Gallia, as Julius Caesar famously remarked and every schoolboy used to know, omnis in partes tres divisa est: "All Gaul is divided into three parts." Likewise, Disraeli suggested a tripartition of mendacity, asserting that "there are three kinds of lies: lies, damned lies, and statistics". And now we in the space age can similarly categorise the celestial population of artificial satellites: the vast majority fulfil one of three major functions.
Of the several hundred artificial satellites in working order at any given time, by far the most numerous are the communications satellites. They are mainly used as telephone and television relays, the latter purpose being the more demanding, and consuming more than 75 per cent of the available capacity.
The second category comprises the remote-sensing satellites. These are essentially cameras in the sky, intended to observe the ever-changing appearance of the Earth's surface, or the evolving condition of the atmosphere above it.
Weather satellites, perhaps, are the most obvious examples: they not only relay visual images of the clouds and areas of rain, but also measure temperature, humidity and wind speed, and provide information about the distribution of ozone and other chemical constituents of the atmosphere. They are used, not only for day-to-day weather forecasting, but also for detecting any signs of global warming, or to monitor the progress of anomalous meteorological occurrences like El Nino.
Remote-sensing satellites, however, are also widely employed to study land use, since they can detect those areas that are given over to specific varieties of crops, and analyse the different kinds of vegetation on the surface of Earth.
They are used to observe the seasonal distribution of ice upon the oceans, to measure the advance and the retreat of glaciers, to keep a watchful eye on floods or forest fires, and to monitor the encroachment of the desert on previously fertile areas in the arid regions of the world.
The third major category comprises satellites that might be described as navigational. The most widely known is the Global Positioning System, or GPS, a network of spacecraft whose coded signals to ground-based equipment allow the exact location of the receiver on the surface of the Earth to be calculated to within a yard or two.
The system was originally developed by the US military authorities, and then became widely used by marine and aviation interests. More recently, the GPS has been adapted for use in private motor cars, and the receivers have now been miniaturised sufficiently to allow their convenient use for hiking and other purely recreational pursuits.