A planisphere is a star chart device that helps you learn the sky, identify the brighter stars, star patterns, deep sky objects and more. For many who are new to astronomy, this will be their first-star chart even though it probably won’t be the last. It is an inexpensive astronomy tool that you can use for a lifetime.
The beauty of the planisphere is that it is simple to use once you get the hang of it. I keep mine in my car so I can grab a few observing minutes with just my eyes or the binoculars I keep in the car. I have often had it next to me as I sat at my telescope, picking out targets of interest.
The image shows the planisphere I use, Guide to the Stars, by Ken Graun. This is a typical planisphere, but there are many other brands. This is a 16” diameter version. I have seen planisphere in 12, 8 and down to about 5-inch units that can be carried in your pocket. Naturally, the larger ones can provide more information and are more easily read. The smaller ones are more convenient and easier to take with you. However, their use and operation are similar. They will contain different helpful information on the front and back. This one is loaded with good information.
Note that a planisphere is usually specific to a particular latitude. For example, this planisphere is good for north latitudes of 30 to 60 degrees. If you are above or below that latitude you would need a different planisphere as your view of the sky is different.
How It works – The Bottom Disk
Planispheres are typically made of two circular disks that are joined in the center with a rivet. They can be paper or cardboard, but I always recommend one that is made of plastic. Plastic is more durable and is not damaged by dampness when you are outside on a humid night or when dew starts to accumulate.
The bottom disk will have the months and dates printed around the edge. These will be used to set the planisphere to show us the sky as it will appear tonight.
The bottom disk also has a star chart printed on it. The better ones have the stars printed in such a way that you can tell which will be the brighter stars, shown as bigger disks, and which are the dimmer stars, smaller disks. Often the bright stars will have their name next to them. The dimmer stars may not be named, but their presence will help you recognize asterisms or patterns in the sky. The Big Dipper, for example, is an asterism, a collection of stars that is easily recognized. The belt of Orion would be another asterism.
If you are in a fairly light polluted location, you may not be able to see all of the stars, naked eye, that is shown on the planisphere. If your sky is darker you may be able to see more stars that are shown on the planisphere. Naturally, as you move from location to location this will vary.
The constellations will likely be named on the planisphere. Usually, the main stars of the constellation will be shown, sometimes joined by a line. Or the constellation may be outlined to help you visualise the area it encompasses. This can be very helpful in learning the sky and learning to recognize the constellations.
Also shown on the typical planisphere will be symbols for double stars and the brighter deep sky objects, DSOs, such as star clusters, nebulae, galaxies, globular clusters and other of deep sky objects. There should be a key printed on the planisphere to show you what each symbol represents.
Many will also have the brighter Messier objects identified on the star chart. For example, the Andromeda Galaxy is also known as Messier 31 or M31. The Messier list of 110 deep sky objects is one of the most popular among armature astronomers and often the first observing list those who are new to the hobby will try to observe.
The stars and these deep sky objects remain fixed in their location relative to each other for all practical purposes. This enables the planisphere to be used as a roadmap of the sky.
What you won’t see shown on the planisphere is the Sun, the Moon, and the planets as these move through the sky against a background of fixed stars. However, the planisphere will likely show the ecliptic line which is the path that the Sun, Moon, and planets travel through the sky, making it easy for you to know where to look for them when they are in the night sky.
To understand the ecliptic, just think of the location where the Sun rose this morning. Now consider the path that it travelled through the sky. Identify where the Sunset. The picture that path and you have the ecliptic line. The Moon and planets will follow a very similar line through the sky.
Because this path is predictable, spotting the visible planets, Mercury, Venus, Mars, Jupiter, and Saturn becomes quite easy. Neptune and Uranus also travel the ecliptic but require some optical assistance to be seen as planets rather than looking like distant stars.
Just remember that the planets are not always in the evening sky. Sometimes the planets are in the morning sky. As the Earth and planets travel around the Sun, a planet may be on the other side of the Sun and so, not visible to us.
How It Works – The Top Disk
The top disk is a little smaller. Along the edge, you will see the time of day shown. We will use these markings to set the planisphere to the date, along with the bottom disk, and the time we wish to observe.
There will be a transparent window in the top disk which allows us to see part of the star chart on the bottom disk. When we rotate the top disk so that today’s date and time line up, the opening will show the sky as it will appear on that date and time.
The Earth rotates at 15 degrees per hour and so the sky appears to move, even though it is really us that is moving. By sitting with the planisphere and progressing the time you can see how the sky will change over the period of an evening.
Advance the date over days and months and you can see how the sky will change. Stars and deep sky objects, DSOs, will come into view and leave the view as the year progresses. This is a very valuable lesson as it helps you internalise how your sky will change over time.
The planisphere can help you plan your observing sessions. You can use the planisphere to help you plan what you want to see and when things will be at their best observing position in the sky. You will see if they are best observed in the evening or in the morning.
The Trick to Using a Planisphere
Many people miss the key element of how to use a planisphere. It is not used flat on a table as a book. You hold it up to the sky.
Usually, there is a centre rivet that joins the two disks. At that rivet, there is a marking for the North Star, Polaris. This is the only star that is fixed in the sky as it is aligned with Earth’s axis of rotation. Everything else in the sky appears to move, but Polaris remains fixed.
The Planisphere should have an indication of how to hold it up to the sky. Likely there will be markings to indicate which part of the disk should be up when facing north.
If you face north and hold the rivet so that it aligns with Polaris you will have a representation of the entire sky before you. In fact, if the rivet is hollow, as many are, you hold the Planisphere out at arm’s length and sight Polaris through the rivet.
The second tricky part is that you are looking at a flat representation of the sky, but the sky is more like looking at the inside of a dome. So you have to use your imagination to help you place the various bright objects in the sky.
For example, if you set up the Planisphere for March 25 at 9 pm, as shown in the photo, the window will show the sky for March 25th at 9 pm. Now, face north and place Polaris in the rivet.
Just to the right of Polaris, you should see the Big Dipper in the constellation of Ursa Major on the planisphere. Now identify the Big Dipper in the sky. Get your orientation.
Still facing north, to your left, high in the sky, you will see the group of bright stars that form the W, or M, which is the constellation of Cassiopeia. To your right, toward the eastern horizon, you will see the bright stars Arcturus and Spica.
Now, here is the second tricky part, as you face north, behind you is the southern horizon. You realise that the top of the planisphere window is actually the southern horizon which is behind you.
Behind you and slightly to your left, as you face north, you will see the constellation of Orion, one of the most easily recognized constellations. See where it is on the planisphere.
The key is to start to pick out those bright or easily recognized formations that will help you orient yourself and help you mentally project the flat planisphere against the celestial dome.
Of course, I don’t know what date and time you are reading this, so the exercise I just described is only for illustration purposes. But it shows you how to use your planisphere, how to position it up in front of you pointing toward Polaris. And it illustrates how, using bright marker stars or shapes, you can get yourself oriented to the sky.
You can do this facing in other directions but north, toward Polaris, is the easiest to understand and to perform. Try to find a location where you can see Polaris in order to learn how to use your planisphere.
What Else Does a Planisphere Offer?
What I have described would be true for any planisphere. However, each publisher adds their own value by including all sorts of other information. The image at the right is the back of the Guide to the Stars planisphere. It includes:
- information on the phases of the moon
- suggestions on good targets for binoculars vs. telescopes
- information on the planets
- all sorts of other useful information
Other publishers may choose to include different information. When you take this additional information into account, the planisphere becomes a very useful observing tool to help you learn the sky and to identify interesting things to see with your eyes, your binoculars, and your telescope.
Naturally, the larger the planisphere the more room the publisher has to add these extra tidbits. So pick your planisphere size according to how and where you will use it.
I chose a 16” planisphere which is a large one. I like all the extra information that is included. You may prefer a small one that can be carried more easily in your equipment bag. But whatever size or brand you choose you will have a powerful observing tool that can enhance your observing experience tonight and for years to come.