How to Read a Star Chart

6th December 2019

When stargazing, the naked eye can make out over 6000 stars, 88 constellations, tons of faint fuzzies, 5 planets and one moon. However, you may struggle to see what's what if you're unfamiliar with the star map. A star chart helps you to learn the constellations and the stars and faint fuzzies within them. But how do you read one? In this article, we explore how to read a star chart.

What is a Star Chart?

Also known as a star map, a star chart is essentially a map of the night sky. People have studied the stars for thousands of years, dividing the sky into grids. The grids help people to locate and identify constellations, stars, and galaxies. By mapping the sky on a star chart, humans have been able to travel the globe just by using the stars.

It's worth noting that a star chart isn't the same as an astronomical catalog. A star chart is a visual tool used to navigate the night sky, while an astronomical catalog is a book that lists astronomical objects. Star charts are particularly useful when used alongside a planisphere and an astrolabe.

The oldest known star chart was discovered in Germany in 1979 and is believed to be 32,500 years old. Worlds away from today's star chart, the map was carved onto an ivory mammoth tusk and contained the constellation, Orion.

How to Read a Star Chart

If you're new to star charts, they can seem like a confusing arrangement of dots, lines, and shapes. However, they're simpler than they look. You only need to know a few things for everything to make sense. When reading your map at night, use a red torch so that you don't ruin your night vision. As you won't be able to see colors with a red torch, a black and white star map is all you'll need.

Circles

On star charts, stars are represented by circles. Most star charts use different sized circles to show the brightness of stars. As you'd imagine, the biggest circles are the brightest stars, which remain visible regardless of the light pollution level. The smallest circles are the faintest stars that you'll only make out from a rural sky.

Some star charts have a wider range of circle sizes, to include very faint stars that can only be seen in certain conditions, or with a telescope or binoculars. These charts are great for those looking to explore the sky in detail.

Constellation Lines

On a star chart, you'll see lines joining certain stars together. These show the general outline of a constellation. Depending on the map, these may just show the brightest asterism within the constellations, such as the Summer Triangle or the Big Dipper. While all constellation boundaries are well defined, the way the stars are joined may vary from chart to chart.

Multiples and Variables

Certain stars are more interesting than others. For instance, some are actually multiples. When looking through a telescope, you'll see that more than one star is making up the circle visible to the naked eye.

Variable stars change in brightness over time, brightening and dimming throughout the year. As stargazers find these stars particularly interesting, they're often marked differently on star charts.

You'll normally find the following types of stars marked differently on a star chart:

  • Multiple stars

    Some multiple stars, such as Alcor and Mizar in the handle of the Big Dipper, are two stars far enough apart to be marked individually on the star chart. In this case, there is no need to use a special symbol. If multiple stars overlap, a black dot is placed on top of another black dot. In this case, the smaller dot usually has a white border for clarity.

    In most cases, however, multiple stars are too close together to mark both on the chart. In this case, the star is given a different label. Instead of a standard dot, it's marked as a dot with a line through it. As Alcor and Mizar are both double stars, they are marked with a line through them.

  • Variable stars

    Variable stars are stars that change in brightness. These are often marked with a dot inside a circle. The size of the outer circle represents the maximum brightness, while the dot inside shows the minimum. Sometimes the star will be bright enough to fill the circle, while other times it's as faint as the inside dot.

Coordinate lines

Some star charts include coordinate lines. These help with navigation, allowing you to more accurately find a faint fuzzy. Sometimes, the coordinates right ascension and declination are marked on a star chart. These are roughly equivalent to longitude and latitude on Earth. Referred to as RA (right ascension) and dec (declination), these coordinates mark the position of a star using two points in the sky: The celestial equator – a line in the sky directly above the Earth's equator, and the First Point of Aries – situated in the constellation of Pisces.

Dec is measured in degrees and tells you how far away the star is from the celestial equator. When people say that a star has a declination of –52°41', it means that it's that many degrees south of the celestial equator.

RA is measured as the angular distance on the celestial equator from the First Point of Aries. Instead of measuring this angle in degrees, it is measured in hours, minutes, and seconds, with a whole circle around the Earth taking 24 hours. When people say a star has an RA of 06h24m, it means that the RA coordination is just over one-quarter of the way around the Earth from the First Point of Aries.

So, there you have it – how to read a star chart. If you're just getting started, the most important things to remember are the circles and constellation lines. Once you get the hang of these, you'll be stargazing in no time.