Stars are scattered across the night sky like sequins on velvet. Over 2000 of them are visible to the unaided eye at any one time under the clearest conditions, but most are faint and insignificant. Only a few hundred stars are bright enough to be prominent to the naked eye, and these are plotted on the monthly sky maps in this book. The brightest stars of all act as signposts to the rest of the sky, as shown on pages 14–15. It is a welcome fact that you need to know only a few dozen stars to find your way around the sky with confidence. This book will introduce you to the stars month by month, without the need for optical aid, so that you become familiar with the sky throughout the year.

What is a star?

All stars are suns, blazing balls of gas like our own Sun, but so far away that they appear as mere points of light in even the most powerful telescopes. At the centre of each star is an immense natural nuclear reactor, which produces the energy that makes the star shine. Stars can shine uninterrupted for billions of years before they finally fade away.

Many bright stars have noticeable colours – for example, Antares, Betelgeuse and Aldebaran are reddish-orange. A star's colour is a guide to its temperature. Contrary to the everyday experience that blue means cold and red is hot, the bluest stars are actually the hottest and the reddest stars are the coolest. Red and orange stars have surfaces that are cooler than that of the Sun, which is yellow-white.

To find your way in unfamiliar territory, you need a map and signposts. This book provides the maps. The signposts are in the sky, once you know where to look. Start with an easily recognizable pattern, such as the Plough in spring or Orion in winter, and work your way outwards from it to locate other constellations and bright stars, a technique known as star- hopping. While star-hopping around the sky you will find that there are many natural ‘pointers’ that direct you from constellation to constellation. Also, bright stars often form distinctive patterns of lines, triangles and squares that you can remember. The four small charts on these pages demonstrate some of the best ways of locating prominent stars and constellations in each season. As you navigate your way among the stars you will discover many more signposts of your own.

Signposts of spring

Start with the familiar saucepan-shape of the Plough or Big Dipper, which rides high in the sky on spring evenings. The seven stars of the saucepan are actually the most prominent members of the constellation Ursa Major, the Great Bear. From the diagram you can see that two stars of the saucepan's bowl – the ones that lie farthest from the handle – point towards the north pole star, Polaris. These two stars in the Plough are popularly known as The Pointers. If you extend the distance between them by about five times you will reach the pole star. Opposite Polaris from the Plough lie five stars that form a distinctive W-shape, which is the constellation of Cassiopeia.

At Jupiter's orbit the light from our Sun is so spread out that it illuminates with a mere one twenty-seventh of its strength at the orbit of our Earth. Of course, even that is nowhere near the outer limits of our Solar System. There are still three mighty worlds and a vast number of very much smaller bodies orbiting much further out where the sunlight is even dimmer. The next major planet is Saturn, where sunlight shines a mere one ninetieth of the brightness we are used to. Beyond that is Uranus and beyond that is Neptune. From these worlds our Sun has very roughly one four hundredth and one nine hundredth of its intensity, respectively. Their great distances also mean that the time light takes to cross from the planet to us is significant, despite the fact that a pulse of light travels three hundred thousand kilometres every second. Whenever we look at Saturn we are actually seeing it as it used to be just over an hour before. Make that two and a half hours in the case of Uranus and fully four hours in the case of Neptune! It is little wonder that these twilit worlds appear wonderfully unearthly and ethereal through our telescopes. In this chapter we will consider each major planet in turn along with what observations we can usefully make of them.

SATURN IN THE SKY AND THROUGH THE TELESCOPE

One starry winter's evening early in 1971 I was in the garden with the 3-inch (76 mm) reflector my parents had recently bought for me. On a few previous nights I had already had my first close-up views of the Moon's magnificent vistas through it. On this night I remember looking up and seeing an intriguing little group of blue stars twinkling in the sky. Only later did I learn that that group was the Pleiades star cluster. Not far from the cluster was something that immediately struck me as very different – a prominent star that didn't twinkle like the rest. It had a leaden yellow hue that looked totally unlike the colours of any of the other stars in the sky. I wondered if it could be a planet – I very much hoped so as it would be my first sighting of one through my new ‘proper’ astronomical telescope.

Preface

I still have my copy of the second edition of Practical Amateur Astronomy, the ‘how to’ book on astronomy that my parents bought me for the Christmas of 1971. The book was edited by Patrick Moore and contained chapters on practical astronomy written by British expert practitioners of the day covering their specialist fields. At the time I was thirteen years old and had already been interested in all the sciences, and particularly astronomy, for many years. However, my practical experience of astronomy at that time was limited to locating some of the more prominent astronomical objects and peering at them through the eyepieces of binoculars, a small terrestrial refractor, and a 3-inch (76 mm) Newtonian reflector. Back then that book seemed very advanced but I took what instruction I could from it and continued to pursue my interest in practical astronomy.

Practical Amateur Astronomy was a very useful book in 1971, giving the reader instruction in the then current activities of the more advanced amateur practitioners. In the main, the chapters were written by the directors of the various observing sections of the British Astronomical Association (BAA). Today that book looks very simple and quaintly old-fashioned, thanks to modern advances. A year earlier, I remember borrowing an early edition of Patrick Moore's The Amateur Astronomer from the local library. That book struck me as being extremely light on practical advice but it nonetheless provided a narrative of elementary general astronomy told from the perspective of the amateur. Despite my young age I had no trouble in understanding everything in that book, so it also helped me on my way. Other borrowed books, such as Nigel Calder's Violent Universe rounded out my knowledge and gave me some inkling as to professional astronomical researches carried out in the world-class observatories of the time.