Universe: The story of the Universe, from earliest times to our continuing discoveries. Peter Grego

features and to map its surface. The Moon’s landscape really did resemble parts of the Earth – Galileo had likened it to parts of Bohemia – so did it have an atmosphere and could it support life? Large dark patches visible with the unaided eye were discovered to be relatively flat grey plains. These areas became known as ‘maria’ (Latin: seas), but it was plain to see through the telescope eyepiece that they did not represent bodies of water.

      The mid-17th century saw the publication of a number of detailed lunar maps. One by Johannes Hewelke (Hevelius) was published in his Selenographia, complete with names for lunar features based upon geographical landmarks, like Sicily, Mount Etna and the Mediterranean Sea. From his private observatory, Hevelius also made accurate measurements of star positions and produced the Uranographica, the most advanced star atlas of its time; the names for his seven new constellations are still used by astronomers today.

      At around the same time, Giovanni Riccioli published an accurate lunar map which incorporated nomenclature that is still current, including his names for the Moon’s seas, such as Mare Tranquillitatis (the Sea of Tranquillity) and many of the larger craters like Copernicus (Mount Etna on Hevelius’ map) – familiar names to modern watchers of the Moon.

       Giovanni Riccioli’s Moon map of 1651.

       Christiaan Huygens’ explanation of Saturn’s rings and their changing appearance.

      Planetary perceptions

      Fresh telescopic revelations answered some age-old questions about our immediate planetary neighbourhood but raised many more new ones.

      Aerial adventures

      In the Netherlands, Christiaan Huygens constructed lengthy refracting telescopes whose lenses were suspended on frames in the air and moved by means of ropes and pulleys. Such unwieldy devices – some of them measuring more than 8om (260ft) from objective lens to eyepiece – were required to overcome a defect inherent in simple optics known as chromatic aberration. Caused by the inability of a single objective lens to focus all the colours within white light to a single point, chromatic aberration causes bright objects to appear surrounded by coloured fringes. The longer the objective lens, the less evident the degree of aberration. Huygens also improved eyepiece design by introducing the twolensed Huygenian ocular – a design still provided today with many budget telescopes. Using these ungainly aerial telescopes he discovered Jupiter’s equatorial bulge – the consequence of Jupiter’s rapid spin and its gaseous composition. Bright polar caps were discovered on Mars, in addition to a dark V-shaped feature known as Syrtis Major. Almost half a century after Galileo had discovered the Solar System’s first planetary satellites, Huygens discovered Saturn’s largest satellite, Titan, in March 1655. A conundrum which had baffled astronomers since Galileo – mysterious appendages that seemed to cling to Saturn’s side and vary in size over the years – was finally solved by Huygens, who explained that Saturn had a flat ring system that nowhere touched the planet.