The Transits of Venus of 1631 and 1639

Introduction

During the months April and May 1607 the German astronomer Johannes Kepler (1571-1630) had been engaged in observing the planet Mercury from Prague in the evening sky. Although current planetary ephemerides predicted that Mercury would be in conjunction with the Sun on 29 May, a violent thunderstorm in the evening and the night of the 27th suggested that Mercury was closer to its solar conjunction than the calculations had indicated.

On the following day (28 May), Kepler observed the Sun with a camera obscura and when he detected a small spot on the solar disk he was convinced that he was observing a transit of Mercury. He published his discovery in a short treatise on the comet of 1607 (Halle, 1608) and in more detail in his Phænomenon singulare (Leipzig, 1609).

Only a few years later Kepler realized that his 1607 observation could not have been Mercury but had been a large sunspot, similar to those seen on the solar disk by other observers using the recently invented telescope. 

The 1631 Transits of Mercury and Venus

Johannes Kepler appears to be the first astronomer who made detailed predictions for past and future transits of Venus, In a chapter on mutual occultations of the heavenly bodies in his Optical Part of Astronomy (Frankfurt, 1604), Kepler stated that no transit of Venus would take place during the 17th century although it had been possible two centuries before his time.

In 1629, as Kepler was preparing a set of astronomical ephemerides for the years 1629 to 1636, based on his new laws of planetary motion published in his Rudolphine Tables (Ulm, 1627), he noted that a transit of Mercury would take place on 7 November 1631 and a transit of Venus on 6 December of the same year. Although his calculations indicated that the latter transit would best be visible from the American continent, he cautioned European astronomers in his pamphlet De raris mirisque Anni 1631 (Leipzig, 1629) to be watchful as well.

The transit of Mercury was successfully observed by the French astronomer Pierre Gassendi (1592-1655) in Paris, by Jean-Baptist Cysat (1588-1657) in Innsbruck and by Johannes Remus Quietanus in Rouffach. Their observations provided the first clear proof that the apparent diameter of Mercury was much smaller than had hitherto been believed.

However, despite careful vigilance, Gassendi and other astronomers failed to see the transit of Venus in the following month. According to modern calculations, the transit actually ended about 50 minutes before sunrise (7:35 UT) at Paris although observers in most of Italy and along the Eastern Mediterranean should have been able to view the last stage of the transit.

The 1639 Transit of Venus

In his 1629 pamphlet, Kepler had predicted that after the 1631 transit of Venus, there would be no other one until in 1761. In 1639, the young English astronomer Jeremiah Horrocks (c. 1619-1641) realized that Kepler could have been in error as his own calculations with the tables of Philips of Lansbergen indicated that there would be a transit of Venus on 4 December of that year. He warned his younger brother in Liverpool and his friend WiIlliam Crabtree (1610-1644), a cloth merchant in Salford (near Manchester), of the coming event.

On the day of the event, the sky in Britain was largely overcast but just before sunset Horrocks was able to observe the planet Venus on the solar disk. His brother was less lucky, but his friend William Crabtree was also able to observe Venus on the solar disk just before sunset.

Jeremiah Horrocks wrote a detailed description of his and Crabtree’s observations and its implications in several drafts with the intent to publish this but his death in 1641 prevented this. A copy from one his drafts was obtained in 1661 by the Dutch scientist Christiaan Huygens (1629-1695), who passed it on to the Polish astronomer Johannes Hevelius (1611-1687) and had it published in 1662.

Visibility Regions for the Transits of Venus of 1631 and 1639

The following diagrams depict the regions of visibility for the transits of Venus on 7 December 1631 and 4 December 1639.

Circumstances of the transit of Venus of 1631
 
Click on the image for a larger view Click on the image for a larger view
map legend   map legend  
 
Begin of the transit (7 December, 03:52 UT) End of the transit (7 December; 06:47 UT)
Sub-Venus point: λ = +120º 11'; φ = –22º 25' Sub-Venus point: λ = +76º 15'; φ = –22º 22'
Apparent diameter of the Sun = 32.50' Apparent diameter of the Sun = 32.50'
Apparent diameter of Venus = 1.05' Apparent diameter of Venus = 1.05'

This transit took place just after Venus passed its ascending node (6 December; 5:45 UT) and occurred on the northern half of the solar disk.

Circumstances of the transit of Venus of 1639
 
Click on the image for a larger view Click on the image for a larger view
map legend   map legend  
 
Begin of the transit (4 December, 14:57 UT) End of the transit (4 December; 21:55 UT)
Sub-Venus point: λ = –46º 18'; φ = –22º 32' Sub-Venus point: λ = –151º 16'; φ = –22º 26'
Apparent diameter of the Sun = 32.49' Apparent diameter of the Sun = 32.49'
Apparent diameter of Venus = 1.05' Apparent diameter of Venus = 1.05'

This transit took place just before Venus passed its ascending node (5 December; 7:32 UT) and occurred on the southern half of the solar disk.

References

I: Kepler’s Observation of a ‘Transit of Mercury’ in 1607

II: Prediction and Observations of the 1631 Transits of Mercury and Venus

III: Observations of the 1639 Transit of Venus

IV: Other 17th-Century Transit of Mercury Observations

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