What is a Star Chart and How Do You Use One?

Charles Messier's star chart of 1764, showing the curved path of a comet
Charles Messier’s star chart of 1764, showing the curved path of a comet. Image Credit: Charles Messier

Things have changed a lot over the past ten thousand years or so. Nowadays we can travel from point A to point B by simply opening an app on our phone and following directions. “Continue straight for five miles. Turn left at the next intersection.”

Twenty years ago, the technology was still in its infancy and thirty years ago it was just a twinkle in Steve Jobs’ eye. You had to navigate your way with the aid of a paper map. It took time and it took planning and you had to know what you were doing.

There were days when you’d go out for a drive and just explore, just to discover previously unseen sights, but how many of us do that anymore? It’s the same with the stars.

Today we have computerized telescopes and apps that tell us where we should look. We don’t need those paper star charts, right? They’re a relic of bygone days that, like the road atlas you used to carry in your car, are simply a reminder of a time when getting lost was easy and finding your way was hard.

Imagine that no such maps or apps exist. Imagine trying to get from point A to point B by simply heading in the right general direction, somewhere “over there.” Imagine trying to find the Whirlpool Galaxy when you only know that it’s in the constellation of Canes Venatici.

Who Created the Earliest Star Charts?

Our ancestors, of course, had no choice. They didn’t have the luxury of smartphones and apps. Instead, they typically recorded their observations of the stars on cave walls.

The earliest depictions of the stars date back tens of thousands of years and typically show constellations like Orion, Taurus or

This star chart originates from 8th century China and depicts the stars around the north celestial pole
This star chart originates from 8th century China and depicts the stars around the north celestial pole.

the night sky’s most famous star cluster, the Pleiades. In fact, back then, the constellations as we know them today didn’t even exist. They were simply groups of stars in the sky that people wanted to record for posterity.

The oldest civilizations – the Egyptians, the Mesopotamians, the Greeks – all had star charts depicting the night sky filled with their constellations. No longer designed to simply record the stars as they saw them, these charts allowed others to see them too.

Over the course of time, as trade and travel opened up previously unexplored areas of the world, the original 48 Greek constellations became adopted internationally. There was just one problem: the further south you went, the less those star charts and constellations were relevant.

As the curve of the Earth took explorers to new lands, those same explorers found themselves under new skies, with strange stars and no identifiable constellations. Just as the ancient Greeks created constellations of creatures and characters familiar to them, so the astronomers of 17th and 18th century Europe added new, contemporary constellations.

This star chart from 1825 shows several constellations that no longer exist. For example, Noctua the Owl is clearly visible to the left while Felis the Cat is over to the right.
This star chart from 1825 shows several constellations that no longer exist. For example, Noctua the Owl is clearly visible to the left while Felis the Cat is over to the right. Image Credit: Sidney Hall, Richard Rouse Bloxam and Adam Cuerden

Forty new constellations – such as the sextant, the compasses and, appropriately, the telescope – now became celestial companions to the original forty-eight. None of those were officially defined until 1922 when the International Astronomical Union determined the final list of 88 recognized constellations.

Until that time, many star charts had been produced depicting constellations that have since simply ceased to exist. For example, Noctua (the Owl), Aranea (the long-legged spider), Felis (the cat) and Hirudo (the leech), never gained enough popularity to survive.

All these charts helped observers to find and identify the stars and, until the invention of the telescope, they served their purpose. The telescope allowed astronomers to see thousands of deep sky sights that were simply invisible to the naked eye.

What is Right Ascension and Declination?

Cartographers could, of course, mark the positions of these objects on the star charts too, but there needed to be some way of recording those positions accurately and with no ambiguity.

Astronomy needed a coordinate system. Hence, the development of right ascension and declination.

If you’re familiar with the concept of latitude and longitude, then understanding the right ascension and declination shouldn’t be a problem for you. On maps and globes of the world, we use latitude and longitude to specify an object’s location upon the Earth.

While both are measured in degrees, latitude represents how far north or south of the equator the object is. Longitude then,

This detailed star chart shows the stars and deep sky objects around the three stars of Orion's belt. Degrees of declination are marked along the sides while hours and minutes of right ascension are along the top and bottom.
This detailed star chart shows the stars and deep sky objects around the three stars of Orion’s belt. Degrees of declination are marked along the sides while hours and minutes of right ascension are along the top and bottom. Image Credit: Roberto Mura

measures how far east or west the object is, as measured from the Greenwich Meridian line in London.

The astronomers of the 18th century adapted this system to pinpoint the positions of the stars and deep sky objects. Instead of latitude, they used declination. Like its terrestrial equivalent, declination is measured in degrees, minutes and seconds and represents how far north or south of the celestial equator an object is.

Right ascension, the celestial complement to longitude, is a little different. Measured in hours (from zero to twenty-four), minutes and seconds, it denotes an object’s position from the point where the Sun moves north across the celestial equator in March.

(That point has a right ascension of 0 hours, 0 minutes and 0 seconds. Similarly, it has a declination of precisely zero degrees. It gets a little more complicated in that the point moves over the course of thousands of years, but that’s a topic for another time!)

For example, Sirius, the brightest star in the sky, has a right ascension (R.A.) of 6 hours, 45 minutes and 9 seconds. Its declination is -16 degrees, 42 minutes and 58 seconds. Its negative declination places it in the southern celestial hemisphere.

Incidentally, if you want to know if an object is (theoretically) visible from your location, there’s an easy way to figure this out. Simply deducting the absolute value of the declination from 90 will tell you how far north or south that object is visible.

For example, we know Sirius lies in the southern half of the celestial sphere because it has a declination of -16 degrees. If we take that absolute value (16) and deduct that from 90, we get 74. So Sirius may be visible from anywhere in Earth’s southern hemisphere, but it will never rise above the horizon for observers north of 74 degrees north. You will never see Sirius from the north pole!

Similarly, the bright star Arcturus has a declination of +19 degrees, 10 minutes and 57 seconds. This means it lies in the northern half of the celestial sphere. If we deduct 19 from 90, we get 71 – so the star will never rise above the horizon for observers south of 71 degrees south on Earth. That star will never be seen from Antarctica.

It might sound a little complicated at first but it’s relatively straightforward in practice. Having said that, you might be tempted to simply use a GoTo telescope instead. These computerized scopes have the coordinates of thousands of objects stored within them. By telling the scope which objects to look at, its computer looks up the coordinates and points the telescope in the right direction.

Easy, but where’s the fun in that? It’s far more satisfying to hunt your target and find it by star hopping – which we’ll come to shortly. If you find the idea of coordinates daunting, it might be wise to familiarize yourself with a star chart that’s a little easier to use.

Planispheres – The Night Sky at a Glance

Flip through almost any astronomy magazine and there, in the center pages, you’re sure to find a chart of the entire night sky. Since these magazines are typically published monthly, it’ll show you which stars and constellations are visible that month. It’ll also have a key on the same page telling you when to go outside and use the chart.

What you have is called a planisphere and it’s the ideal way to learn the night sky. By going outside and turning the magazine toward the appropriate horizon, you can get a good idea of what’s visible that night. For example, if the magazine says the chart is good for the 15th at 9 pm, you can go outside at that time and then turn the magazine so that whichever direction you’re facing is at the bottom.

Unfortunately, magazines can’t provide accurate depictions of the night sky for every location in the world. That, of course, would be simply impractical. They’re typically designed for the locale of their largest readership. Astronomy and Sky & Telescope magazines have charts for North American observers while The Sky At Night magazine has charts for those in the United Kingdom.

The good news is that they’re good enough to be used in most major towns and cities in that part of the world – and many are now printing equivalent charts for the southern hemisphere too.

If you want a little more accuracy, it’s worth spending a small amount of money and buying your own planisphere. These are

This planisphere, produced by Sky & Telescope magazine, allows you to "dial up" the night sky for any time on any date.
This planisphere, produced by Sky & Telescope magazine, allows you to “dial up” the night sky for any time on any date. Image credit: Richard J. Bartlett

large discs that comprise of a star chart with a mask that overlays it. Around the edge of the disc are markings denoting the date and time. These allow you to “dial up” a specific date and time so you can see how the night sky should appear at that particular moment.

These are typically produced for various latitudes and you simply need to buy the planisphere that’s closest to your own. For example, if you lived at a latitude of 42 degrees north, you’d look for a planisphere that depicted the night sky at either 40 degrees or, failing that, 50 degrees north.

Planispheres, to some extent, are no longer as popular as they used to be. Smartphone apps have made a valiant attempt to replace them, but planispheres have two big advantages over smartphones: they don’t need power and they won’t ruin your night vision.

If you’re using your phone to identify the stars and your battery runs down, you could well be out of luck. And although many apps now have a “night” feature that turns the screen red (thereby protecting your night-adapted vision) if you push the wrong button, you could close that app and suddenly be dazzled by your screen.

A planisphere is a great, quick-fix solution to seeing what’s up in the sky tonight, but if you want to discover some deep sky wonders, you’ll probably be lost. Some planispheres may show you the positions of the brighter star clusters, nebulae, and galaxies, but unless you already know where to look, you’re going to need something a little more detailed.

Star Hopping With a Star Atlas

You’re going to need a star atlas – and this is where you need to know your right ascension from your declination. A star atlas works in much the same way as an atlas of the world; open it up and you’ll find charts of the night sky with degrees of declination along the side and hours of right ascension along the top.

You just need to know the coordinates of the object you’re looking for and then turn to the relevant page in the atlas. By carefully studying the approximate area of the chart, you can usually find the object you’re looking for on the page.

But again, we have apps for that, right? And we can tell our GoTo telescope what object we want to observe and it’ll take us there. Why go to all that bother, especially when the atlas might tell us where to find an object, but not necessarily how?

This is where the challenge lies – not to mention the fun. There’s no sense of achievement when you tell your GoTo you want to look at the Whirlpool Galaxy and it takes you right to it. What have you done to deserve that prize?

With a star chart, you can star-hop to your destination and truly discover something new, like the astronomers of centuries past. Star hopping is when you start with a bright star and then progressively hop across the sky, from star to star, until you come to your target.

Let’s take the Whirlpool Galaxy as an example. It lies at right ascension 13 hours 29 minutes and 52 seconds. Its declination is +47 degrees 11 minutes and 40 seconds, placing it well within the northern celestial hemisphere.

Its nearest bright star is Eta Ursae Majoris, commonly known as Alkaid. This magnitude 2 star is the star at the very end of the big dipper’s handle. By looking at a star chart, we can see that there’s another, slightly fainter star nearby – 24 Ursae Majoris (24 UMa).

If we center Alkaid in our finderscope’s field of view, we should be able to see 24 UMa nearby, just to the south-west. Another,

A close-up view of page 32 of Sky & Telescope's Pocket Sky Atlas, showing the location of the Whirlpool Galaxy
A close-up view of page 32 of Sky & Telescope’s Pocket Sky Atlas, showing the location of the Whirlpool Galaxy. Image Credit: Richard J. Bartlett

slightly fainter star should appear just to the north of 24 UMa, but if we look to the south, we should see a triangle of three faint stars. Just beside the southernmost star, if we look very carefully, we should be able to see a tiny, faint patch.

This is the Whirlpool Galaxy. It might not look like much – and it certainly doesn’t look like the amazing images you see in books and magazines – but if this is your first time tracking down this famous galaxy, nothing beats knowing that you found it yourself.

It may already be well known to others, but this is a new discovery for you. This is it. This is the famous galaxy. It’s real. And you found it. You didn’t need a GoTo to lead you there, you did your homework, you studied the chart, you planned your route and you reached your destination.

So the next time you venture out under the stars, leave the smartphone behind. You don’t need it. Give the GoTo the night off. Take a different telescope outside instead, grab a star chart and go explore. You never know what you might discover.

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