Once you’ve set the chart to match the current date and time, you’ll be able to see which stars are in the sky. Image Credit: www.secretstudio.net
For budding stargazers, a planisphere is one of the greatest aids to helping you find your way around the night sky.
They don’t look like much, just two discs of cardboard and plastic fastened together with a central pin.
But this deceptively simple design belies the fact that a planisphere allows you to work out which bright stars are in the night sky and where on any date and at any time throughout the year.
This basic knowledge is useful for casual stargazers and more serious amateur astronomers alike.
For example, a planisphere can help you to learn the constellations or even just identify a bright star you can see at a particular time.
It can also be a useful aide-mémoire when planning an observing session.
Although the two discs are pinned to each other, they can still be rotated independently of each other.
Printed over most of the lower disc are the stars, constellations and brighter deep-sky objects that you can see from the UK’s latitude.
Marked around the outside of this lower disc are the days and months.
A window to the night sky
The upper disc is slightly smaller than the lower one, so you can still see the day and month markings on the larger disc beneath.
It also has an oval window in it, revealing part of the star chart on the lower disc.
The edge of this window represents the horizon with appropriate north, south, east and west markings, and everything within it is the visible sky.
Just like the lower disc, the upper disc has markings around its edge.
In this case, they denote the time of day.
By lining up the date and time, the stars visible in the window will match the ones in the night sky at that time.
Planispheres are cheap, easy to use, robust (plastic ones more so), lightweight, portable and – best of all – they don’t need electricity.
The one important point to keep in mind when using one is that planispheres are designed to work at specific latitudes.
If you try using one too far north or south of the location it has been designed for, you’ll find that the stars don’t appear in the right positions.
UK latitudes vary from 50ºN (southern England) to 60ºN (northern Scotland).
Why can’t I use a planisphere to find the planets or the Moon?
Planispheres show objects that are ‘fixed’ in the night sky relative to Earth – that’s why they can be used year after year.
However, this means that they can’t predict the location of planets or the Moon.
Some manufacturers try to overcome this by printing details of planetary locations for several years on the back, but there is also a line printed on the chart itself that can help.
The ecliptic, often shown as a dotted line, marks the plane of the Solar System, in which most of the planets orbit the Sun.
If you discover a ‘star’ in the sky that’s not shown on the planisphere, then it is probably a planet.
How to use a planisphere
Get your bearings
There’s one thing you need to know before using a planisphere, the cardinal points from where you live.
If you don’t have a compass, use the Sun.
It rises in the southeast and sets in the southwest in January.
Set the planisphere
Let’s say you’re heading out at 9pm on 15 October.
Align the 9pm marker on the upper disc with the 15 October marker on the lower disc.
The stars in the oval window should now match those in the skies above.
Hold it up
To start with, look north, holding the planisphere so that the word ‘north’ is at the bottom.
If you change the direction you’re facing, move the planisphere round so that the corresponding compass point is now at the bottom.
The central pin represents Polaris and the north celestial pole.
Just to its lower right will be the seven bright stars of the Plough.
Use these and the five stars forming the W shape of Cassiopeia to get to know the constellations.