kw: book reviews, nonfiction, science, astronomy, planets
During the early decades of the Eighteenth Century, several factors came together. The philosophical and scientific ideals of the European Enlightenment were loosening the nationalistic and political ties that "natural philosophers" felt toward their countries, even as political tensions mounted; the great empires of England, France, and Russia were settling down (though squabbling) and global transportation was now possible; advances in astronomical calculations had led to a great ability (with much laborious paperwork) to predict the positions of the planets and moons of the solar system; the achromatic telescope objective, invented in 1730, became affordable in useful sizes (up to about 50 cm diameter); and a hortatory essay by Edmond Halley, published in 1716, was being circulated.
That essay predicted the first of a pair of transits of Venus across the Sun on June 6, 1761, and called upon astronomers everywhere to observe it from as many places as possible around the globe, so as to obtain measurements that would permit more accurate determination of the scale of the solar system. Thus, the few years prior to 1761 saw a series of discussions, proposals, and requests by scientists for funding (from their various governments and other patrons), that led to astronomical expeditions from South Africa to Newfoundland, and from Siberia to India.
The events that led up to the heroic expeditions in 1761 and 1769 (for the second transit of the pair), the observations themselves, and the aftermath, are ably chronicled for us in Chasing Venus: The Race to Measure the Heavens, by Andrea Wulf. The men who "chased Venus" to all corners of the Earth were up against incredible odds. Not all succeeded. Some died. Some managed to transport half a ton or a ton of instruments to their remote targets, only to be clouded in and miss the entire transit.
In the First World, we think nothing of hopping in the car and driving several miles in a few minutes to buy groceries, or perhaps dozens of miles in an hour or less to try out a new restaurant. With a little more preparation, we travel by jet to "civilized" locales on six continents (getting to Antarctica is still a bit of an ordeal). Some years ago, I found that returning from my in-laws' home in Japan, to our own home in Oklahoma, took just under 24 hours, and we thought of that as a difficult trip.
During the heyday of the British Empire, a fast sailing ship could travel between 40 and 60 miles per day. If you were lucky enough to sail straight through without encountering any storms, you could cross the Atlantic in about two months. Land travel in areas without good roads (nearly anywhere outside a major city) was slower than that; 5-10 miles daily was typical. Getting from France to Tobolsk, in Siberia, took an astronomer named Chappe about a year in 1761; getting to California in 1769 took even longer, and he died there after making his observations.
I was quite interested to find that one pair of observers, who observed the 1761 transit from the Cape of Good Hope at the tip of Africa, was Charles Mason and Jeremiah Dixon. They observed the 1769 transit also, but from different locations. In between, they spent a few years in America surveying the boundary between Pennsylvania and Maryland, now called the Mason-Dixon line.
What makes a transit of Venus so special? It is one of the rarest of predictable phenomena. A pair of transits will occur eight years apart, minus about 3 days, but the next pair will occur more than a century later. The period from pair to pair alternates between 113.5 years and 129.5 years, for a total cycle of 243 years. A transit is also an excellent opportunity to take measurements to establish the solar parallax. This parameter is the difference in angle (properly, the half angle) between observations of the Sun from exactly opposite edges of the Earth; knowing the size of the Earth, one can then compute the Sun's distance.
Why aren't the transits more frequent? The orbit of Venus is at an angle of about 3.4° to that of Earth. Eight Earth years are just slightly longer than 13 Venus years. Thus, when the Earth crosses the plane of Venus's orbit, Venus usually isn't anywhere nearby. These crossings aren't exactly opposite one another because Earth's orbit is a little bit elliptical (The orbit of Venus is the most nearly circular of all the planets). So while conjunctions occur a couple times yearly, they are usually not transits.
In 1761 Europe was in the midst of the Seven Years' War. The astronomers sailed across the earth at the risk of their lives. Amazingly, none were lost to hostilities. In 1769, that war was over, but tensions remained. Getting Chappe to Baja California was a political nightmare for the French Academy…and for him. He had to travel under guard on a Spanish ship. Those who traveled to the frozen North, whether in Siberia or northern Scandinavia, had to go when travel was physically possible, in winter when the rivers were iced over. It is amazing that plump, well-fed astronomers did so, and generally prevailed. In the south, in 1769, Tahiti was the target of an expedition led by James Cook, who became the first European to visit Hawaii in 1778. The entire trip took three years. Southern observations in general were hampered by the lack of land in the southern Pacific Ocean.
It was necessary to observe the transit, and time it accurately, from locations as far-flung as possible, so that many partial parallaxes could be combined into a composite, theoretical equatorial parallax. This required measuring the longitude, and the exact moment of local (not statutory) noon, using tall case clocks and observations of the position of the Moon against reference stars, and doing hours and hours of calculations.
From this amazing body of work, the results in 1761 were a little disappointing. One of the biggest problems was that Venus has an atmosphere, which refracted sunlight, making it very tricky to determine exactly when a transit began or ended. One had to time the first moment of ingress, then the first moment light could be seen between the rear edge of Venus and the outer edge of the Sun a few minutes later; then the reverse sequence as the planet approached the other edge of the Sun and slid off the disk six hours later. Venus's atmosphere led to confusing phenomena such as the black drop effect, and to variations in the timing by ten seconds or more, sometimes much more, from observer to observer. As a result, the scatter of calculated values for the parallax ranged from 8.28 to 10.6 arc seconds. That led to a distance range of 77 to 99 million miles. Not very accurate.
1761 had been a practice run, many decided. They would do better in 1769. They did, with three times as many observers in about twice as many places. Although the weather was cloudier than it had been eight years before, larger numbers of better observations were performed. The "best value" as judged by the most competent astronomers was 8.78 arc seconds, which compares well with the value we accept today of 8.794. That, and a less accurate value for Earth's size, led to a distance about 1% too long, a tremendous achievement for the time.
The cosecant of 8.78 arc seconds is 23492.6; multiplying by the radius of Earth (3,960 miles) we get just over 93 million; the average distance to the Sun that we accept these days is 92.96 million miles. The distance calculated in 1769 was actually 93.73 million, based on a slightly larger value for the Earth's radius. So the angular measurement of the Solar parallax was actually in error by only 0.15%.
By 1772, most of the scientists had returned home, the results had been published, and life was settling back to normal. But a seed had been planted. Ms Wulf traces modern "big science" such as the Large Hadron Collider and the great observatories, both on Earth and in orbit, to this first global collaboration between scientists from a number of countries. One could say, though nations may war, among scientists peace prevails, as it must for civilization to continue.