These days people are not really used to looking up for the stars and in many places, due to man-made light pollution, the night sky looks pretty empty anyway. For many, navigating the night sky can be overwhelming at first, particularly from dark locations.
Traditionally, star charts and planispheres were the tools of reference to help amateur astronomers to orient themself during their stargazing sessions. But they also were an invaluable tool for sailors, enabling them to navigate at sea by night.
Today, planispheres are a popular gadget among amateur astronomers: you can buy one for a few bucks or get one for free when subscribing to an astronomy magazine. Or, you can even print one by yourself.
But star charts, and planispheres in particular, can be a bit off-putting if you have never used them before.
So, let’s learn how to use it.
Are Star Charts Useful?
They sure were, for astronomers as well as for sailors sailing through the night. In today’s digital world, though, many outstanding astronomy apps are just one tap away on your smartphone.
Nonetheless, a planisphere is still a nice educational instrument for learning about the sky. It gives you a quick overview of the stars and constellations in the entire night sky, horizon to horizon, as visible for your location.
What Are The Limitations Of Star Charts And Planisphere?
Planispheres and star charts do have some limitations.
While star charts and planispheres pack a lot of info, apps such as Stellarium and SkySafari provide a ton more information. With those apps you can get detailed info on stars, planets and deep sky objects, simulate the exact field of view of your instrument, easily move back and forward in time, and even control your mount.
Astronomy apps for smartphones are also much easier to use: simply point your phone at the sky to see on screen what is in the part of the sky you are framing.
Because of the small font often used in star charts and planispheres, they could prove rather hard to read at night.
Finally, planispheres are only accurate for a small range of latitudes. I live in Brussels, Belgium, about 51º N and I had to get a planisphere for 50º N latitude, which is accurate between 45ºN and 55ºN. And if you want the best precision, you should correct for your local longitude as well.
Apps for smartphones don’t have these limitations.
What Info Does A Planisphere And A Star Chart Show?
Planispheres depict the whole night sky, which is printed on a disc and a mobile window will rotate on the disc to only show the part of the sky that is visible from your location at a particular time and date.
If you don’t already own a planisphere, you can print one from In-The-Sky.org.
Star charts usually focus on smaller regions of the sky at a time and are usually collected together to form a star atlas.
Ed Vazhorov has created a beginner star atlas you can download for free.
Both star charts and planispheres display a number of information about the night sky.
On the disc, stars are shown as dots, and the size of the dots represents the apparent magnitude of the stars: the smaller the dot, the higher the apparent magnitude and the fainter the star is.
For example, my Omegon Planisphere shows stars ranging from magnitude 5 to -2 and it shows some 700 stars, all visible to the naked eye and with a binocular.
Greek letters are often used to identify stars within a constellation, like γ for γ-Cassiopeia, the middle star in the Cassiopeia constellation.
Constellations are also shown with solid lines connecting the stars. Both constellations and the brightest stars have their name printed next to them.
Deep Sky Objects
A number of easily observable deep sky objects are also shown with their Messier or NGC catalog designation. For example, the Andromeda Galaxy is named on my planisphere as M31, while the North America Nebula is NGC7000.
Usually, different symbols are used to identify the type of objects, such as:
- Double Star
- Variable Star
- Open Cluster
- Globular Cluster
- Planetary Nebula
- Diffuse Nebula
- Special points or faint but interesting stars, such as Barnard’s Star in Ophiuchus, one of the best studied red dwarf
My Omegon Planisphere contains some 300 deep sky objects easily spotted with a binocular.
Sun, Moon, And Planets
The Sun, the Moon, and the Planets do not stand still in the sky as distant stars appear to do. Instead, they move along (or nearby) the ecliptic, the imaginary line in the sky traveled by the Sun throughout the year.
The ecliptic is usually printed as a dotted/dashed line in a planisphere, with each dot representing one day.
One other limitation of a Planisphere is that it can’t show you what planets are visible and the Moon phase. All you can have is a rough idea of where the Sun, the planets, and the Moon might be in the sky.
How Are Star Charts Used To Find Constellations And Deep Sky Objects?
Star Charts are not really used to find where things are in the night sky. They are helpful to know in which region of the sky a target is, or what is visible in a given region. They may also tell you the angular separation between a visible star and the intended target so that you can quickly hop from the star to the target you want to observe.
To locate the stars and constellations in the night sky, you should use a Planisphere.
How Do You Use A Planisphere?
A planisphere is formed by two disks that can rotate one with respect to the other. The bottom disk is the reference one. It has printed, for a large portion of the sky, the stars, constellations, and deep sky objects. Make sure you are using a planisphere that is made for your latitude (+/- 5º or so).
On the outer rim of this reference disc, a calendar is printed.
The magic is done by the upper disc. On this disc, several pieces of info are printed:
- The time of the day
- The south, west, east, and north directions
- The horizon line, running from east to west and crossing the north sud direction at the proper latitude
- And a transparent window to show the portion of the sky visible at a given time and date
Detailed instructions and other info are often printed on the planisphere itself.
Here is how I use the popular Rob Walrecht planispheres, on which designs are based on many commercial planispheres.
Setting The Date And Time
First, you have to know if your time is standard time or daylight saving time: in the first case you have to use the large numbers for the hours, and in the second case the small ones.
So let’s say I want to observe for tonight, September 12th, at 23:00 in Brussels.
- At this time of the year, here in Belgium, we are using daylight saving time. Therefore I have to use the small numbers for the hours;
- I have to rotate the upper disc so that the selected time (23:00) will be opposite to the selected date (September 12th). For this, I use the long North-South line to select the date, and look at the date the 0-hour (1 for daylight saving time) is pointing at: March 10th in my case.
- I then rotate the upper disc to have the 23-hour mark (daylight saving time) pointing at March 10th. At this point, the transparent window in the upper disk will show the visible portion of the sky.
Orient The Planisphere
Now you have to properly orient the planisphere in space, and here is where things may get confusing.
If you place your planisphere on the table, as you would for a geographic map, you will see that North and South point Up and Down as usual, but East and West are swapped, which is, well, odd.
Cardinal points are named considering you are in the sky, looking down on Earth. Planispheres assume you are on the ground, looking up at the sky.
All this can be quite confusing, and it helps to think of North, East, South, and West as just labels. What is important is to remember that when you move clockwise, 90 degrees at a time, starting from the true magnetic North, you are going real East, real South, and real West, where the real directions are those used in the geographical maps.
Ok, the instructions for the use of the planisphere say:
Hold the planisphere above your head so that the points of the compass on the planisphere correspond to the real directions.
And when I do that, I end up with the constellation I see at my right (East) being on the opposite side of the planisphere.
Orient The Planisphere: The Easy Way
This always confuses me, and I think it is over complicated for the simple tasks of identifying constellations and stars you are looking at. This is my easy personal way to orient my planisphere, and it goes like this:
- I face true North (if you don’t know where it is, you can use a compass)
- With the disc parallel to the ground, I orient it so that the label North points to the true North directions in front of me.
- I forget about the South, East, and West labels and simply compare the constellations I see drawn on the planisphere to that I see in the sky.
Planning Your Session Under The Stars With A Planisphere And A Star Chart
We saw in this article how with a planisphere, you can know ahead of time what constellation will be visible from your location for a particular date and time. You can then use a star atlas, which usually contains many more targets than those shown in the planisphere) to plan your astronomy observations or astrophotography sessions.
For the example used in this article, if I were to observe/photograph on September 12th at 23:00, I would see the busy Perseus, Cassiopeia, and Cepheus constellations (and more) will be visible. With a star atlas, I can go to the star chart containing those constellations and easily choose and research possible targets.
Planispheres and star charts, although a bit quirky and off-putting at first, make great educational tools you can use to have fun exploring and learning the constellations visible in your sky.
Although less intuitive, universal, and informative than astronomy apps for a smartphone, they still have a place in the toolbox of the amateur astronomer and it is always good to have them around.