No terrestrial observing site is perfect and any one site can only be a compromise between the many, often conflicting requirements of a so-called site. What makes a good site for an optical or infrared observatory? Firstly, it must have many good, clear nights. Then it must be as dark as possible. This means that the site should not be too near the polar regions if the aurorae are to be avoided. A major component of the light of the night sky is man-made street lighting. There are always enough dust or water droplets in the atmosphere to ensure that street and advertisement lighting cause a bright glow over a city which can have a substantial effect on the sky brightness many tens of kilometres from the city There are many parts of the world where towns and cities are so close together that there are few truly dark sites over large areas For example, urban development has been so rapid over the last thirty years in California that the great observatories on Mount Wilson, Mount Palomar and at Lick are substantially affected. However, Kitt Peak in Arizona is less affected because the Tucson city authorities have installed baffles on street lighting to minimize the effects to the telescopes 80km away. In recent years the substantial growth of city lighting (10 per cent per annum) has forced astronomers to choose observing sites on small mountainous islands with a limited potential for growth.

The next requirement of a good site is that the atmosphere above it should be stable, to ensure that the seeing is as good as possible. Seeing generally improves with altitude and the best sites are those which are above the INVERSION LAYER in the atmosphere. This is the height above which hot air from the surface of the Earth does not normally rise. Above the inversion layer the atmosphere is particularly steady, clear and free from dust or water vapour. This last aspect — freedom from water vapour – is of particular importance to infrared astronomers and for this the most important single factor is altitude. Mauna Kea on Hawaii (4500m) is, therefore, an excellent infrared site. It is also a good optical site along with the Chilean sites (Las Campanas, Cerro Tololo. La Silla) and the Canary Island sites (Tenerife and La Palma).

Radio observatories may be sited in poor northern climates since radio observations are largely unaffected by cloud or rain, except at wavelengths below 10cm. The largest, most sensitive telescopes have to be built away from sources of man-made interference such as that produced by electrical machinery. In addition, there are a number of radio wavebands which are protected from use through¬out the world and which are used for radio astronomy.

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