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How to Read a Star Chart

If you are new to astronomy, finding your way around the night sky can be scary, overwhelming, and exciting all at once. 

Don’t worry! Even the most experienced eye takes time and effort to compare blurry dots on the star maps to stars, constellations, and other objects in the night sky. 

This post will be your first step to understanding what these maps are, learning how to read a star chart, and most importantly, what you can do with them. 

What Does a Star Chart Look Like? 

Similar to how we use terrestrial maps to find destinations on Earth, a star chart is a map of the night sky and one’s guide to navigating the stars. It represents the stellar dome with only the brightest stars and objects and the constellations.

Normally, a star chart only shows what’s visible to the naked eye under dark skies. It’s similar to a planisphere but they have different uses.

Stars are represented as black dots on a white background, which makes it easier to read when observing at night. The size of the dot reflects the brightness of the star, the bigger the dot, the brighter the star.

In addition to dots, there are gray circles that refer to star clusters and ovals to galaxies. Since not every night sky object is present here, astronomers use these brighter objects as references for star hopping to fainter ones.

The Milky Way is often shown too, either as an outline or a slightly grayed out area. 

A Sky & Telescope Star Map

Night Sky Coordinates

Before we get into the details of reading star charts, it’s helpful to understand a little bit about night sky coordinates.

Let’s say that you wish to locate Arizona on a map, but you only know that it is somewhere in the bottom left in the US. Wouldn’t it be so much easier if you were given exact coordinates, that AZ is 34° North and 111° West? The state can now be located in a jiffy. 

Likewise, locating objects in the night sky requires a reference frame for astronomers to navigate. The reference frames we use on star charts are equivalent to longitude and latitude used on maps of the earth. They are called right ascension and declination respectively. 

The celestial equator mirrors Earth’s equator and the north and south celestial poles are directly above those on Earth.

We developed an in-depth guide to the celestial coordinate system, if you’d like to learn more detail. 

Reading Your Star Chart

It helps to think of the sky as a dome around the Earth upon which stars and planets reside. As the night progresses, this little dome turns so that the stars seem to rise in the east and set in the west. 

Since the sky is above our heads, your star chart is designed to be held overhead when you read it. The point directly overhead is called the Zenith and can be found in the center of the chart. The outer edge of a circular map represents the horizon.

Atlas of the Night Sky (source)

Directions on the map

Charts can be obtained for the northern hemisphere and southern hemisphere. Wherever you are, the chart is read in the same way.

To use the map, make sure you are facing the right direction. Turn the map in your hands such that ‘N’ on the map is oriented towards the north. You’ll see that East and West in the map are the exact opposite of what we have here on Earth, which is because Earth maps are designed to be put on the ground whereas your star chart is to be held up in the sky.

Hands-on Usage of Star Charts

A good way to start to read a star chart is to identify the bigger, brighter stars and constellations. You can try starting your search with the Big Dipper- the group of seven bright stars in the constellation Ursa Major.

Ursa Major Chart (source)

Once you locate it in the night sky, try spotting it on your star chart. When you’ve found it, orientate the chart so that it matches your view of the night sky. From there, the Big Dipper can be used as your anchor to come back to if you get lost, and it’s a great jumping-off point to see other stars. 

Remember, the chart only shows naked eye objects but if you suffer from light pollution you may not be able to see everything on it. Keep in mind too that you’ll need your night vision intact to see the sky chart effectively, so only do so with a red flashlight.

Try experimenting with fainter objects, what can you see with your sky chart?

A little DIY project

We can create a little tool to help show how much of the star chart we can see in our eyepiece field of view. This makes it much easier to use the chart for star hopping.

To begin, you need to know the size of your finderscope’s field and then your lowest power eyepiece. First, make sure that your [finderscope is all aligned] and ready to be used. 

Ready to start? Great! 

First, try locating two stars (preferable belonging to a constellation like the Big Dipper) that fit just inside the edges of your finderscope. Locate the same constellation on your map to check the distance between the stars in degrees (the scale is mentioned on the map’s margins). Make a note of this measurement. 

Repeat this process to measure the field diameter of your telescope’s lowest power eyepiece. If you find it hard to pick a pair of stars to focus on, fix upon any star in the celestial equator such that it appears in the center of your field of view. 

From here, note the time taken (in seconds) for the star to drift from center to the edge. Dividing this by 120 gives the diameter of the field in degrees. 

Next, make little wire rings, from fuse wire, or similar, that correspond to these field sizes you just calculated. Traversing these rings across the map will give you a better understanding of what area of the sky you can expect to see while star-hopping (don’t be surprised if it is very little!). 

If DIY projects do not sound fun and you are short on time, no problem! You can simply look into Sky and Telescope’s Pocket Sky Atlas.

The inside of its cover contains rings for 0.5°, 2°, and 4° (matching what can be seen through a Telrad finderscope), these can be copied onto tracing paper and used with your star chart to navigate through the night sky. 

Telrad finder rings inside the Pocket Sky Atlas

Things to remember and other helpful tips: 

  1. The map might very well look like the remains of spilled ink. Take a step back to look at the bigger dots. These are the brighter stars and constellations that you should be able to spot easily. 
  2. If you find it hard to keep comparing the star chart with the night sky, go ahead and draw lines on your map so that you don’t miss your constellation among the thousands of visible dots. Once you get a better handle on this, you will swiftly go through these constellations with ease.
  3. Always try starting your search by knowing what to look for, including the right ascension and declination coordinates. It makes the entire experience more rewarding and less frustrating. It is easy to get lost in the vastness of space, especially if you are looking through a tiny eyepiece! 
  4. Once you get comfortable identifying the brightest stars and constellations from your star chart, try hopping from bright stars to fainter, more exciting objects.
  5. Observe under the darkest skies you can, where the big constellations are easy to see. Go armed with a red LED flashlights to read your maps in the dark nights!

Resources

Sky & Telescope Pocket Sky Atlas – The go-to choice of star map for backyard astronomy

Star Chart maker – Create your own star chart with In-The-Sky.org

Software – Generate your own guides using Stellarium (free) or SkySafari 6 (paid)

Virtual Astronomy Club – Join our monthly club giving you objects to hunt for and the maps you need to find them


Written by Sharmila Kuthunur