uranus, and Pluto, indicate where these objects are in the chart and in the sky.  

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Finding and Exploring the Deeps of Space

Deepsky objects on star charts are denoted by various symbols. Carolyn Collins Petersen

Many charts also show you how to find what are commonly referred to as "deep-sky objects". These are star clusters, nebulae and galaxies. Each of the symbols in this chart refers to a distant deep-sky object and the shape and design of the symbol tells you what it is. A dotted circle is an open cluster (such as the Pleiades or the Hyades). A circle with a "plus symbol" is a globular cluster (a globe-shaped collection of stars).  A thin solid circle is a cluster and a nebula together. A strong  solid circle is a galaxy. 

As you study the star charts, you'll start to notice that a lot of clusters and nebulae are located along the plane of the Milky Way, which is also noted in many charts. This makes sense, since those objects are INSIDE our galaxy. The distant galaxies are scattered everywhere. Look at the constellation Coma Berenices, for example, to see many galaxy circles. They are in the Coma Cluster (which is a galaxy herd).

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Get Out there and Use Your Star Chart!

A typical chart you can use to learn where things are in the sky. Carolyn Collins Petersen

Here's a challenge for you. Use an app or online star chart to explore the sky. If it's interactive, set your location and time to get YOUR sky. Next, simply step outside and stargaze. Compare what you see with what's on your chart. Focus on small parts of the sky each night, and build up an inventory of sky sights you can recognize.  That's really all there is to it! 

Happy Stargazing!