by Thomas Haeberle on Thursday, May 31, 2012
There is no question about the importance of the transit of Venus. It has inspired early astronomers to accurately measure the solar system by determining the value of the astronomical unit (AU). Measure it, they did, but its accuracy remained questionable for a long time.
The possibility that Venus may on rare occasions pass in front of the sun was first given serious thought by Johannes Kepler years before he died in 1630. His predicted transit of 1631 was not observed due to uncertainty of observation locality. He also failed to predict the 1639 transit.
Fortunately the young English astronomer Jeremiah Horrocks fined-tuned Kepler’s calculations and realized transits occurred in pairs of eight years. Barely months before the event, Horrocks contacted his friend William Crabtree to watch the event on and around December 4, 1639. Both men were successful and they became the first two human beings to see the event.
Keplers third law of planetary motion predicted a planet’s distance from the Sun in relation to its orbital period. But the exact distance in miles were completely unknown. Distances are measured in AU’s which is the Sun-Earth distance being equal to one and an orbital period of one year; all the other planets distances and periods are in relation to the Earth.
Edmond Halley realized that the Venus transits could be the solution to the problem in determining the size and scale of our solar system. In the next transit years of 1761 and 1769, international scientific expeditions were assembled and disbursed to all parts of the world. Observations were also made from Tahiti on the first voyage of Captain Cook. But many of these transit parties would not come home unscathed. Cook had lost prominent astronomer Charles Green to illness in 1769. The French lost members to diseases at the Baja California post.
The first major effort by arch-rivals France and England to observe and record the transit was done during the Seven Years War. The two country expeditions, all of them adventurous and all of them ending somewhere different than originally planned, launched six excursions to various parts of Asia, Africa and the Pacific. There are some mildly amusing stories, particularly of the Frenchman with the very long name: Guillaume Joseph Hyacinthe Jean-Baptiste Le Gentil de la Galaisière or simply Guillaume Le Gentil.
In 1760 Le Gentil would leave France in attempt to observe the Venusian transit in Pondicherry, India the following year. It was decided prior to his arrival not go to port there because the British had occupied Pondicherry when war broke out. So he was sent on his way to Mauritius near Africa only to be lost at sea for several weeks. As June 6th transit day arrived Le Gentil would be stranded on the rolling sea in the Indian Ocean and would not be able to make useful scientific observations and timings of the transit.
In the words of Richard Proctor, one would think that having traversed such a large portion of the globe and to have endured all the weariness and perils of a long sea-voyage, the only thing to do is to give up and go home. But not so Le Gentil. He was determined to make good of a bad situation by staying put in the eastern hemisphere for 8 years!
He crisscrossed the Indian Ocean from Mauritius to Madagascar to Manila exploring the wildlife, plant life and natural history of places he visited. He decided that Manila would be the place to observe the transit. But the French Science Academy had other ideas and instructed Le Gentil to go to Pondicherry now that the war was over.
As luck would have it, as June 3, 1769 transit day arrived Le Gentil would again be frustrated, this time by clouds. The sun would be completely covered at the crucial moment when Venus was leaving the solar disk. To add insult to injury, Le Gentil would learn that Manila had perfectly clear skies.
But his troubles were not over; having stayed a while longer he slowly prepared his journey home. Having no chance to see the next transit 105 years in the future he left for home after 12 years, but only to be shipwrecked twice and barely making it to land in Spain.
Forced to travel on foot over the Pyrenees Mountains, he arrived home in France to find he had been declared dead and his heirs were busy dividing up his estate. He would take legal action to regain his property and later he would marry and spend the next 20 years raising a family and publishing his exploits in the Indian Ocean. He died in 1792 at the age of sixty-seven.
Despite some successes, the accuracy of start (ingress) and exit (egress) times was to be foiled by the “black drop effect,” a dark elongated projection linking Venus with the sky when Venus entered or left the sun’s disk. The results of the 1700’s expeditions yielded an astronomical unit value of 95 million miles. “New” technologies of the 19th century would be employed to better capture and measure ingress and egress times, utilizing telegraph communication between scientists, maritime chronometers to accurately time events, and improvements made with astrophotography to supersede drawings.
The Earth and Venus line up every 584 days; but they must be properly aligned at the node intersections for a transit to take place. This happens rarely every 105 years or 122 years but in pairs separated by eight years. Note also that Venus likes to visit the “seven sisters” in the Pleiades every eight years around April 1st; last time this conjunction occurred was this year and 2004. The next conjunction takes place in 2020.
This eight-year cycle was noted in the The Da Vinci Code; its regular interval occurrence forming a pentagram around the Sun. This progression of Venus was also observed by the Mayans who probably based their calendar on this cycle. It may be no coincidence that the Mayan calendar comes to an end at the final pairing in this transit year.
One hundred and five years after the 1769 transit the world prepared for the 1874 transit. One last note about 1769; the famed Captain James Cook was chosen to sail to Tahiti in the Pacific to record the event. He setup at a post he called Point Venus, which is what it is still called today. Astronomers who were with him were again foiled by the black drop effect which upset the timing of the Venusian contacts on the solar limb.
Cook would go on to his second voyage to survey Australia and his third would be his last. Having explored the Antarctic coast, he went on to discover the Hawaiian Islands. Unfortunately he got in to a dispute with the natives over a stolen boat and was killed and reportedly cannibalized.
A 19th century space race. The world was a much different place in 1874 and the changes were revolutionary, most notably the industrial revolution that brought significant changes to transportation, communication and other technologies of that era. Astronomy, particularly solar, was also evolving. International expeditions proceeded in the following century and attempts were made to photograph the transiting planet.
German born English astronomer William Herschel was the first to tinker with filters to study the Sun. In doing so he concluded that there must be an invisible form of light that we now know as infrared radiation. His son John Hershel would later become interested in geomagnetic disturbances and there connection with the Sun. He would encourage a wealthy businessman; Warren De la Rue to oversee the development of the photoheliograph. The initial design by De la Rue was built under his supervision by Andrew Ross and Company who had in turned brought in German optician John Henry Dallmeyer as science advisor. This first photoheliograph was installed at Kew Observatory, and successfully photographed the sun in March of 1858, but it took 2 years to solve the many operational problems. It would also be used to take the first photographs of the solar eclipse of July 18 1860 in Rivabellosa, Spain. The photoheliograph was taken to English observatories in Cranford, Middlesex; then in 1861 to Kew where De la Rue and Kew director Balfour Stewart photographed the sun every clear day until 1872. A total of 2,778 images were made over a complete 11 year solar cycle. The transit of Venus of December, 1874 motivated a new generation of photoheliographs. John Dallmeyer completed construction in 1874 of about 6 photoheliographs for the transit. In 1884, other Dallmeyer photoheliographs were sent to Mauritius, Dehra Dun India, and the Cape of Good Hope. This device would be used by various astronomers to capture larger images of the sun in 1874 and again in 1882. In the latter year, American astronomer David P. Todd went to the Mt. Hamilton observatory, California, to view the Transit. The photographic plates he took were regarded as one of the best obtained from that era.
The 1882 transit was the first to lie within the reach of the common man. As noted in the NY Times, “This the first time that the unlearned common people have been permitted to view a transit … the first to be seen through smoked glass.” Observers in the eastern United States had the advantage of watching the transit in its entirety in 1882, encompassing a large population. Amateur astronomer Lewis M. Rutherfurd is said to have observed the transit from Second Avenue in New York City.
U.S. Naval observatory astronomer Simon Newcomb led the Transit of Venus commission for both transits of 1874 and 1882. For the latter year, he chose New York born astronomer David P. Todd to go to the Mt. Hamilton observatory in California. Using what was called a photoheliostat he was able to obtain pictures of the event using the observatory’s twelve-inch refractor.
Probably regarded as the best photographs seen to date; they were the sharpest and clearest of any taken for both 1874 and 1882. Yet when meticulous measurements were made the results were less than perfect. Venus when magnified, instead of being crisp, was instead fuzzy around the edges.
To Newcomb’s disappointment, Todd would never complete his measurements for the solar parallax for the 1882 observations as he done for the 1874 transit. Few parallax results were published at all for 1882 observations. The two men, who were once close, grew further apart.
Todd ‘s wife, Mabel Loomis was an author in her own right. She had had an affair with William Austin Dickinson, the (married) brother of famed poet Emily Dickinson. Todd seems to have accepted the affair willingly and may have partaken in a ménage à trois (who says astronomers are all work and no play).
Todd went on thirteen eclipse expeditions making him one of the most prolific eclipse chaser of his time. During one such expedition to Algeria in 1900, he met Percival Lowell who had become celebrated for his observations of the Martian canals.
In 1924 Todd fronted a radio experiment by the U.S. navy to ease drop on the planet Mars. Perhaps not wanting to look foolish, the old army game of heads we win, tails you lose became the navy’s game in this instance. One night in August of that year, radio stations around the world were urged to cease transmissions. Todd was older then and may have begun to reach senility. He alluded to claims of possible contact with Mars. He died in 1939 after spending many years in hospitals and nursing homes.
The nineteenth century expeditions and their results proved unfortunately more disappointing than the previous century’s AU values. Astronomers were becoming fast aware that there were better methods of determining the AU distance, which will be discussed in the last segment of this series.
Several other methods were tried to determine the AU distance. The planet Mars was used during a close opposition in 1877. Then there was a close approach by the asteroid Eros at the turn of the 19th century were a solar parallax was measured. The latter gave a more satisfactory result for the AU value.
In the end Venus would provide a meaningful measurement of the AU, but not by the transit method. In March, 1961, radio astronomers bounced signals off the planet and received its echo in 6.5 minutes. Even this wasn’t considered accurate. Over time, the AU has become a “derived constant” with its value being equal to an ideal body and not the actual planet Earth. Also, measurements made of positions of space probes provided a more absolute value. So the mean distance between the Earth and the Sun has yielded a modern value of 92,955,807 miles.
A few months after the 2004 transit, Dr. Jay Pasachoff spoke at the American Museum of Natural History regarding his research in to the black drop effect. His concluding remarks where that the “effect” came from both the blurring by “older” terrestrial telescopes and solar limb darkening as Venus entered the disk. It is Earth’s atmosphere that pronounces this effect from the ground and that the refraction through Venus’s atmosphere wasn’t a factor in causing the effect.
Days of Future Transit Pass: After 2012, the next transits of Venus will be in December, 2117 and December, 2125. So we’ll have to wait at least 105 years for the next one unless we can find a way to get to Mars sooner. Venus transits happen more frequently there, roughly at 32 year intervals. But a transit of Venus from Mars is like seeing a transit of Mercury from Earth; being the same apparent size from their respective planets.
After 2012, one can only ponder worlds we will never know and events unseen. But mark your calendar; there will be a transit of Mercury on May 16, 2016. Then there is the greatest transit of all coming our way on August 21, 2017, a Total Eclipse of the Sun is to cross the contiguous United States for the first time in 38 years.
I prepare my telescopes and equipment for TOV day and I will be along the Hudson River in New York City to view the event. As I listen to the weather reports I wonder if I will suffer the same fate as Le Gentil, not that I will be declared dead when I return home, but that I will be clouded out. I also wonder what stories you will tell about the June 5th Venusian transit of 2012.
History of the Transit of Venus – post observations
The transit of Venus came and went and now it is lost to the ages. Time was spent preparing and planning; calendars were marked for event day and suddenly it came and went. The fruits of your labor either bore results or your plans are a wash by uncontrollable circumstances such as the weather.
Tuesday, June 05, 2012
It was great — we all had a great time. The High Line, a park on an old elevated train line in New York City, had a big crowd, it was huge but wonderful. I was there as part of an effort by the Amateur Astronomers Association of New York to show the public the Transit.
We got just over a half hour of viewing out of a 2 hour east coast transit but it was worth it. You might call it the second miracle on the Hudson (in reference to Captain Sulley’s US Airways Flight 1549 emergency landing on the river in 2009). We had a window from about 6:15 to 6:45 and I managed to show over 30 people and children, Venus crossing the sun. Unfortunately it remained cloudy toward the end yet at my scope I had a line of 50 people waiting for over an hour to view the transit which never again materialized. I estimated there was close to a thousand people that evening to see the event. We had a second viewing at the 70th street pier on the Hudson River. Estimates there put the crowd at fifteen hundred people.
“With transits of Venus occurring in pairs separated by over 100 years, it is interesting to reflect upon the changes in society at each pair, and to imagine what society might be like at the next pair in 2117/2125.” – Dave Herald, (SEML) from Murrumbateman, Australia.
1639 observation by Horrocks and Crabtree: Occurred before the industrial revolution and before the start of the Thirty Years War. Kepler discovered planetary laws of motion. Telescope introduced to astronomy in 1609 by Galileo and discovered sunspots. He was tried by the inquisition in 1614. (He died in 1642 under house arrest. I wonder what his thoughts were on the transit?)
Industrial revolution is starting. Harrison’s chronometer just developed. Australia is discovered by Europeans (James Cook, following the 1769 transit of Venus); first efficient steam engine (Watts 1775) produced.
Telephone just invented. No radio invented; just before the invention of motor cars and airplanes. Scientist had utilized telegraph communication during the transit events along with the improved maritime chronometers to accurately time events, and improvements were made with astrophotography to supersede drawings.
Society as we know it today… Social media and internet communication utilized through computers, smart phones, iPhones and iPads. Superior photography employed. Mystery of the Black Drop effect resolved.
2084 – November 10 – Solar Transit of Earth as seen from Mars. (Arthur C. Clark wrote an interesting short story called “Transit of Earth” which takes place in 1984.)
“Given the huge changes that have occurred between transit pairs the only thing I think we can say about society at the next transit pair it will be hugely different from what we know today, in ways we cannot even imagine.” – Dave Herald. But here are some guesses:
The world population should be between 10 to 11 billion by 2100. Water level expected to be 4 decimeters higher than it was in 2000.
The Mary Shelley novel The Last Man portrays the year 2100 as the extinction of humanity following a deadly pandemic.
In the Star Trek universe, Mars is colonized in the year 2103.