Venus, the second planet in order of distance from the Sun, is almost a twin of the Earth in size and mass; it is only very slightly smaller and less dense. However, in all other respects it is quite unlike the Earth. Only during the past 40 years have we been able to find out what Venus is really like; its surface is permanently hidden by its thick, cloudy atmosphere, and before the Space Age Venus was often referred to as ‘the planet of mystery’. Data are given in Table 5.1.

Venus is the brightest object in the sky apart from the Sun and the Moon. At its best it can even cast shadows – as was noted by the Greek astronomer Simplicius, in his Commentary on the Heavens of Aristotle, and by the Roman writer Pliny around 60 AD. Venus must have been known since prehistoric times. The most ancient observations which have come down to us are Babylonian, and are recorded on the Venus Tablet found by Sir Henry Layard at Konyunjik, now to be seen in the British Museum. Homer (Iliad, XXII, 318) refers to Venus as ‘the most beautiful star set in the sky’ and the name is, of course, that of the Goddess of Love and Beauty.

It may have been Pythagoras, in the sixth century BC, who first realised that the evening and morning apparitions of Venus relate to the same body – though like almost everyone else at that period, he believed Earth to be the centre of the universe.