Without any doubt the biggest impact on the discussion of astronomy and cosmology at the beginning of the seventeenth century was made by the invention of the telescope in 1608 and the subsequent discoveries that were made by astronomers with the new revolutionary instrument. That the Moon was not smooth and perfect as claimed by Aristotle but had geological features like the Earth, that the Milky Way and some nebula resolved into separate stars when viewed through the telescope, that the Sun had spots, that Jupiter had four Moons orbiting it and lastly that Venus displayed phases showing that it must orbit the Sun and not the Earth. All of these, for the times, amazing discoveries were made between the end of 1609 and 1613 then the stream of new discoveries dried up as suddenly as it had begun, why? The problem was a technological one.
All of these initial discoveries had been made using so-called Dutch or Galilean telescopes that consisted of a simple tube with two lenses a convex objective at the front and a concave eyepiece at the back.
A simple instrument with a serious drawback, by adjusting the focal lengths of the lenses one can increase the magnifying power of the instrument but the greater the magnifying power the smaller the field of vision. Most of the discoveries were made using telescopes with a magnifying power of between twenty and thirty. With such telescopes, for example, Galileo could only view about one quarter of the Moon at a time. With magnifying powers above thirty the Dutch telescope becomes effectively useless as an astronomical instrument. The discoveries that had been made by 1613 marked the limit of discoveries that could be made with the simple Dutch telescope, another instrument had to be found if new discoveries were to be made.
The solution to the problem had already been presented by Johannes Kepler in his Dioptrice published in 1611.
In this important contribution to the science of optics Kepler not only explained, for the first time, how the Dutch telescope functioned but also what became known as the Keplerian or astronomical telescope with a convex objective and a convex eyepiece. He also described the function of the so-called terrestrial telescope with three convex lenses. The astronomical telescope had a much bigger field of view than the Dutch telescope and could thus be constructed with a much higher magnification.
It, however, suffered from the problem that whereas the image in the Dutch telescope was upright, in the astronomical telescope it was inverted. Thus the terrestrial telescope the third lens functioning as an inverter, righting the image.
Christoph Scheiner constructed astronomical telescopes for his work observing the Sun.
However, Scheiner remained an exception, if a prominent one, and in general it took three decades before other astronomers turned from the Dutch telescope to telescopes with convex lenses. This of course raises the question, why? The inverted image in the simple two lens astronomical telescope was one problem, however not for Scheiner, who projected the Sun’s image onto a sheet of paper and could thus simply invert his drawn image when finished. There is, however another reason for the very protracted move away from the Dutch telescope to the astronomical telescope and that reason bears the name Galileo Galilei.
Since the publication of his Sidereus Nuncius in 1610, Galileo had become the authority for all things connected with telescopic astronomy.
Galileo was also arrogant enough to reject anything that he didn’t discover or originate. He made rude noises about the astronomical telescope praising the advantages of the Dutch telescope against the astronomical telescope, even though they didn’t exist. He was also very rude about and dismissive of Kepler’s Dioptrice claiming that it was unreadable. His authority was sufficient to hinder the adoption of the astronomical telescope.
One of the first to go against the authority of Galileo and construct and observe with an astronomical telescope was the Italian astronomer Francesco Fontana (c. 1558–1656), who as we saw earlier made the telescope with which Zupi first observed the phases of Mercury.
Fontana drew a new more accurate map of the Moon, discovered the bands visible on Jupiter. He made the first drawings of Mars and discovered its rotation also inferring the rotation of both Jupiter and Saturn. He published a book of all of his discoveries Novae coelestium terrestriumque rerum observationes, et fortasse hactenus non vulgatae in 1646.
This turned out to be a major problem as the book also contained discoveries that Fontana claimed to have made, for example new moons of Jupiter Saturn and Venues, which simply didn’t exist. The charitable explanation is that these were optical artefacts produced by his telescope. This highlights another major problem of early telescopic astronomy, the quality of the early telescopes ranged from bad to abysmal.
The quality of the glass used to make the lenses was usually fairly poor. Often discoloured and equally often containing inclusions, bubbles created during the cooling of the glass, which interfered with the optical quality of the glass. All the early lenses were spherical, i.e. their curvature was segment of the surface of a sphere. This was the only shape that could be ground and polished with the technology available at the time. Even so, the further one got from the centre of a lens the more it tended to deviate from the correct form. This meant that the image formed by such lenses tended to be fairly severely distorted. The current theory is that the invention of the telescope occurred not when somebody succeeded in grinding and polishing lenses, spectacle makers had been doing that for three hundred years before the telescope emerged, or when somebody came up with the right combination of lenses, there is evidence that the magnifying property of the combination of a convex and a concave lens was known sometime before the breakthrough, but when somebody (Hans Lipperhey?) first came up with the idea of masking the outer edges of the objective lens reducing the available area to the truly spherical centre and thus creating a sharp image at the cost of a loss of light. Another problem was so-called spherical aberration. A spherical lens doesn’t actually focus light to a single point but the image is spread out over a small area causing it to blur. This was already known to Ibn al-Haytham (c. 965–c. 1040), who also knew the solution, lenses shaped according to the surfaces of ellipsoids or hyperboloids but lens makers in the seventeenth century were incapable of grinding such shapes. A much bigger problem was chromatic aberration. This is caused by the fact that simple lenses focus different wavelengths and thus different colours of light at slightly different points, causing coloured fringes on the images. However, the discovery of chromatic aberration by Isaac Newton still lay in the future and its solution even further in the future. Over time the telescope makers discovered that making objective lenses with very long focal lengths reduced the problem of spherical and chromatic aberration and so throughout the seventeenth century the telescopes got longer and longer. Given all of these optical problems it is not surprising that astronomers made discoveries that were illusions; it is to a certain extent a wonder that they discovered anything at all.
The major breakthrough in the use of the astronomical telescope came with the invention of the multiple lens eyepiece by Anton Maria Schyrleus de Rheita, born Johann Burkhard Schyri (1604–1660), an Capuchin monk, who had studied optics and astronomy at the University of Ingolstadt, the university of Christoph Scheiner and Johann Baptist Cysat, which, although they were no longer present when he studied there, still maintained a high standard in these disciplines. Schyri built his own telescopes and made astronomical observations. In 1643 he published his observations in his Novem stellae, which was full of new discoveries but like those of Fontana they mostly weren’t. Much more important was the publication in 1645 of his Oculus Enoch et Eliae in which he describe, without illustrations, a terrestrial telescope with a three lens eyepiece, as well a description of a pair of binoculars.
Beginning in 1643 he had already begun to manufacture his new telescope together with the Augsburger instrument maker and optician Johann Wiesel (1583–1662), Germany’s first commercial telescope maker.
The Wiesel/ Schyri terrestrial telescope, which had an upright image, a wide field of vision and high-level magnification, was a huge success throughout Europe. Not only did they sell well but they were soon copied and used not just on land but also as astronomical instruments. In his book Schyri also coined the terms ocular and objective for telescopes.
The Wiesel/ Schyri telescope broke the dam and opened the market for convex lens, astronomical telescopes. In Italy Eustachio Divini (1610–1685) a clockmaker began to manufacture optical instruments becoming by 1646 the leading optician in Italy selling astronomical telescopes throughout Europe.
In 1649 he published his first book of observations centred round a spectacular selenography.
He would later go on to make detailed observations of Jupiter, the changing shape of the belts, the big red spot and the shadows cast by the satellites. His observation confirmed the axial rotation of the planet.
Divini’s reputation as Europe’s leading telescope maker/astronomer was usurped in 1656 by the still young Dutch polymath Christiaan Huygens (1629–1695), who designed his own astronomical telescope, which he constructed with his brother Constantijn (1628–1697) and with which he discovered Titan the largest of Saturn’s moons.
The year before he had already staked his territory by explaining that the strange observations made by various astronomers of Saturn were in fact differing views of rings surrounding the planet. He explained this in his Systema Saturnium in 1659, which also contained the first telescopic sketches of the Orion Nebula. His explanation of the rings led to a major dispute with Divini, who was convinced that they were a belt of satellites.
In the same year he made the first observations of a surface feature of another planet, Syrtis Major, a volcanic plain on Mars, using it to determine the length of the Martian day.
Divini lost his status as Italy’s prime telescope maker to the Campani brothers Matteo (1620–after 1678) and Giuseppe (1635–1715) in a series of contests staged the Accademia del Cimento to test the quality of their telescopes in 1664, which the Campani brothers won, although largely through skulduggery. Of interest is that the quality of the telescopes were compared by reading printed letters though them, a forerunner of the letter charts in the practice of every ophthalmic optician.
Although Giuseppe Campani was an active astronomer, who made his own observations and discoveries it is their most famous customer, who made the biggest impact, Giovanni Domenico Cassini (1625–1712), who became Jean-Dominique when he moved to France in 1669.
Employed as an astronomer at the observatory in Panzano by the Marquis Cornelio Malvasia (1603–1664) from 1648, Cassini was able to study under Giovanni Battista Riccioli (1598–1671) and Francesco Maria Grimaldi (1618–1663), themselves important telescopic astronomers, who produced an important lunar map, at the University of Bologna.
In 1650 he was appointed professor for astronomy at the university. During his time in Bologna Cassini was able, with the assistance of Riccioli and Grimaldi, using a meridian line in the San Petronio Basilica to prove that that either the Sun’s orbit around the Earth or the Earth’s orbit around the Sun was an ellipse thus confirming a part of Kepler’s astronomical system. The experiment was unable to determine if the system was geo-heliocentric or heliocentric.
As Europe’s leading telescopic astronomer Cassini discovered and published surface markings on Mars, determined the rotation periods of Mars and Jupiter, discovered four satellites of Saturn–Iapetus and Rhea in 1671 and 1672 followed by Tethys and Dione in 1684–he is also credited with the co-discovery with Robert Hooke of the big red spot on Jupiter. He was able to determine the orbits of the moons of Jupiter with enough accuracy that they could be used as a clock to determine longitude, as originally suggested by Galileo. A spin off of this research was the determination of the speed of light by Cassini’s assistant, Ole Rømer (1644–1710). He also showed that both the moons of Jupiter and Saturn obeyed Kepler’s third law, a fact used later by Newton in his Principia Mathematica.
The problem of aberration and the semi-solution of having objectives with ever-longer focal lengths led to the development of the aerial telescope. These are extremely long focal length telescopes that have an objective lens and an eyepiece but no tube, instead having some mechanism to keep the two lens units aligned. Christiaan Huygens constructed one with a cord between the objective and the ocular.
The most famous aerial telescope, however, was that of Johannes Hevelius (1611–1687), a wealthy beer brewer and amateur astronomer who lived in Danzig.
Hevelius constructed a telescope with a focal length of 150 feet, which became a tourist attraction.
He also built a fully equipped observatory on the roof of his brewery and undertook extensive astronomical observations. He, like other, produced a very detailed map of the Moon, discovered four comets and hypothesised that comets obit the Sun on parabolic orbits, created an extensive star atlas in which he described and named ten new constellations, seven of which are still included in official star maps.
With the exception of the discovery of the five largest moons of Saturn, this second wave of seventeenth century telescopic astronomy, starting in about 1640 and continuing till the end of the century, was not as spectacular as the first one. However by the end of the century the small discoveries had accumulated to create a completely different picture of the heavens to the one that existed at the beginning. Planets were no longer Aristotle’s perfectly smooth, spherical bodies but had satellites and surface features, rotated on their axes and had determinable day lengths. The Moon had been accurately mapped by several independent astronomers and there was absolutely no doubt in the minds of the observers that it was fundamentally earth like. The position of many more stars had been accurately mapped and the orbits of the newly discovered satellites had been accurately determined. The celestial spheres of Aristotle and Ptolemaeus had been totally banished. During this second wave of telescopic observation and discovery telescopic astronomy came of age and became a recognised scientific discipline.
In 1669 Cassini was appointed the first director of the Paris Observatory, which had been founded in 1667 by the French minister of finance, Jean-Baptiste Colbert (1619–1683).
The founding of the Paris observatory was followed in 1675 with the founding in England of the Royal Observatory in Greenwich by Charles II, with John Flamsteed appointed in the same year as the first Astronomer Royal.
Berlin came somewhat later in 1700 with the appointment of Gottfried Kirch (1639–1710) but who never lived to see his observatory, which first opened in 1711. What we see here is a radical change in the status of astronomy. Whereas for most of the seventeenth century astronomy had been the province of either private citizens or university professors it now became the province of governments with astronomers appointed as civil servants required to deliver astronomical data for cartographical and navigational purposes.