William Sheehan & John Westfall
The Transits of Venus
(Prometheus Books, Amherst, NY) 2004
Reviewed by Frederic Jueneman
It may be much too late for readers of this review to observe the rare transit of Venus across the disk of the Sun, which took place on June 8, 2004, unless they already had been aware of the phenomenon and made prior arrangements to view the spectacle. As it so happens, the entire transit would only be visible in much of Europe and Asia, with some of the best viewing being in--of all places--Iraq. The eastern seaboards of Asia and Australia would only see the ingress of the transit, while eastern North and South America would only see the egress. The last time this transit occurred was December 6, 1882.
But, fret not my friends, for this rare celestial alignment will occur one more time in this century on June 5-6, 2012, as the entire transit passes across the International Dateline in the mid-Pacific. It's next two appearances then won't be until December 11, 2117 and December 8, 2125.
Curiously, these transits of Venus come in doublets spaced eight years apart minus some two days (or approx. 2920 days, with the appropriate allowance for leap years). However, the long intervals in between each pair of transits alternate between 105 and 122 years. Moreover, astronomers have grouped these transits into series, and which themselves recur every 243 years. (The number `243' is interesting as it coincidentally is the retrograde rotation of Venus in days.)
But, I digress.
The story itself begins with the dilemma of parallax, an age-old problem of viewing a body from two or more positions and estimating its size and distance. The Moon was such an object that had puzzled astronomers for millennia, although in the third century BCE Aristarchus of Samos had closely estimated the Moon's size and distance based on Earth's shadow during lunar eclipses. This problem is compounded by an apparent parallax-shift of some 2° when viewed from widely separated positions on the Earth. However, it becomes even more critical when extremely small angular displacements are encountered while estimating the size and distance of other objects such as the planets and Sun, not to mention distant stars.
Kepler's Third Law states that the cube of a planet's distance from the Sun is proportional to the square of its period of revolution about the Sun. Thus, it was thought that knowing the period of revolution of Venus, an estimate of its size and distance during a transit would give an indication of our own distance from the Sun. Such Cytherean transits across the Sun can take anywhere from three to six hours, depending on the specific planetary alignments and to a great extent the geographical positions of the observers, whereas a precise timing of the crossing is critical to each observer at a given latitude and longi¬tude. Such observations would be a triumph of science over the 45 known transits of Venus over recorded history since the days of Ammisaduqua,, the penultimate king of the First Dynasty of Babylon.
In the fourth century BCE, Heraclides of Pontus, a pupil of Plato, suggested that a lot of problems would be resolved if planets as Mercury and Venus were thought as orbiting the Sun and that the earth itself rotated on its own axis, but this was swept aside as more important concerns took precedence. This idea was later taken up by Aristarchus, but rejected on religious grounds that epistemologically declared that the Earth is the center of the universe and that the planets orbit it in perfect circles. The Alexandrian astronomer Claudius Ptolemy in the second century CE carried this to a fine art. Indeed, for the sake of convenience, today's astronomers often refer to a body's deferent--average orbital figure--as circular, and we ourselves speak of the Sun's literally rising and setting, although it does no such thing.
However, by the time of Copernicus, Kepler, and Galileo, it was clearly apparent to the intelligentsia that "saving the phenomena" wasn't cutting the mustard, as traditional views and pontifical hand-waving couldn't substitute for direct observation. In 1631 French astronomer Pierre Gassendi observed the transit of Mercury and hoped to see Kepler's prediction of that of Venus later the same year, except that it was over before sunrise in Paris. The first recorded observation of a partial Venus transit was carefully but hurriedly made eight years later by the 20-year-old English as¬tronomer Jeremiah Horrocks in 1639, as the next one was not due until 1761, and in so doing was able to correct the ratio between Venus' and Earth's orbits to a value still used today, The illustrious Gassendi could also have seen it from Paris, but he was otherwise occupied and had apparently lost interest.
An alternative solution to the solar parallax problem was taken up by Giovanni Cassini and the Dane Ole Rømer at the Paris observatory by way of Mars close approach to Earth in September 1672, with a second observation post manned by the Jesuit astronomer Jean Richer in Cayenne, French Guiana, on the South American coast, finding a value of some 25 arc-seconds of displacement of the planet's image against the background of stars. From this measurement the solar parallax implied a distance from the Earth to the Sun within about eight percent of to¬day's figure.
This all occurred during a period of almost global scientific revolution in the late 17th century, when it was thought that measurements of Venus transits would underscore Isaac Newton's clockwork universe and define the absolute values of the scale of our solar system--the solar parallax itself. Edmund Halley proposed that widely separated observers time the interval between transits from ingress to egress, so that the angles of observation could more accurately define the parallax. It was a suggestion that he himself would not see in his lifetime, nor that of his prediction of the comet of 1682 now bearing his name returning in 1758.
The French astronomer Joseph-Nicholas Delisle proposed an alternative method of observing the transit of 1761 over four essential moments, by noting first contact of Venus with the Sun's limb, then when it was just within the solar disk, when it was about to leave the disk again, and when it finally separated from the Sun proper. The main difficulty with this procedure was knowing one's precise longitude, a problem alleviated by the invention of the marine chronometer by John Harrison and independently by the lesser known Pierre Le Roy and Ferdinand Berthoud.
With Europe finally settling down after a series of conflicts and other political upheavals, dozens of astronomers and their retinues scattered across the globe to witness the event, most using Delisle's methodology. However, bad weather conditions, accidents, and of course politics--the Seven Years War just broke out--daunted many observers, but there were a few preciously good sightings that made the effort seem worthwhile. Moreover, the Russian astronomer Mikhail Lomonosov was the first to discover and recognize that the haze appearing around Venus as it crossed the Sun's limb was its atmospheric envelope.
It was this selfsame haziness that hampered the accurate observations of the transit, making the exact moment of crossing indistinct and error-prone, a phenomenon which cast serious doubt on the precision of parallax determination, and by inference the uncertain masses of the other planets. Further, a dark blob appeared to attach itself to the Sun's edge as the image of Venus dissociated itself during ingress and again when leaving during egress, exacerbating the difficulty of timing the transit. Ah well, there would be the upcoming transit of 1769 when they would be better prepared to adapt and mitigate such problems with improved techniques.
It didn't happen.
Preparations for the 1874 crossing by interested countries were lackadaisical in coming, despite otherwise serious planning by the astronomers, most only a few years before the event. However, new instru¬ments and techniques were involved in the mix, such as the heliometer, spectroscope, improved chronometers, telegraphy, and the advent of photography, notably collodion film and daguerreotype. And yet, with all the technological advances, the selfsame problems of clarity and sharpness remained. The photographs upon enlargement were even fuzzier than before. The uncertainties, if anything, were compounded by the advances.
According to authors, Sheehan and Westfall, the December 1882 was "the last hurrah" for the Halley and Delisle methods. Nevertheless, it was the first time that the Americas were in the spotlight since 1639, when Harvard was merely a fledgling university, having been founded just three years prior. The disappointments of 1874 left most previous contributors to the safari-like excursions less than enthusiastic, But then again, the next transit would skip the 20th century entirely, so some forty-odd voyages were dispatched to the western hemisphere for that last hurrah.
As is normally the case, newer and more revolutionary techniques were even then being forwarded that didn't rely on transit observations to ascertain solar parallax, and hence distances to the various planetary bodies, and subsequent determination of planetary masses, including that of Earth. Celestial mechanics was in its infancy, but was making such rapid inroads in these determinations to the point where transit observations would no longer be necessary or even desirable, except perhaps as recreational exercises.
One touching story from out of the 1882 event involved David Peck Todd, his wife Mabel, and Austin Dickinson, a town administrator of Amherst, Mass. It concerned an interesting ménage à trois in a so-called May-December romance between Mabel and Austin (the brother of poet Emily Dickinson). David Todd was the first to identify Phobos, the inner satellite of Mars during its opposition in 1877 (Asaph Hall is otherwise credited with the discovery of the outer moon Deimos), but Todd was actually more interested in studying Jupiter. However, he accepted the invitation to use the James Lick Observatory on Mount Hamilton in California when the American doyen Simon Newcomb declined.
This site, just over the line in the transit egress zone, wasn't considered by the Washington, DC, Transit Committee because of generally poor winter weather conditions, besides it wasn't on their list for funding anyway. So, private subsidy, at least in this case, succeeded where public funding came up deficient. The facility had procured the latest photoheliostat, and Todd managed to get 125 photos good enough for micrometric analysis out of 147 plates. They were discovered in Lick's vault just a couple of years ago still "in mint condition," and enough to make a short motion picture of the series accompanied by John Philip Sousa's Transit of Venus March.
Mabel Todd herself single-handedly collected and collated the poems and letters of Emily Dickinson, rescuing them from oblivion, who in turn wrote:
"What are stars but Asterisks.
To point a human life!"
The transits of Venus in 2004 and 2012 will no doubt be irregular but enticing prey for the computer and camera-toting eclipse hunters, who will themselves be joined by tens of millions of onlookers from all walks of life. The problems of parallax won't perturb the scientific community as once they did, and so these events will now bring to a close another interesting chapter in the history of astronomy.
It's been fun.