Out of Diaries: 6 May 1761

Posted on May 6, 2012 by Andrea Wulf

On 6 May 1761, Charles Mason finally sat down and wrote a letter to the Royal Society – we can only imagine how worried he and his partner Jeremiah Dixon must have been because they had ignored their orders and decided to observe the transit from the Cape of Good Hope instead of in Sumatra. There were several reasons: they were much delayed – first by contrary winds that had left them stranded in Portsmouth for several weeks, and then by the disastrous battle with the French frigate only four days into their voyage – they were already late when they made a stopover at the Cape on 27 April 1761. Not only that, they had also received news that the French had taken the British trading port of Bencoolen (today’s Bengkulu) in the ongoing battles for colonial possessions in the East Indies.

Mason’s letter to the Royal Society explained their reasoning and emphasised that it had been the ship’s captain who had made the final decision, smoothly diverting any subsequent blame from themselves.

They had hired a carriage to transport their instruments and the Dutch governor did all he could to assist the scientific endeavour by providing materials and workmen to help construct an observatory but, so Mason complained, ‘the Dutch are so slow’ and did not understand a word of English. As the astronomers tried to explain how to build the observatory by pointing and sketching, they began to despair ‘of getting it completed in time’.

But, so they told the Royal Society, they had convinced their ship’s captain to send his carpenters for a few days. Without their help, Mason wrote, they would never have completed the observatory in time. When the carpenters were finished, they had a small tent-like structure – circular with a diameter of six-and-a-half feet, covered with canvas and a conical roof that could be opened ‘to any part of the heavens’. They fixed their clock against two pieces of timber which were buried four feet in the ground. They were ready – but the weather was very unpromising because it was cloudy ‘near all the time’. Unlike other observers however, Mason and Dixon were doubly nervous because only a successful observation would justify their clear breach of instructions and decision to remain at the Cape of Good Hope.

 

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June issues of Astronomy and S&T

Posted on May 5, 2012 by Steven van Roode

Both Astronomy and Sky & Telescope feature the transit of Venus in their June issues. In Astronomy (pp. 50-52), Richard Talcott delves into viewing the Venus transit. Planning your observation of the transit involves a couple of preparations, which shouldn’t be left to the last minute. First, you need to know where to observe. Our transit calculator can be of help, but you should also check the site for obstacles blocking your view of the Sun. Then, it is important to know how to watch the transit. There are several way to safely view the transit, and Richard explores three basic methods. Finally, you should know when to watch. Special phenomena like the black drop effect and the aureole occur during ingress and egress, so make sure that your equipment has been set up on time.
Richard also explains how the Hubble Space Telescope will observe the transit of Venus indirectly by turning to the lunar crater Hipparchus, and how the transit of Venus can help with detecting exoplanets.

Sky & Telescope has three articles about the transit. The first (pp. 28-33) is written by Eli Maor, who is the author of Venus in Transit. Eli tells the fascinating stories of the 1761 journeys of astronomers interrupted by war, their trials and tribulations and their bad luck with the weather gods. Despite these drawbacks, new discoveries were made, most importantly the atmosphere of Venus. Yet the main goal of the expeditions – finding the distance to the Sun – was hindered by another unexpected phenomenon: the black drop.
On page 50-51 Alan MacRobert explains the sequence of events during ingress and egress, with a striking 1874 illustration by Henry Chamberlain Russell showing both the aureole and black drop effect. Alan also gives some good tips about June’s weather forecast.
On page 60 Gary Seronik features the Sun Funnel, the nifty closed-loop device that projects the sun’s image without the risk of having one of the spectators looking into the light beam leaving the telescope.

Both magazines stress that you don’t want to miss your last chance to see the transit of Venus. Next time Venus will cross the Sun’s face will be in 2117, and unless you live another 105 years, you won’t have another opportunity to see Venus silhouetted against the brilliant solar disk. Be prepared – run to your local newsstand and get yourself these two issues!

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Presenting the transit of Venus

Posted on May 4, 2012 by Tim Cole
Tim wearing a Transit of Venus T-shirt

Tim visiting a high school to talk about Transits of Venus

I recently visited a local high school to give a presentation on Transits of Venus. High school audiences can be difficult – there’s often a lot of attitude to deal with. In this case, the venue was good. The teacher had arranged this for her enriched class, and more students in the school had asked to come to spend their lunch hour. All very good signs!

A standard problem with giving presentations on astronomical subjects is that they tend to be somewhat abstract. Focusing on the pure science of any technical subject is almost guaranteed to bore a general audience within minutes. Your first task is to find some attention-inspiring hook. The rareness of a Venus transit is a good place to start, but you need something more to hold an audience’s attention.

Fortunately, the history of Venus transits is jammed full of incredible stories of adventure, intrigue, romance, disaster, and triumph. You’d have a hard time finding any other astronomical subject salted with so many human interest angles. With the 2012 transit of Venus, we can take advantage of an immense number of photographs, videos, and stories from the 2004 transit.

If you can find a personal angle to your talk, work with that. Since I’m planning a trip to Hawaii to observe this transit, I added a lot of material on Hawaii’s natural history and astronomical connections. Tying this to the historical transit expeditions turns an astronomical curiosity into a story. People have always enjoyed stories, so tell a story when you can.

Remember that your audience has chosen to spend time with you. Make sure they feel that time was well spent. The closer you can come to giving an impression of having a chat with room full of people, the more successful you’ll be with your presentation. The point of any presentation is to get people interested in your topic, so let your enthusiasm show.

It’s important to strike a balance between seeming knowledgeable and seeming pedantic. You have to be fairly well versed on your topic, but nobody can reasonably expect you to know everything about it. Don’t be afraid to admit you don’t know the answer to a question. I usually try to suggest ways we could find an answer. In any case, don’t make a big deal about it. Just move on.

Sometimes, I’ll get a question I’ve never considered. Better yet, I might get a question for which nobody knows the answer. This sort of thing shouldn’t be an embarrassment. Instead, it can be a great opportunity to let your audience see that you’re always looking for opportunities to keep learning.

Reading your audience can be tricky, but it helps you fine-tune your presentation. There will always be a few bored people. With high school groups, there are always kids who are just too cool to look interested. Don’t let that get to you. Focus on the people who are interested. Be prepared to change your presentation to meet the demands of your audience.

As an example of changing a presentation on the fly, I discovered the school was considering setting up a class trip to Hawaii for their Earth Sciences program. With that in mind, I spent extra time on the geology of the Hawaiian Islands and, in particular, the Big Island and Mauna Kea. That let me work with their natural enthusiasm for a cool class trip and gently steer the presentation back to transits of Venus.

There are no set rules for giving good presentations. Over many years as a museum educator, I’ve found that my best tools are enthusiasm for my topic, a willingness to be seen making mistakes, and a sense that the presentation belongs to my audience. Perhaps that will work for you too.

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Fred Watson on the transit

Posted on May 3, 2012 by Steven van Roode

In this episode of Self Improvement Wednesday, Dr Fred Watson, Astronomer-in-Charge of the Australian Astronomical Observatory at Coonabarabran tells you about the transit of Venus. Venus is a dim world of intense heat and volcanic activity. A transit of a planet occurs when the planet passes directly between the Earth and the Sun so that as seen from the Earth, the planet appears to pass across the face of the Sun. Hear more about the historical significance of how and when this occurs in this week’s lesson.

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Out of Diaries: 2 May 1769

Posted on May 2, 2012 by Andrea Wulf

In Tahiti in the morning of 2 May 1769 Captain James Cook and astronomer Charles Green discovered that the large quadrant that they had brought from the Endeavour to Fort Venus was missing. They peered into every corner of the fort and on board the Endeavour. Without the quadrant, the expedition would be worthless. Green would not be able to measure the altitude of the sun, which was needed to set the clock (and to calculate the longitude). He would also not be able to take the altitude of planets, moons and satellites necessary to determine their latitude. The observation of the transit of Venus was only useful in combination with the exact geographical position of the observer – the quadrant was essential to the success of the voyage.

The twelve-inch portable astronomical quadrant (made by London instrument-maker John Bird) that was taken by Cook to Tahiti

One of the Tahitians must have taken it. It was not the first time that they had stolen. They took ‘every thing that was loose about the ship’, from knives, snuff boxes to even the glass panes from the portholes. Cook had not sailed across the globe through storms and dangerous seas to fail now. He ordered that some of the ‘Principle people’ be locked up until the precious instrument was found. As Cook began to question his prisoners, Green together with botanist Joseph Banks and the midshipman scoured the island. They rushed from one hut to another, questioning and cajoling. By late afternoon and seven miles from the fort, they finally discovered the person who had taken their quadrant.

After some discussion (and a display of the pistols that they had brought), Banks and Green persuaded the Tahitians to return the stolen goods. Green watched in horror as they brought out one part after another: the islanders had dismantled the delicate instrument to divide it between them. But after examining the pieces, Green was relieved to see that there seemed to be only minor damage. They packed the quadrant in a box and padded it with grass before beginning their long walk home. By the time they returned to Fort Venus it was dark. It had been a long, hot and tiring day – they were exhausted but triumphant. Everything they needed for the transit observations was in place. They had only to guard their instruments carefully for another month.

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From a lunar perspective

Posted on April 30, 2012 by Steven van Roode

When you look at the visibility map of the 2012 transit of Venus, you see that there are some large populated areas where the transit of Venus won’t be visible: especially South America and the western African continent are deprived of this priveliged view. But that doesn’t mean they can’t see the transit at all.

Over at the Solar Eclipse Mailing List (SEML), an interesting technique is being discussed by which the transit of Venus is detected by the diminishing of the lunar brightness. The idea is very simple: during the transit of Venus, the moon is nearly full – only two days earlier there was a lunar eclipse. So, if the sun is below the horizon at the time of the transit, the moon will be visible. As the sunlight reflected by the lunar surface will slightly decrease when Venus moves across the face of the sun, the moon’s brightness will be reduced accordingly.

Though the idea is simple, the practical execution is quite complicated. The drop of intensity is very small – no more than 0.1% – and hardly detectable. Additionally, the lunar brightness varies because of other factors too: the change of phase and height above the horizon are to be taken into account as well. The best way to detect any decrease of brightness is by measuring the amount of light received from the moon during ingress (or the increase of brightness during egress). Though the technique has many hurdles and pitfalls, it’s interesting to see if specialised amateurs in South America and Africa are able to catch a hint of the transit of Venus in this way.

For sure, the Hubble Space Telescope will also attempt to measure the variation of the moon’s brightness caused by the transit of Venus.

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Out of Diaries: 28 April 1769

Posted on April 28, 2012 by Andrea Wulf

On 28 April 1769, Maximilian Hell’s expedition in Vardø almost came to a premature end. Since October 1768 Hell and his assistant had waited on the remote island in the Arctic Circle for the day of the transit. They had survived their dangerous journey across Norway’s empty landscape and the wild sea along the coast. For months they had been enveloped by continuous darkness but now they were getting ready for Venus. Then, on 28 April (which was also the one–year–anniversary of their departure from Vienna) some stray pellets went through one of the observatory windows – a few young locals were shooting birds nearby. The pellets only missed Hell narrowly.

On the same day, William Bayley (Nevil Maskelyne’s assistant at the Royal Observatory in Greenwich) arrived at the North Cape – a barren rock at the northern tip of Norway. Following Maskelyne’s instructions, he immediately began to set up his portable observatory. Coincidentally but with perfect symmetry on the other side of the globe on the very same day, in Tahiti, Captain Cook ordered their prefabricated observatory to be carried from the Endeavour on shore.

Cook had arrived on 13 April 1769 and had ordered fifty of his men to dig trenches and ramparts to built a fort ‘for the Defence of the Observatory’ at the northern end of Matavai Bay. One side of the fort was bordered by the river while the other sides were protected by a high fence. Inside the compound were several tents: one that functioned as the observatory (topped with the British flag), one for the kitchen and another for the guards, amongst several others. Cook also installed some swivel guns and cannons. They called it ‘Fort Venus’ in honour of their mission. On the palm-fringed island dotted with groves of trees and small open huts, the European fortifications that enclosed the fort must have looked completely out of place.

Fort Venus. The observatory tent is the round structure in the middle with the flag

 

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Trade routes to the transit

Posted on April 26, 2012 by Steven van Roode

The transit of Venus is one of the rarest predictable astronomical events: every 120 years you have only two chances eight years apart to see Venus move slowly across the sun’s face. Especially if observing the transit can solve astronomy’s greatest problem, you don’t want to miss a single opportunity. When the transit of Venus is viewed from different locations on earth, the duration of the entire event will vary between these places because of parallax. Combining the timings of the start and end of the transit could then yield the distance to the sun – a holy grail to eighteenth century astronomy.

When in the eighteenth century astronomers were planning for the 1761 and 1769 transits of Venus, they had to pick with care the locations from where the transit would be observed. Two main factors were at play. First, the global visibility of the transit. Joseph-Nicolas Delisle in 1761 and Jérôme Lalande in 1769 prepared and distributed a map showing the areas where the transit was visible. Secondly, the means of travel. The challenge of the eighteenth century observations was to position astronomers at far flung places across the globe within the area of visibility without too much trouble.

Reading Andrea Wulf’s book Chasing Venus, it suddenly struck me that the choice of locations was often limited by the trade routes of commercial ships. For scientific institutions like the Royal Society or the Académie des Sciences it was much more convenient to board the astronomers and their equipment on a commercial ship than to fit out ships especially for each expedition.

As the following visualisation shows, trade routes where quite constant over time. The animation shows the ship paths in the seas, as recorded in hundreds of ship’s log books, by hand, one or several times a day. It compresses a century of ocean shipping onto a single span of January-December, to reveal seasonal patterns. It loops through the same data a couple of times.

The video reveals the stop-overs and end points of the trading routes, and at these places many astronomers disembarked (or were supposed to disembark) to set up their temporary observatories: Cape Town, Mauritius, Jakarta, Pondicherry and Hudson Bay. But the animation also shows the seasonal patterns, which also had to be taken into account when planning an expedition. Ships to Hudson Bay, for example, only sailed in the summer. Because the transits occurred in June, the British dispatched astronomer William Wales already at the end of May 1768, to be in Hudson Bay on time for the transit a year later. As a result, Wales had to spend a long winter in Hudson Bay. From the animation it is also clear that the winds in the Indian Sea shift directions to north-east in November, only to become south-west again in April. Guillaume Le Gentil, on his way from Mauritius to Pondicherry in India had to cope with this transition. Andrea Wulf writes in her new book (p. 29):

On 11 March 1761, Le Gentil left Mauritius and, after a short stopover at the nearby island of Réunion (then called Île Bourbon), they set course for Pondicherry. At first, an exuberant Le Gentil noted, everything seemed to go smoothly. Each day the ship covered between thirty and forty-five miles, proceeding swiftly until they hit the north-east monsoon, north of Madagascar. Instead of being able to sail in a diagonal line across the Indian Sea towards India, the winds pushed them towards Africa. They made little headway. Because April and May mark the transitional period of the monsoon, when the winds begin to turn and become the south-west monsoon during the summer months, Le Gentil woke each day, praying that their direction had changed. But his prayers were in vain. Instead of encountering strong westerly winds that would have carried them to Pondicherry, the vessel was enveloped in a lull, as if someone had suddenly pulled the brakes.

Planning an expedition and getting the astronomers on their destinations on time was a real puzzle at the time! What other stories of the eighteenth century transit of Venus expeditions can you link with the patterns revealed by the animation?

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An introduction to the transit

Posted on April 25, 2012 by Steven van Roode

On June 5, 2012, for the last time in 105 years, until December of 2117, the planet Venus will pass directly between Earth and the Sun, in an event called a transit. Dr. Kevin McGouldrick details the history of transits and observations of the planet Venus in this LASP Public Lecture from April 4, 2012 at the University of Colorado Boulder. He spotlights the pivotal role that observations of the planet Venus — with special emphasis on those of previous transits — have played in the development of our current scientific world view.

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Out of Diaries: 23 April 1769

Posted on April 23, 2012 by Andrea Wulf

Georg Moritz Lowitz from Göttingen was struggling. He was travelling on behalf of Catherine the Great to Guryev on the Caspian Sea – but having left St Petersburg (after an audience with the empress) with seven carriages and sledges which were pulled by eighteen horses, he was not making much progress. By the time he arrived in Moscow his vehicles were so damaged that they could only be burned – leaving him to purchase new ones. On his long journey he had to cross several frozen rivers and with spring arriving, it became a race against the thaw. It can’t have helped that the widowed astronomer was also travelling with his eleven–year–old son!

By early April, the ice was breaking everywhere and the heavily loaded sledges were in constant danger of crashing into the rivers. On the 23 April 1769, the expedition almost came to an end as they had to cross a river on precariously thin and floating ice sheets. Lowitz just made it but the problems continued and a month later he decided that his entourage and instruments slowed him down too much. He sped ahead to build the observatory while his men followed at a slower pace. The team and the instruments arrived only a week before the transit – but Lowitz had already built an observatory.

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