THE RENAISSANCE IN ASTRONOMY

In 1330 AD the Italian scholar Francesco Petrarca coined the term ‘dark ages’ for the centuries of cultural decline in Europe after the fall of Rome in the fifth century. Intellectual development did not resume until the start of the Italian Renaissance in the fourteenth century. During this interregnum, the works of classical Greece and Rome were available only in Arabic translation. On being ‘rediscovered’, they were translated from Arabic into Latin.

In 1505 Leonardo da Vinci, who had exceptional eyesight, drew an impression of the face of the Moon. He interpreted the brighter part to be water, the dark areas as land, and believed that there were clouds. He was the first to explain the old-Moon-in-the-new-Moon’s-arms effect that occurs when the Moon is a narrow crescent. At such times the majority of the Earth’s disk in the lunar sky must be illuminated, and the dark part of the remainder of the lunar disk is dimly lit by sunlight reflecting off Earth. Late in the 13th century, it had been realised that light was bent by passing through a glass lens. The term ‘refraction’ was not invented until some time later. In 1490 da Vinci had speculated upon whether lenses could be used in combination to make an enlarged view of a distant object. In 1504 he conducted experiments, and by 1510 had the optical principle of the telescope.

After further experiments, three years later he described how a concave mirror could produce a magnified image.

As the Renaissance progressed, some of the ancient beliefs were questioned. By the Ptolemaic system, all celestial bodies travelled around Earth on a daily basis, but Nicolaus Copernicus, a Polish canon, realised that this was not entirely true. In his book De Revolutionibus Orbium Coelestium he revived the heliocentric system of Aristarchus of Samos. Copernicus said only the Moon travels around Earth, but he retained circular orbits, deferents and epicycles. The planets, including Earth, are in orbit of the Sun. But knowing that the Church of Rome would construe this to be heresy, he kept silent, and his book was not released until after he died in 1543. His caution was justified, as in 1600 Giordano Bruno was burned at the stake in Rome for arguing in favour of the heliocentric hypothesis.

Johann Kepler was born near Stuttgart in Germany in 1571. He went to Prague in 1600 to assist the Danish astronomer Tycho Brahe, who held the title of Imperial Mathematician to the Holy Roman Emperor Rudolph II. Over a period of 20 years Brahe had compiled a highly accurate catalogue of planetary motions. When Brahe died in 1601, Kepler inherited the title of Imperial Mathematician, together with the archive of observations, which he set about analysing – something that Brahe had never attempted. Brahe was convinced of the view that Earth was central, but Kepler found otherwise. In his book Astronomica Nova, published in 1609, he announced that a planet pursues an ellipse with the Sun at one focus and the other focus vacant. The same applies to the Moon, but with Earth at one of the foci instead of the Sun. Whilst this rendered obsolete the Ptolemaic system with its circular orbits, deferents and epicycles, the Church was reluctant to concede the point.

In fact, Kepler also realised that the speed of a body in its orbit is proportional to its distance from its primary. In the case of the Moon, with Earth at one focus of its orbit, it travels more rapidly at perigee than at apogee. As a result, whilst the rate at which the Moon turns on its axis is fixed and is synchronised with its orbital period, the Moon is sometimes leading and sometimes trailing the mean position of its orbit, at which times we can see a portion of the otherwise hidden hemisphere around first one equatorial limb and then the other. Similarly since the Moon’s orbit is inclined to the Earth’s equator, when the Moon is in the southern sky we can observe slightly beyond its north pole at a time when that is illuminated, and when the Moon is in the northern sky we can see beyond its south pole when that is illuminated. This effect is known as libration. As for the Moon as a body, Kepler introduced the terms ‘terrae’ and ‘maria’ to describe the light and dark areas respectively.