DID YOU KNOW? The first photograph showing the entire disc of Earth was taken by a Russian satellite in 1966

Our knowledge of other worlds in the Solar System has grown enormously since the dawn of the Space Age, along with ever more detailed photographs

WORDS ANDREW MAY

Over 3,000 years ago, Babylonian astronomers discovered that five bright points of light moved across the night sky in a different way from all the other stars. These were the planets we now call Mercury, Venus, Mars, Jupiter and Saturn. In those early days, the only way to observe astronomical objects was with the unaided eye, making it impossible to discern any detail. A planet looked virtually identical to any other star, the only difference being its distinctive wandering motion. The very word planet comes from the Greek planētēs, meaning ‘wanderer’.

The next major step forward came in 1609, when Galileo first observed the night sky with an early telescope. This showed the planets to be extended discs rather than star-like points of light, and it was only with this discovery that people came to realise that the planets might be other worlds like Earth. As telescopes improved over time, two further such worlds were discovered that had previously been too faint to see: Uranus in 1781, followed by Neptune in 1846.

The first person to observe surface features on another planet was the Dutch astronomer Christiaan Huygens, who long before the invention of photography made a pencil sketch of Mars in 1659. Later observers of the Red Planet spotted white areas around the poles, which were speculated to be ice caps, as well as clouds and evidence of changing seasons. Such discoveries led to fanciful portrayals of Mars as an Earth-like world, complete with vegetation and maybe even intelligent inhabitants – the latter idea bolstered by the supposed observation of linear features that were dubbed ‘canals’. These wild speculations only came to an end in 1964 – just 60 years ago – when a NASA spacecraft finally took the first close-up photographs of the Red Planet.