What is a Planisphere?
Planispheres are an essential piece of equipment no matter how experienced an astronomer you are.
To all of us new backyard stargazers though, who can't yet confidently find their way around the night sky, owning a planisphere (opens a list on Amazon) is essential.
Deceptively simple, planispheres are designed to show you the whole night sky above your head for any hour of day and any day of the year you care to choose.
They're brilliant for three reasons: they squeeze a whole sky's worth of information into a really small space; they're perpetual, which means you can use it from the day you buy it to the day you die, and; you don't need any detailed knowledge to use them.
When you've got one, you'll use a planisphere for four things:
- Identifying the constellations in the sky above your head
- Discovering which constellations are visible for any given observing time
- Finding the best time (of day or year) to observe a constellation you want to see
- Locating a particular planet for any given month from now until 2020
In the rest of this article, I'm going to show you how to get the very best out of your star wheel.
Northern Hemisphere Star Chart
As general guidance for buying a planisphere, make sure to buy a plastic one as card/paper ones will get ruined by dew. You should also steer clear of 'glow in the dark' ones, as they tend to be less accurate.
What you must be careful of is buying the right planisphere for your location.
Planispheres are designed to show the sky for a specific band of latitude. For that reason, there are a number of different versions printed and you'll need the one most relevant for your location.
My personal favourite is Philip's Planisphere, although there are many other makes available - a popular US model is made by David Chandler (click the images below to get more info about each on Amazon).
Philip's publish four northern hemisphere planispheres:
Each planisphere is fine to use for around 10° either side of its stated latitude.
For example, Maine is around 45°N so the 42°N edition works from there all the way down to a line stretching from southern California, across Arizona, New Mexico, Texas, Oklahoma, Arkansas, Mississippi, Alabama, Georgia and South Carolina.
If you live in or south of those states, buy the 20° - 30°N version instead. That one is good down to Key West (24.5°N).
How Planispheres Work
Each planisphere is basically two big circles of plastic riveted together at their centres.
The back circle has a complete star map printed on it, whilst the front has an oval window which only lets you see that part of the sky visible at a time you choose.
The first thing you need to do with planisphere is learn how to set it.
How to Set a Planisphere for a Date and Time
To understand how to use a star chart like this, let's pick a time and date that we're planning to look at the night sky.
I'm at my astronomy group tomorrow night, which happens to be the 20th November and, clear skies willing, we'll be pointing our scopes upwards at around 10pm.
To set this date and time, rotate the front disc so that the little blue line under 22h/10pm on the blue disc points to 20 November on the outer white circle, as shown in the first picture.
The chart inside the oval window is the night sky as it will appear at 10pm on 20th November in your location.
To get as much value from your planisphere as possible, we've picked out the important parts of the sky map and explain each of them below.
What You See on a Planisphere's Star Chart
The rivet in the centre marks the location of the north celestial pole. All the stars appear to rotate around this point, and it's where the pole star - Polaris - is located.
Constellations are written in BOLD CAPS and I've circled Orion as an example in the picture.
Notice that lines join stars in the constellations to make their shapes easier to identify.
The stars themselves are shown with different sizes of dot. A bigger dot means a brighter star.
Only naked-eye stars are shown, down to magnitude 5 (if you live in a light-polluted area, keep in mind that you probably won't be able to see the faintest stars).
Planispheres are designed to show the whole, vast night sky above your head in a really small oval. To do that effectively, they only show some objects of interest, like M31 (the Andromeda Galaxy) and the double cluster, both highlighted in the picture.
The Milky Way is shown in gray spreading along the night sky.
The ecliptic is shown as a dotted line tracing a circular track across the planisphere window. This is the line which the sun traces against the sky over the course of a year. It's also the line along which the planets appear to move.
Planets move across the sky so much faster than the static stars, so they can't be plotted into the planisphere window... but you'll see below how to locate a planet using the planisphere in any given month.
The final highlighted part of the window is the declination markers. Declination is the measure from celestial north (the rivet) at 90°, to the celestial equator at 0° and on down to the celestial south pole at -90°.
Declination is used together with right ascension (equivalent to longitude on Earth and marked in hours on the outer ring), to give coordinates for any object in the night sky.
What you might find surprising is that this exact view of the sky we've dialled in for 10pm on the 20th November can be seen at many different times of the year.
We set the planisphere for 10pm on the 20th November, but if you look around the rim of your own planisphere, you'll see that it's also set for 11pm on the 4th November, midnight on the 21st October and even 10am on the 20th May!
At some of these times and dates, the sun will be up of course and so we can't actually see any of the stars in the planisphere's window - but they are there nonetheless.
This demonstrates the fact that the night sky moves on about four minutes every day. So 10:00pm on 20th November is exactly the same as 9:56pm on the 21st November, 9:52pm on the 22nd November, and so on.
After a month, the sky 'moves forward' 2 hours (30 days x 4 minutes = 120 minutes = 2 hours) so 10pm on 20th November is also exactly the same as 8pm on the 20th December.
The great thing about this is that, sooner or later, the part of the northern hemisphere night sky that you want to look at will come into view at a time that works for you to see it.
How to Use the Planisphere
Now we've discovered what a planisphere is made of, it's time to learn how to use one.
Monthly guides are great because they are focussed enough to contain planet positions and moon phases. But, they don't show the actually nightly variations.
At the other end of the spectrum, star atlases are a fundamental piece of kit for a backyard astronomer to plan a night's viewing. But... they've got a large amount of detail in them and they are not time/location specific.
A planisphere sits nicely in between these two.
It's specific for the time and location, shows you the whole sky but not in overwhelming detail, it doesn't need a computer or magazine subscription to work... and they're really cheap!
Using a Planisphere to Learn Constellations
Learning the constellations is a handy skill that any new astronomer should focus on getting.
Once you recognise the main northern hemisphere constellations, finding your way around the night sky becomes a lot easier and you can focus on finding the specific object you're looking for.
Let's return to the planisphere with the time we set earlier - 10pm on the 20th November.
Remember that the rivet in the centre is the northern celestial pole (ncp), right where you'll find polaris in the night sky.
Around the ncp are the circumpolar constellations, or the constellations that circle the pole.
If you turn your planisphere a full loop, you'll see there are some constellations that are always in the window - they never get covered over.
On the planisphere being used for our example, these are all the constellations inside the red circle.
If you learn to recognise the constellations within that circle, you'll always be able to find them at night as they never set below the horizon.
The list of circumpolar constellations on the 51.5°N planisphere is:
- Ursa Minor
- Ursa Major
The further south you are, the fewer circumpolar constellations there are. Click here to discover your circumpolar constellations.
Using your Planisphere to Locate Other Constellations
All the other star wheel constellations are visitors to our night sky for a periods of time across the night / year. The further south they are, the more fleeting the visit.
Look at Orion as an example.
Set the planisphere so that Orion is at its highest point in the sky, which is when midday is on 15th June.
We obviously can't see it at that time, because the sun is up, but looking around the rim of the planisphere you can see that Orion is this high in the sky at 7pm on Feb 01, 10pm on Jan 15, 2am on November 16 and 5am on October 01.
With a clear sky, all of these times work for observing (even if they don't match up with your sleep requirements!)
Now, turn the planisphere's front wheel a half circle so that midday is aligned with 14th December and Orion is no longer visible in the window...
Looking around the rim, note that Orion is not visible at 7pm on August 30, 10pm July 15, midnight June 15, 2am May 14, 5am March 29, etc
Orion is clearly not a circumpolar constellation as there are many times of the year it is not visible to be observed.
This simple demonstration shows why Orion is known as a winter constellation: there are so many more opportunities to see it then.
How to Use Your Planisphere for Observing
We can use the planisphere to find out exactly what we've got above our heads at any given moment.
Re-set your planisphere to 10pm on the 20th November.
Before going any further, there is one last feature of the planisphere which I need to bring to your attention that you might not yet have noticed... it's a little blue cross!
This blue cross is printed onto the front circle of your planisphere and is so hard to see that I've put a piece of white paper behind it in the picture.
That cross marks your 'overhead' point when using the planisphere.
On the 20th November at 10pm, you can see the overhead point is almost in Cassiopeia, as shown in the picture.
Once you know what is directly overhead, it's time to get pointed in the right direction for observing.
We're going to use our star finder wheel to do exactly that: find our way around the rest of the visible night sky.
Use the following steps out in the field to get the best results from your planisphere:
- Face approximately south (your phone's compass will help)
- Hold the planisphere in front of you as you would if reading a book
- Rotate it so that 'south' on the planisphere is ahead of you, i.e. pointing towards the southern horizon you're facing
- Make a note of your overhead point (Cassiopeia for our example)
- Low in the southern sky is Cetus, and about midway between the overhead point and the horizon is Andromeda
- Facing different horizons, carry out the same steps to pick out constellations, e.g. when facing east, rotate the planisphere so east is facing forwards
Following these simple steps will quickly help you become comfortable with how to use your planisphere to find constellations.
If you prefer a visual demonstration, scroll to the video below and watch from about the 9:50 minute mark for about 2 minutes for a great 'how to' use a planisphere demonstration.
There's a Lot of Sky in that Little Window!
Every star between the overhead point and the horizon fits into just two inches of the planisphere's star window, and that can take some getting used to.
There's an added challenge to matching what's in the planisphere's star window to what's above your head: the design of a standard planisphere for the northern hemisphere stretches the constellations further south in the sky, making them appear more spread out on the planisphere window than they are in reality.
That's why it's easiest to start learning the constellations above your head and down to around midway between overhead and the horizon. They're simpler to compare with the image in the planisphere's window than constellations on the horizon.
Armed with a red flashlight, your Planisphere, and this guide, you'll never be lost for naming the stars and locating constellations overhead, no matter the time of day or day of the year in which you are observing.
Let's discover what else this clever little tool can help you with...
Using a Planisphere to find the Planets
I've already mentioned that the planets are not printed on the planisphere because they are not static against the background of stars, they are constantly moving across the night sky.
Thankfully, a Philip's Planisphere provides a way for you to locate Venus, Mars, Jupiter and Saturn for any month between now and 2020.
Turn your planisphere over and on the back you will see four charts of numbers under the heading of 'Planetary Tables'.
To demonstrate how to use these, let's work through an example.
Assume it's January 2017 and you want to find Mars in the night sky.
You can see from the picture, the numbers 341 for January and 3 for February 2017.
These are both measures of degrees on the outermost ring of the planisphere.
Using a straight edge, mark a (removable) line joining the 341° marker with the central rivet.
Where this line crosses the ecliptic is where Mars will be found on the 1st of January 2017.
Do the same again for 3° to find Mars on the first day of February.
Taking the two lines together, you can see the direction of travel over the course of January 2017.
Now we know where Mars will be (in Aquarius) we can use the planisphere to work out the best time to see it.
In this example, we'll be able to see Mars in the early evening throughout January 2017, when it will be low on the western horizon.
Finding Sunset/Sunrise with your Planisphere
One last little 'trick' that I'm going to share with you is how to use your planisphere to learn the approximate sunrise and sunset time on any given day of the year.
Let's stick with the 20th November as our working example.
This time, set the planisphere so that midday is against the 20th November.
Draw an imaginary line from the centre of the planisphere to the 20th November and, where it crosses the ecliptic is approximately where the sun is located on the sky on this date.
Now, move the front disc so that the 'eastern horizon' is just touching the sun's location on the ecliptic and read off the time on 20th November.
In the picture you can read a sunrise time of just before 8am (yours may be different, depending on latitude).
To find the sunset time, move the front disc so the sun's location on the ecliptic is just touching the western hemisphere edge of the viewing window and again read off the time on 20th November.
This time you can see it's just after 4pm.
How to Use a Planisphere: A Video
This YouTube video is really helpful for understanding how to get the most out of your planisphere.