The emergence of modern astronomy – a complex mosaic: Part XXXI

Of all the discoveries made during the first phase of telescopic astronomical discoveries perhaps the more impactful was the discovery by various observers of the phases of Venus, which showed that Venus in fact orbited the Sun and not the Earth. This was the first real empirical proof that a pure geocentric system of astronomy was not possible. Because Mercury displayed the same behaviour as Venus in never moving more than a few degrees from the Sun as viewed from the Earth it was assumed, by analogy, that Mercury also orbited the Sun and not the Earth. This observed behaviour of Venus and Mercury had already led Martianus Capella (fl. c. 410–420) in late antiquity to hypothesise that both of them orbited the Sun and not the Earth. The Capellan system was widespread and well known in Europe during the Middle Ages. Unfortunately due to its comparatively small size and distance from the Earth, and the inadequacies of the most used Dutch or Galilean telescopes it would be almost three decades before anybody succeeded in observing the phases of Mercury.


Naboth’s representation of Martianus Capella’s geo-heliocentric astronomical model (1573) Source: Wikimedia Commons

The knowledge that Venus orbited the sun and the assumption that Mercury also did led to the possibility of observing a so-called transit of one or other of them, i.e. the passage of the planet across the face of the Sun, as observed from the Earth. What would appear to be simply simple is in fact complicated by various factors. Principally the orbits of the Earth and Venus and/or Mercury around the Sun do not actually lie in the same plane. Taken Venus, for example, its orbit is tilted with respect to the Earth’s this means that most times when Venus is between the Earth and the Sun, it mostly passes above or below it rather than in front of it. Transits of Mercury take place approximately 13 or 14 times per century. Transits of Venus are less frequent taking place over a 243 year-cycle, two transits separated by eight years followed by long gaps of 121.5 and 105.5 years.

In order to be able to observe transits of the inner planets an astronomer requires detailed, accurate planetary tables, which allow them to predict the occurrence of the transit. Following the discovery of the phases of Venus it was Johannes Kepler (1571–1630), who, with the Rudolphine Tables (1627), first provided a set of planetary tables accurate enough to predict a transit of either Venus or Mercury.

According to Kepler’s calculations there should have been transits of both Mercury and Venus in 1631. Kepler did his best to draw astronomers’ attention to these occurrences with his De raris mirisque Anni 1631 including anadmonitio ad astronomos (1629), because their observation would they would help astronomers to settle the question of the true size of the planetary orbits, which up till then known relative to the Earth’s distance from the Sun, an unknown distance, and to determine the angular sizes of Mercury and Venus.


Kepler died in 1630 but his son in law, the astronomer Jacob Bartsch (c. 1600–1633) published a printed pamphlet advising European astronomers of Kepler’s information.

Strangely there is no information that Bartsch observed the Transit of Mercury on 7 November 1631 but at least four other European astronomers did. The first was the German astronomer, astrologer, physician and calendar writer, Johann Rudrauf (1588–1654), known Johannes Remus Quietanus, a correspondent of Galileo, Kepler and Johannes Faber. Rudrauf observed the transit from Rufach in the Alsace border region between Germany and France, where he was town physician. The second was the Swiss Jesuit astronomer Johann Baptist Cysat (c. 1587–1657), who had been a student and assistant of Christoph Scheiner (c. 1573–1650) at the University of Ingolstadt and became his successor as professor of mathematics in 1618. Cysat observed the transit from Innsbruck in Austria.


Johann Baptist Cysat Source: Wikimedia Commons

The third observer was the French philosopher, astronomer and mathematician, Pierre Gassendi (1592–1655), who observed the transit from Paris. A fourth unknown astronomer observed the transit from Ingolstadt.


Pierre Gassendi after Louis-Édouard Rioult Source: Wikimedia Commons

Gassendi’s account of the transit was the most widely read and studied so he is usually credited with being the first to observe a transit of Mercury. On 6 December 1631 Gassendi tried to observe the transit of Venus that had been predicted by Kepler unaware that because it took place during the night it wasn’t visible from Europe.


Transit of Mercury on May 9, 2016 Source: Wikimedia Commons

Kepler’s tables predicted no further transits of either Mercury or Venus for the seventeenth century so theoretically nobody would have set out to observe one. However, a young, self taught, English astronomer, Jeremiah Horrocks (1618–1641)did something quite extraordinary. Studying Kepler’s Rudolphine Tables Horrocks’ realised there would be another transit of Venus on 24 November 1639 (os) (4 December (ns)). Horrocks and his friend and fellow amateur astronomer William Crabtree (1610–1644) both observed the transit that Horrocks had predicted, Crabtree from Manchester and Horrocks from Much Hoole by Preston about 40 miles north of Manchester.


Jeremiah Horrocks makes the first observation of the transit of Venus in 1639, as imagined by the artist W. R. Lavender in 1903 Source: Wikimedia Commons

Both of them made their observations by projecting the telescopic image on to a sheet of paper. Horrocks wrote an extensive report of their observations calculating both the size of Venus and the distance between the Earth and the Sun. Unfortunately Horrocks died in 1641 just 23 years old and his report of the first ever observation of a transit of Venus, Venus in sole visa, was first published was first published by the astronomer Johannes Hevelius (1611–1687)in Danzig in 1662.


In 1639 the Italian Jesuit astronomer Giovanni Battista Zupi (1589–1650) succeeded in observing the orbital phases of Mercury using an astronomical telescope (two convex lenses) constructed by the Neapolitan astronomer, mathematician and telescope maker Francesco Fontana (1585–c. 1656).


Zupi and Fontana often observed together and Zupi’s observations of the phases of Mercury were published in Fontana’s Novae Colestium in 1646.


The observations of the transits of Mercury and Venus, and the phases of Mercury was final empirical proof, if it was needed, that both of the inner planets orbited the Sun and not the Earth nailing down the lid on the coffin of a pure geocentric, astronomical system. However, by the time these observations were made the majority of the astronomical community had already decided that only Kepler’s elliptical system or a Tychonic system with diurnal rotation were acceptable as the true system. The observations were of course compatible with both systems, so a final decision was not yet possible.



Filed under History of Astronomy, Renaissance Science

4 responses to “The emergence of modern astronomy – a complex mosaic: Part XXXI

  1. james oberg

    An ancient site in Greece [my wife’s ancestral home at Monemvasia] was the site of one of my most exciting naked-eye astronomical observations of my life. Skywatchers are taught early that while stars twinkle, planets do not – the reason is that stars are so distant they are essentially point sources with near-zero angular diameters, so tiny air ripples can disturb the light beam’s path, but the wider planets [so much closer] provide enough parallel light paths that any small air ripples are cancelled out. This reminds me of my experience a few years ago at a seaside resort in Monemvasia, Greece, with its Gibraltar-like bare-rock fortress hill. On one visit there, we stayed in a hotel at the foot of the citadel. Rising before dawn to enjoy the awesome crisp stars, I realized Venus should be appearing in the east [it was in Leo at that time and I could see the lion’s mane peeking around the edge of the rock], so seeing that the dark mass of the treeless hill was in the way, I waited… until a glimmer, then a brighter and brighter light appeared on the shoulder of the bare rocky hill several hundred meters in front of me. It seemed to take about ten seconds from zero to full, steady brightness. Curious at the prolonged brightening, I slowly walked downhill until Venus was again extinguished behind the rock edge, then knelt and rose to make it come on and off. I realized I was seeing non-telescopic evidence for the non-zero angular size of Venus [actually, of the illuminated portion of Venus]. What was really exciting was when I measured the distance my eyes had to move to make this happen, and then estimated the range to the occulting rock face. Using similar triangles with a guess at the range to Venus, I roughly calculated a diameter of 8,000 to 15,000 km — bracketing the actual value. The Greeks knew the Earth-Moon distance amazingly well, and realized the sun was a lot farther way than the moon, so they might well have made further deductions of the size of Venus compared to Earth – but had they? Weeks later, I made a search in history of astronomy references [and consulted several experts], and nobody could find records that the keen sky-watchers of antiquity had noted [or at least recognized the significance of] this phenomenon. Only Venus and Jupiter would have produced noticeably non-instant brightening emerging from behind the moon [or moving in behind it], or an earthside rock face. Mars and Saturn were much smaller angular sized objects. I wonder if a proper design of a deliberate occulting edge and its orientation [or selection of a usable natural rock edge] would have allowed determination of the non-spherical shape of Venus [crescent], versus the spherical shape of Jupiter [at opposition]. Well, I had cheated by knowing in advance what the answer was, but it was a freak accident that provided me a sharp occulting edge far enough from me to create the noticeable-prolonged effect with one of only two celestial objects that could engender it. Awesomely lucky experience.

  2. Thony,
    I’ve just been listening to P.G. Wodehouse’s angle on the history of mathematics and the measurement of time. Occurs to me you’d appreciate it –

    section begins at 3hrs 18mins 35 secs.

  3. sorry – didn’t know the link would automatically embed.

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