How to Use a Planisphere


We use constellations with signs of the zodiac or animals, because these are convenient easy ways of helping to remember where particular stars are. We name the constellations for the shapes their stars make when joined up. Cygnus is in the shape of a swan with wings outstretched, and Scorpius is shaped like a scorpion. Sometimes a shape will be part of a constellation, such as the seven bright stars in Ursa Major that form the saucepan and handle of the Big Dipper.

But to find your way around the night sky, you need a map, just as you do of an unfamiliar city. But unlike a city, the night sky changes from hour to hour, day to day, month to month and season to season. So you need a map that changes with the night sky. This map is called a planisphere, and no astronomer, beginner, intermediate or advanced, should be without one.


Planispheres can be confusing to the first-time user, but you should buy one. Also purchase a basic book on practical astronomy that can clearly instruct you in using the planisphere, which you can experiment with until you get it right. Your local astronomy club should have supplies of both, or you may be able to get them from where you buy your telescope or binoculars, or a good technical bookshop.


But before you buy your planisphere, you need to get the right one for where you live. This is because the sky is different depending on where on the Earth you are located. In the mainland USA (and other countries on the similar latitude) there are planispheres for the latitudes of 51.5°N (Northern USA, Canada and Northern Europe), 42°N (Central USA, Southern Europe and Northern Japan) and 32°N (Southern USA, Middle East, North Africa and Southern Japan). The planisphere for Hawaii (also Mexico, India, Hong Kong, and Taiwan) is 23.5°N.  Make sure you don’t buy a Southern Hemisphere planisphere – you will get terribly confused and will have wasted your money. 35°S is the planisphere for South America (South), South Africa, Australia and New Zealand. If you don’t know your latitude, you can find it out using an atlas. Latitudes are shown as parallel, evenly space horizontal lines, with the latitude labelled on the left side of the map. Check with telescope stores and astronomy clubs for the right planisphere in your location.


The basic principle of a planisphere is simple enough, but you’ll understand it more easily if you have one in front of you, while reading this article. You match up the date around the edge of the base map and the time of your observation (remembering that the planisphere doesn’t take into account daylight saving, so adjust back one hour from your actual time if this is the case). Although holding your planisphere above your head would seem the thing to do, most observers hold it in front of them and look up at the sky. However, because the sky appears overhead, the points of the compass around the edge of the planisphere’s oval (the window that moves as you turn the planisphere to the correct date and time, and shows the stars that are in the night sky at that moment) are reversed compared with conventional maps on the ground. This means that the eastern horizon is on the left of the oval and the western on the right. The point of the oval closest to the edge of the planisphere is due south (or due north if you are observing in the Southern Hemisphere). Once you have this lined up with due south, you orient the eastern and western horizons. You may have some trouble doing it at first, so ask someone at your local astronomy, of which you should be a member. Once you get used to the planisphere, it will become second nature to you.


As well as using your planisphere to locate any stars above you that can be seen at least with a telescope, you can also use it to find the approximate location of the Sun, and hence calculate the times of sunrise and sunset. To do this place a straight edge between the centre of the dial (a metal ring) and the date, and the Sun is where the line crosses the ecliptic (the path of the Sun through the sky). Note this position and move the dial until it touches the eastern horizon. Read off the time against the date in question and you have the time of sunrise. Do the same on the western horizon to determine the time of sunset.


Many planispheres have tables on the back of them listing the approximate positions of the planets Venus, Mars, Jupiter and Saturn month by month to about eight years ahead. In each case a position in degrees is provided, which you can find on the outermost scale of the planisphere in degrees of right ascension – the sky equivalent of longitude. The tables will assist you to identify any planet you see in the sky.


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