Sirius is now at its best in the evening sky. It is one of our nearest stellar neighbours, and is much more luminous than the sun. It seems to flash all colours but is really a white star, it has a strange, super-dense White Dwarf companion. Patrick Moore and lain Nicolson discuss some of the questions associated with Sirius.

How big is the universe - and does it have a boundary? Distances on the astronomer's scale are very hard to appreciate. Patrick Moore and Dr Ron Maddison discuss them in this programme, and explain how to describe them in everyday terms.

First detailed photographs of Mercury, the innermost planet, were obtained by the American space-probe Mariner 10. Maps of Mercury have been drawn from these photographs by Arthur Cross and in this programme he joins Patrick Moore to explain how the maps were made and what they have told us.

In April 1957 The Sky at Night began. It could not have started at a better time; within months the Space Age opened, with the launch of Sputnik 1, and over the next few years astronomy saw some of the most spectacular advances of all time. Today, 20 years later, men have visited the moon; rockets have flown past the planets and even landed on some of them; new giant telescopes have probed further into the universe than Man has ever done before, and each year brings its new quota of discoveries. Every month, since April 1957, The Sky at Night has presented the changing scene. In this 20th anniversary programme Patrick Moore discusses the past and the future with some of Britain's leading figures in astronomy.

Sunspots are known to increase and decrease over an 11-year cycle. We have just passed through a period of minimum solar activity and the sun-spots should now be increasing, but are slow to do so. Patrick Moore and Dr Ron Maddi son discuss the possible causes of this apparent irregularity. in the solar cycle, and talk about some of the other interesting features of the sun's surface.

For centuries the ringed planet Saturn has been regarded as unique. This year the startling discovery has been reported that Uranus also has a system of rings - though as yet they have not been seen directly. Patrick Moore discusses this new development with Gordon Taylor, of the Royal Greenwich Observatory. who has made special studies of Uranus, and Dr Garry Hunt, who is closely involved with plans for Voyager, the unmanned space-craft due to bypass Uranus in the 1980s.

This month an important new British telescope has been completed. It is destined to be set up in Hawaii, and it is not an ordinary telescope; it is designed to study infra-red radiation from space, which cannot be seen directly but which is all-important in modern science. Professor Vincent Reddish , Astronomer Royal for Scotland, describes the Hawaiian telescope, and Patrick Moore talks to Professor James Ring about its likely role in infra-red astronomy.

Cygnus, the Swan, is one of the most beautiful of the constellations, and it contains some spectacular objects, including the very luminous Deneb - a star at least 10,000 times more powerful than the Sun-and the lovely coloured double star Albireo. Very recently astronomers have found a new object in Cygnus-a star surrounded by a highly luminous disc from which planets may be in the process of formation. In this programme, Patrick Moore shows where to look for Cygnus, and describes some of the interesting objects on view there.

The two Voyager probes to the outer planets are probably the most ambitious of all automatic space-craft launched up to the present time. They should send back important new information about Jupiter, Saturn and their satellites - they may even go on to study Uranus and Neptune before finally leaving the Solar System altogether. Patrick Moore discusses the Voyager projects with Dr Garry Hunt of University College London, who has been involved in the planning of the missions.

How many people living in Britain have seen the Southern Cross? It never rises over Europe, but it is a familiar sight to those who live in the Southern Hemisphere. The stars of the far south are indeed of special interest, and Patrick Moore talks about them, explaining why they are important and the reasons why they are invisible from Britain. Many of the new giant telescopes are being set up in the Southern Hemisphere to study objects of particular significance, such as the Clouds of Magellan, and in this programme PATRICK MOORE talks about these new developments with Dr David Allen , who is carrying out research at Siding Spring Observatory in Australia.

Astronomy is still one of the few sciences in which the amateur can play a useful part. Paul Doherty , who specialises in planetary observations, is an amateur astronomer who has built his own observatory and 16-inch reflecting telescope. Patrick Moore visits him at his observatory to discuss its construction and the observational work being carried out there.

All the bright planets are now visible in the early morning sky. Some of them seem to outshine the stars, and yet they have no light of their own. They shine only by reflecting the light of the sun. The different ways in which they reflect the sunlight tell us a great deal about the planets themselves. Patrick Moore talks about the reflecting powers of the planets and what they can tell us with Dr Peter Cattermole of Sheffield University.

Ever since the days of Jules Verne and H.G. Wells, science fiction and space exploration have been closely linked, and over the last 100 years science fiction has often become science fact. Rockets to the moon, orbiting stations, missions to Mars and the outer planets-all these were forecast decades ago. Patrick Moore talks to Arthur C. Clarke, science-fiction writer, who describes himself as an armchair astronaut, and The Sky at Night welcomes the return of Michael Bentine, best known as a humorist, but who is also a serious and dedicated scientist.