Deciphering Star Charts for Skygazing

Every stargazer discovers what she or he needs to enjoy the sky. Take it easy and all good things will eventually come to you. Halfblue/Wikimedia Commons Share and Share Alike license.

Stargazing can take you across hundreds or thousands of light-years in the time it takes to glance upwards. It opens up a universe of planets, moons, stars, and galaxies to anyone who wants to learn about them. All they have to do is wander outside on a clear dark night and simply look up. It can hook people into a lifetime of exploring the cosmos at their own speed. 

Of course, it helps if people have some sort of guide to the stars. That's where star charts come in handy. At first glance, a star chart may seem confusing, but with a little study, it becomes a stargazer's most valuable tool.  

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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

The first thing that people do when they stargaze is to find a good observing spot, and might even have a good pair of binoculars or a telescope. The best thing to start out with first, however, is the star chart. 

Here is a typical star chart from an app, program, or magazine.  They can be in color or black and white, and festooned with labels.This chart for the night sky for 17 March, a few hours after sunset. The design is pretty similar throughout the year, although different stars show up at various times of the 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

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. The easiest things to start with are those that are visible to the naked eye, or that can be easily spotted with binoculars and/or typical backyard-type telescope (which will extend the view to about magnitude 14).  

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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. People need to know where north is. For Northern Hemisphere dwellers, the North Star is important. The easy way to find it is to 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.  

Once a stargazer finds the North Star, they're facing North. It's a very elementary lesson in celestial navigation that every astronomer learns and applies as they progress. Locating north helps skygazers find every other direction. Most star charts show 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

Long-time stargazers notice that the stars seem to be scattered in the sky 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 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 Across the Sky

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

In Cardinal Points, it's easy to see how to "hop" from the two pointer stars in the Big Dipper to the North Star. Observers can also use the handle of the Big Dipper (which is sort of an arc shape) to star-hop to nearby constellations.  Remember the saying  "arc to Arcturus", as shown in the chart. From there, the viewer can "spike over to Spica", in the constellation Virgo. From Spica, its a leap UP to Leo and the bright star Regulus. This is one of the easiest star-hopping trips anyone can make.  Of course, the chart doesn't show the leaps and hops, but after a little practice, it's easy to figure it out from the patterns of stars (and constellation outlines) on the 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". There's also the term "meridian" used. In the 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. One's horizon may be flat, or it may have landscape features such as hills and mountains. 

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Angling Across the Sky

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

To observers the sky appears 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" which is 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. This is noted on the chart. Also, angle measure line between the 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. Today, astronomers may use the names and same general outlines, but their science has nothing to do with astrological "magic". 

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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 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 how to find "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 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. 

On most star charts, a lot of clusters and nebulae seem to be 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. A quick look at the chart region for the constellation Coma Berenices, for example, shows 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

For stargazers, learning charts to explore the night sky can be a challenge. To get around that, use an app or online star chart to explore the sky. If it's interactive, a user can set their location and time to get their local sky. The next step is to get out and stargaze. Patient observers will compare what they see with what's on their chart. The best way to learn is to focus on small parts of the sky each night, and build up an inventory of sky sights. That's really all there is to it! 

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Your Citation
Petersen, Carolyn Collins. "Deciphering Star Charts for Skygazing." ThoughtCo, Feb. 16, 2021, Petersen, Carolyn Collins. (2021, February 16). Deciphering Star Charts for Skygazing. Retrieved from Petersen, Carolyn Collins. "Deciphering Star Charts for Skygazing." ThoughtCo. (accessed June 10, 2023).