From our vantage point on Earth, it is possible to observe the motions of the planets and to map out their relative distances from the Sun with great accuracy. This was first done over 300 years ago but it required most of the intervening centuries before the key to the absolute distance scale within the Solar System was found. Simple observations of the changing directions of the planets in the sky can give the ratios of their distances from the Sun; the absolute distance to any one member of the Solar System gives the distances to all others. Unfortunately, the major planets are too far away for our method of diurnal parallax to provide an absolute distance, and an alternative was slow in arriving.

Orbiting primarily in a region of space between Mars and Jupiter are the minor planets, known as asteroids. Some of these periodically swing close enough to the Earth for trigonometrical parallax to be employed to find their distances. At the turn of the twentieth century, the asteroid Eros passed within 40 million kilo¬metres of Earth, sufficiently close to fix the absolute scale of the Solar System. The mean distance of the Earth to the Sun is well established and even the small variations about this mean are well studied. Today the fundamental place of the asteroids has been superseded by radar-ranging techniques using signals sent from Earth to Venus. Reflected from the surface of Venus and timed in their round-trip journey at the speed of light, these signals also measure the distance directly. With confidence we can write down that the mean distance from the Earth to the Sun is 149 597 893 km, the ASTRONOMICAL UNIT.

With centuries of technological expertise on our side, we have actually left the Earth. We have seen our planet from afar; we have followed our robot electronics to the Moon and have scraped at its surface; we have sent cameras in advance to study the atmospheres and surfaces of Mars, Mercury, Venus and Jupiter. We have landed spacecraft safely on the surfaces of Venus and Mars, and searched for traces of life in the Martian dust. While the Solar System was once the remote preserve of the astronomer, it is now fast becoming a conscious part of the daily experience of us all.

If we move away from the Sun, we pass its rocky companions, the inner planets and the asteroids, we pass beyond the giant outer planets of Jupiter and Saturn, still shrouded in their massive primordial atmospheres, and slowly we leave behind the icy comets which inhabit the outermost reaches of our Solar System. And then we-.face the seemingly limitless void of space. No sooner have we assimilated the duration of our lengthy journey as a measure of the size of our Solar System than the Galaxy looms beyond, even larger still

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