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How to Use a Planisphere
When you need to find your bearings in the night sky a planisphere is indispensable, even in our digital age.
They don't look like much — usually a planisphere is simply two discs of cardboard or plastic fastened together with a central pin — but as a new stargazer, you'll soon discover that this tool is one of the greatest aids to helping you navigate the night sky. In fact, this deceptively simple design will allow you to work out which bright stars are in the night sky on any date and at any time throughout the year.
Although the two discs are pinned together, 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 a given latitude. Marked around the outside of this lower disc are the days and months.
The upper disc will be slightly smaller than the lower one or will have a clear rim, so you can still see the day and month markings underneath. 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 indicate 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.
The crucial point to keep in mind when using a planisphere is that they are designed to work at specific latitudes. If you try using one too far north or south of the location it has been intended for, you'll find that the stars don't appear in the right positions.
Follow these simple steps and you'll soon be navigating the night sky like a pro
- Find Your Bearings
Before you can start using your planisphere you need to know the cardinal points from where you live. If you don't have a compass, use the Sun. It rises roughly in the east and sets roughly in the west. You can also download a free compass app for most smartphones.
- Set the Time and Date
Let's imagine that you are heading out to observe at 9pm on October 15th. Spin the upper disc to align the 9pm marker on this disc with the October 15th marker on the lower disc. The stars in the oval window should now match the stars in the night sky above you.
- Look to the North
Look north to begin with, holding the planisphere up so that the word 'north' is at the bottom. If you change the direction you're facing, you need to move the planisphere round so that the compass point sitting at the bottom corresponds with the direction you're facing.
- Start at the Big Dipper
Helpfully, the central pin in your planisphere represents Polaris and the north celestial pole. Just to its lower right will be the seven bright stars of the Big Dipper asterism. Use the Big Dipper and the five stars forming the distinct 'W' shape of Cassiopeia to get to know the constellations.
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