June 2023

May 30, 2023 Posted by: Richard Fefferman


The Lewis & Clark Expedition is one of the most amazing stories that Gateway Arch National Park commemorates.   Without modern technology but with the aid of outstanding leadership, teamwork, courage, skill, and good luck, the Corps of Discovery managed to achieve nearly all the objectives that President Thomas Jefferson had set out for it.   Although they found no all water route to the Pacific Ocean, Lewis & Clark contributed greatly to the store of knowledge regarding the geography, flora and fauna of the west, while doing their best to establish relations with the American Indians.   A number of explorers, most famously Zebulon Pike, followed in the footsteps of the Corps of Discovery, working to fill in the map of the west.   Not all of these adventures were completely successful, and this was also true in the world of astronomy.

Somewhat like Thomas Jefferson, the famous astronomer Edmond Halley (1656-1742) also inspired a flood of expeditions traveling the world.   Halley is most famous for realizing that past observations of comets about 76 years apart actually represented multiple returns of the same object.   Although in the end Halley lived to the the ripe old age of 85, he knew that he would not live to see the next return of the comet, which he predicted would take place in 1758.   He was proved correct, and as everyone knows, the comet named after him is the most famous of these objects.  Although the 1986 return was a poor one, I was still lucky enough to view Halley’s Comet many times and show it to many people.  Since I was 24 then, my chances of doing so again in 2061 are not good. 

Halley realized that another rare event, the transit of Venus, would give astronomers a chance to determine the distance from the Earth to the Sun.   About every 19 months, Venus, Earth, and Sun all line up.   Although usually Venus passes above or below the line, it actually can appear as a small black spot on the Sun, as shown in the picture below.   Transits of Venus occur in pairs eight years apart, with more than a century in between.   I saw both the 2004 and 2012 events, but hardly anybody alive today will see the next transit, since it won’t take place until December 11, 2117.  

In another “beyond the grave” suggestion, Halley proposed that expeditions from multiple countries travel to view the June 6, 1761 and June 4, 1769 Venus transits from various locations within the path.  His plan was that these groups arrive well in advance and accurately determine their latitude and longitude.  On transit day, they would then accurately time the moments when Venus appeared to arrive on to and then leave the Sun’s disk.   The difference in times of these widely separated observation points could be used to determine the parallax of the Sun.   The best way to understand parallax is to consider that one object might appear in front of another as seen from one spot, but not when seen from another location.   In the case of the Sun, it was realized that the distance was so great that this parallax would be tiny, but the Venus transit would provide a good opportunity to calculate it.   Once the parallax was determined, it would be relatively simple trigonometry to then calculate the distance to the Sun.

Orange ball with tiny black dot inside, surrounded by black space.
The transit of Venus, June 5, 2012.  NPS Photo/RF

The individual Venus transit expeditions experienced varying results.  The Frenchmen Jean-Baptiste Chappe d’Auteroche led journeys to Siberia in 1761 and then Baja California in 1769, and successfully observed both events. However, the group was then beset by an outbreak of yellow fever, with d’Auteroche dying.  Just one person survived, returning to Paris to transmit the 1769 observations.

The next time that you are having a really bad time, consider the journey of Guillaume Le Gentil.  Subject of both a play and an opera, his saga is incredible only for its almost unbelievably bad luck compounded by reasonable decisions that ended up proving utterly wrong.  An experienced astronomer who discovered several clusters of stars and nebulae (gas clouds) in the sky, Le Gentil led a voyage to Pondicherry, India to view the 1761 transit.   Upon approach, he learned that the city had been captured by the British during the course of the Seven Year’s War between France and Britain, and he was not allowed to land.   Le Gentil attempted to return to the island of Mauritius in the Indian Ocean, but was unable to make it in time and was thus still at sea on the date of the transit.   Although he and his party could see the disk of Venus superimposed on the Sun, with the rolling ship, they were unable to make the accurate observations necessary.   While waiting eight years for the 1769 transit, Le Gentil successfully mapped the coast of Madagascar and then headed to the Philippines to view the event from Manila.   Upon arrival, he was threatened with arrest by the Spanish governor, possibly as a spy, and decided to return to Pondicherry, which by now had been restored to French control.   As it happened the skies in Manila were clear on June 4, 1769, but those in Pondicherry completely cloudy, so Le Gentil saw nothing! 

Even this is not the end of Le Gentil’s travails.   Delayed by dysentery and storms, he finally returned to Paris in 1771, having been gone for eleven years.   On arrival, he found that letters he had sent to the Royal Academy of Sciences and to his family about his progress (or the lack thereof!) had never reached their destination.   Le Gentil had been found to be legally dead, with his relatives squabbling over his possessions and his wife having remarried.   Yikes!  To the good side, other expeditions to the Venus transits were more successful, and the distance to the Sun was calculated to within 2% of the correct value.

Although there will be no more transits of Venus within almost any of our lifetimes, much of the United States will witness an annular eclipse of the Sun on October 14, 2023, followed by a total eclipse of the Sun on April 8, 2024.   Future editions of this blog will certainly discuss both of these events, but we will have a free in person preview at the Gateway Arch on June 23 at 7:00 pm.   Missouri Eclipse Task Force member Don Ficken will discuss both events in our Education Classroom, followed (weather permitting) by night sky view of the Moon and other objects.


Stargazing, SkyRanger

Last updated: May 30, 2023

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