The sky on a clear day is an expanse of monotonous blue, may be with a few whiffs of cloud. But after sundown things change dramatically As daylight fades into dusk and then into night, the beauty of the star-studded sky reveals itself in its full glory. If it is a moonless night and the sky is clear, and if we are far away from city lights, we will be able to appreciate this beauty.
At first we may feel bewildered at the sheer number of stars that we see. We may wonder if we can ever recognise them individually. But we needn't get disheartened. We can learn how to recognise the stars with the help of sky maps and a few tips given in this book.
Almost all newspapers publish a sky map on the first of every month showing the constellations and planets visible during tl <_ month, but these maps are difficult to use unless we know how to use them. This is because, firstly, they try to show on a flat surface what actually is seen as a hemisphere. This greatly distorts the shapes of the constellations and also their relative positions. Secondly, and this is important, these maps are drawn for only a particular hour of the night and can be used only at a particular latitude. por example, the sky map shown on the opposite page shows the night sky at Delhi (latitude 28° 39'N) at 9 p.m. on January 1, or at 8 p.m. on January 16, or at 7 p.m. on February 1. If we live in a more northernly latitude, in Srinagar (lat. 34°N), for example, then we will not be able to see the stars near the south horizon shown in the map. On the other hand, a viewer in Kanyakumari (lat. 8°N) will see a completely different sky. From there, the pole star (Polaris) will be seen almost on the northern horizon, while stars and constellations unseen from northern latitudes will become visible.
These difficulties can be overcome to a large extent by sky maps which can be designed for use at any hour at any latitude which undoubtedly will be rather complex. But there is an easier way: to first learn to recognise a few prominent stars and constellations and then use them as pointers to identify others. It is like trying to find an address in a new city. A city map is helpful, but we can also do it by asking the way from the railway station, or bus terminus, using various landmarks such as parks, hotels, markets, post offices, etc. as pointers.
Whether we use the sky maps published in newspapers or those we find in this book, we have to remember one thing: the relative orientations of the cardinal points (north, east, south, west) shown in a sky map are different from those we find in a conventional geographical map. For exan pie, in conventional maps the top usually denotes north, bottom south and the left and right denote west and east respectively. In a sky map it is different because we look up at the sky. In fact, the correct way to read a sky map is to hold it overhead and when we do that and look up, we will find if we keep north at the top (actually pointing to our back) then east will be to the left and west to the right, the opposite of what we find in a geographical map.
The constellations in the sky will appear as shown in the sky maps only if we are looking at them facing the right direction. While rising and setting, some of the familiar constellations may appear strange and we may not be able to recognise them till they are well up in the sky. The best way to get familiar with the constellations is therefore to observe them when they are culminating, that is, when they are at the highest point during their passage across the sky. The time of culmination of most of the important stars and constellations are given in this book.
Of course, before we try to identify the constellations we must get familiar with the directions north, south, east and west. A magnetic compass is handy and will help us in facing the right direction. But it is better try to find some landmarks — a tree, a pole, a chimney or a building — to mark our direction.Once we get familiar with the constellations, the stars themselves will become our direction finder.
How Many Stars?
If we are blessed with a good eyesight we should be able to see about 3,000 stars with unaided eye on a clear moonless night. But usually many of them cannot be seen because they are near the horizon where atmospheric haze blots them out. Glare of city lights obscures some more of them so that a city dweller is able to see only a few hundred stars even under the best viewing conditions.
Wherever we may be, even if we look casually at the night sky we will notice that not all stars are equally bright. Some are so bright that we can see them clearly even under city lights while some are so faint that they are hardly visible. In sky maps these various brightnesses are shown as different 'magnitudes' in the shape of large or small dots. Each step on the magnitude scale represents a factor of 2.5. For example, a star of magnitude 1 is 2.5 times brighter than a star of magnitude 2. Similarly a star of magnitude 1 is 6.25 times brighter than a star of magnitude 3 and so on. (Stars of magnitude one or less and up to magnitude 1.5 are known as first magnitude stars; stars fainter than magnitude 1.5 but brighter than magnitude 2.5 are second magnitude stars, and so on.) We must remember here that the brightness of a star as seen from Earth is only its apparent brightness and depends on both the star's actual or 'absolute' brightness and its distance from Earth. So, an apparently faint star may be actually very bright but too far distant, and an apparently bright star may not be really very bright but may appear so because it is very close to us.
Stars may even have a negative magnitude. Obviously these would appear even brighter than the first magnitude stars. In all there are about 20 stars of first magnitude or brighter. The brightest of them, Sirius, has a magnitude of -1.46. On clear nights stars of up to magnitude 4 are normally visible in the city sky, but fainter ones (up to magnitude 6) can be seen from the countryside where there is no glare of city lights.
The First Magnitude Stars
Sirius Canopus Alpha Centauri Arcturus
Aldebaran Acrux An tares Spica
Piscis Austrinus Gemini
-1.46 -0.72 -0.27 -0.06 +0.03 0.08 0.12 0.38 0.50 0.51 0.68 0.77 0.85 0.87 0.96 0.98 1.16 1.20 1.25 1.36
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