How to Read a Star Chart: A Q&A with Astronomer Ian Ridpath
Navigating the Sky
Not sure how to read a star chart? Help is here! Ian Ridpath is a seasoned astronomer who will answer our questions about how to find what you're looking for in the night sky. We're consulting the August star chart in his Monthly Sky Guide, 9th edition.
I want to locate stars and constellations, but I’m not sure how to read a star chart. What are the differences between charts, sky guides, and atlases, and which do you recommend for a beginner? What features should I be looking for?
A star atlas is different from a whole-sky chart since it does not show you how the sky appears at any given time. The Monthly Sky Guide has both types of chart—monthly all-sky charts and individual detail charts of constellations of particular interest.
The monthly charts in The Monthly Sky Guide show the stars as they appear at around 10 pm (or 11 pm in summer time) for each month. The sizes of the dots are an indication of the brightnesses of the stars. Some stars show a slight tinge of colour, depending on their surface temperature, so some of the dots are tinted accordingly.
Once you have located your constellation of interest, you can turn to one of the detail charts in The Monthly Sky Guide to find its features in more detail.
As a beginner, what’s the first thing I should try to identify with a chart?
Finding your way around the sky is like finding your way around a town or city you haven’t visited before. You start from a major landmark and work your way outwards. You can always return to the landmark and start again if you get lost, or head off in another direction.
The main celestial landmark in the northern sky is the Plough (in red on the map), part of the constellation Ursa Major, the great bear. It consists of seven stars that make up the outline of a saucepan. Another popular name for this shape is the Big Dipper.
In the summer, the Plough is fairly low down in the north and might be difficult to find. As all Boy Scouts should know, a line drawn from the two stars of the bowl of the saucepan farthest from the handle point to Polaris, the north pole star. This star isn’t exactly at the north pole of the sky, but it’s near enough to orient yourself. When you are facing Polaris, you are facing north.
If you continue the line beyond Polaris, you will come to a W-shaped group of stars. This is the constellation called Cassiopeia (in green on the map), and is another good reference point for finding your way around the sky.
When I take my map outside at night, how do I orient myself so that I can read it accurately?
You can find north at night by reference to Polaris, as just explained. Turn the chart accordingly.
You might think that East and West are marked on the wrong sides of the chart, but that’s not so. If you hold the chart above your head, East and West will be the correct way round!
How can I use this map to find a particular constellation?
If you look at the August chart in the Monthly Sky Guide (pictured above) you will find a light-blue star called Vega almost at the centre. Vega is in the constellation Lyra and appears almost overhead from mid-northern latitudes as the sky darkens in the summer. It is one corner of a large triangle of stars, called the Summer Triangle (in yellow on the map). The other two stars in the triangle are Deneb in Cygnus and, lower down, Altair in Aquila. You can find all these on the monthly chart for August.
Cygnus represents a swan, one of the mythological guises of the Greek god Zeus. It is popularly termed the northern cross, since it is shaped like a large cross. You can see a detail chart of Cygnus on page 55 of the Monthly Sky Guide and read about its many wonders on the facing page. No summer night is complete without a sighting of the summer triangle and Cygnus, the swan, flying overhead along the Milky Way.
Go forth and consult your star charts! Summer Stargazing is flying by—now that you know what to look for and how to find it, you can tackle a new challenge…how about astrophotography? David Eicher will be on the blog next week answering our questions about how to take fantastic photographs of the night sky. Don’t miss it! In the meantime, tell us why you stargaze on Twitter—we’ll put our favorite reasons on the blog! Use the hashtag #WhyIStargaze.