Mercury Transit, November 9th 2006, Dargaville High School.
On Thursday 9 November, from 8:10 am to 1:10 pm Mercury will transit across the Sun. A transit is the phenomenon of a planet appearing to cross the disc of the Sun, from our point of view on earth. Transits are fantastic events for the whole family to watch and you needn’t be an experienced observer because the Sun and the event takes all morning so it is easy to appreciate. However, it isn’t safe to look at the Sun except through the correct filters, so if you want to see this event, check the website of the Royal Astronomical Society of New Zealand to get details of the Astronomy Club nearest to you and the observatories that will be open that day for viewing.
We are lucky to have a resident astronomer, Deborah Hambly, founder of Astronomy Adventures. She will be bringing telescopes from the Skydome observatory, Baylys Beach and setting up for safe solar viewing at Dargaville High School on the day. One of the telescopes boasts an extremely rare solar filter, of which there are only a handful in New Zealand, none of which at the time of writing are being used for public viewings. Through this filter it is possible to see prominences “ejections of flames” around the edge of the Sun, as well as detail on the Sun’s surface. The start and end of the transit will be open to the public, by prior reservation, in conjunction with a fully catered breakfast and lunch put on by the DHS hospitality students. Junior classes are queuing for observing sessions during the transit.
Did you know that it was because of a transit that James Cook and the Endeavour were sent to the Southern Hemisphere? Transits are exciting to watch because they have a huge historical and scientific value, not to mention the experience of watching an event with the tens of thousands of people. Transits of Venus in particular are incredibly rare; only six have ever been seen! The next Transit of Venus takes place in 2012 but New Zealand will be the best country in the world for viewing the transit, as it is the only country where the whole transit will be visible – from start to finish, and astronomers and tourists will travel here from around the world to catch this very special event. It will not be visible again until the 22nd century! Therefore very few, if any of the children who will be old enough to appreciate the event (age three) will alive to see the next one! However, before we get carried away with the future, let’s take a step back four centuries in time to appreciate the relevance and importance of this event and to learn how an observation of a transit of Mercury first started it all off.
17th Century
An Introduction
A Scottish mathematician James Gregory first theorised that transits could be used to investigate the distance to the Sun and the size of the solar system. At the time of Kepler, a famous physicist whose tables enabled the prediction of the transits, the distance from the Earth to the Sun was thought to be about 22 million km, about 6 times smaller than it actually is (150 million km)! Finding the distance from the Earth to the Sun, would help astronomers to calculate the distance to every other planet and determine the real size of the solar system.
First observed Transits
The first ever observed transit was of Mercury on 7 November 1631. It was made by four observers spread around Europe and Asia. The first observed transit of Venus was made in 1639 by a curate in rural England in between his church service duties and passing clouds! His best friend, observing from another village was so entranced by the event that he forgot to take any notes!
Halley’s journey to the Southern Hemisphere
In the 1670’s Halley (who predicted the return of the comet that is now named in is honour) travelled to the Southern Hemisphere and catalogued 300 stars. More importantly for us, on 7 November of 1677, nearly 329 years to the date of the upcoming transit, and a day before his 21st birthday he also observed a transit of mercury under perfect conditions at his temporary observatory.
Simple equipment – a telescope and a clock
This transit of mercury was pivotal because Halley saw split second detail, and figured that calculations could therefore be done with preciseness. All that was needed was telescope, and a clock that could keep time accurately for 6 to 8 hours.
Calculate the size of the Solar system
Multiple observers stationed around the world could all record the timings of the transit. By putting this information together, the distance to the Sun could be calculated and in turn the size of the solar system. Halley believed a transit of Venus would be most suitable as it was considerably larger. Unfortunately for Halley, the next Transit of Venus wasn’t for another 84 years! However, that didn’t stop Halley from making sure that the astronomers of the next generation took full advantage of the opportunity.
Transits in the 18th Century leading to the charting of New Zealand’s coastlines
First ever International Scientific Expedition
Halley’s 17th Century ideas to send observers around the world inspired the “first worldwide scientific expedition of its kind.” His work and recommendations informed the expeditions, much as his predictions that a bright comet would return (also after his death) gave the comet his name. Amazingly, Halley also went on to predict future transits of Venus up to the present day with amazing accuracy. He predicted the transit of Venus in 2004 to within 50 minutes (over 4 centuries).
Warfare & weather - 1761
Observations of the first transit of Venus in the 18th Century were made extremely difficult to the risks caused by the several warring nations. Of the few observers who managed to reach far flung outposts for observations, most did not necessarily know accurately where they were! Without precise latitude calculations, the results were meaningless. Astronomers faired a little bit better, with extra practice and preparation for 1761.
Cook’s first transit – Venus in Tahiti - 1769
King George the III of England, agreed to send a ship to the South Pacific, but insisted that his ship was under the command of a naval officer and not an astronomer. (In the previous century, Halley had not been a popular commander and England had nearly lost its ship due to mutiny!) Lieutenant James Cook was commissioned for the job.
Cook submitted his observations of the transit in 1769, made from Tahiti, along with more than 120 astronomers who had travelled around the world. Unfortunately for Cook, the poor quality optics of the day combined with the atmosphere of Venus (unlike Mercury which has no atmosphere) obscured the precise measurements needed for the calculations.
Cook’s second transit – Mercury - in New Zealand 1769
After observing the transit of Venus, Cook received secret orders to hunt for what is now known as the South Island, which had been sighted 130 years previously by Abel Tasmen. On the way, he anchored in a Bay in the Coromandel Peninsula, which had fresh water and botanical specimens to keep the botanists happy while the astronomers were busy. The location is now called “Mercury Bay”. On 9 November (exactly 337 years before our next transit) Cook observed a transit of Mercury, and used his calculations to assist with the first accurate calculation of Longitude. The details were used for mapping the rest of what was then known as “Terra Australis”. Cook sailed the Endeavour on, charting many of New Zealand’s and Australia’s coastlines before returning to England in 1771.
Transits in the 19th Century
Massive preparation but weather failure - 1874
Astronomers had to wait until 1874 to see another transit of Venus. Given the huge importance several teams travelled to New Zealand and set up at various locations to observe the event. A British team set up near Christchurch, and a US team set up near Queenstown. It was the first time that the event could be photographed. One astronomer started preparations years before the event, and built the largest telescope in New Zealand of the time with a mirror size of 11”. He was provided with a chronometer by the New Zealand government and a telegraph cable was laid between Thames and Christchurch for the event. Unfortunately, New Zealand was under cloud that day, though one astronomer was able to get partial timings of the event through the clouds and the American team was able to get photographs.
Transits and Timezones - 1882
Even though the weather had not co-operated for the first transit of the Century, two couples travelled from England to the same site in Christchurch for 1882. Although the transit was only partially visible from New Zealand; when the Sun rose, the transit was well underway, the astronomers made satisfactory observations throughout the remainder of the transit. Visitors to the site today will find pillars and plaques to commemorate the success of 1882. Of particular relevance today, the observation point is the same one used to calculate New Zealand Standard time as compared with the international time line in Greenwich, UK.
Scientific Advancements – but more to come
By the end of the 19th century, given the length of time between transits of Venus, and the poor seeing presented through the optics thought to be due to the atmosphere on Venus, it was no longer the optimal way to calculate the size of the solar system. While observations with Mercury were clearer due to its lack of atmosphere, it was so much smaller and so much closer to the Sun, there were margins or error. Astronomers agreed to base their estimates on a variety of measurements, of which the transit observations were just one. Unfortunately, there were no transits in the 20th Century so it was going to be a long wait!
Transits in the 21st Century
My first ever transit – Mercury
On 7 May 2003, I woke up at 5:30 am to witness my first ever transit, visible in full from the UK and part of Europe. Most astronomers concentrate on looking at the sky after the Sun has set, so I didn’t own the necessary equipment to look at the Sun. In fact, I had never seen the Sun except with eclipse glasses using safety film. I was keen to study it through a telescope (possible only with specialised solar filters). I was entranced by the massive size of the Sun by comparison to Mercury and enjoyed feeling the warmth of its rays while observing. It was such an enjoyable experience by comparison to my nightly sessions spent wrapped in layer after layer to keep away the cold!
My second transit - Venus
Just over a year later, I had the incredible fortune of seeing the first transit of Venus in the 21st century (visible only from the Northern Hemisphere). My observing partner and I camped on the highest nearby hill (the Vale of the White Horse with its famous chalk drawings) in order to have the best chance of catching the start of the transit (just after sunrise). We were the only ones in the county who had saw the start as the rest of Oxfordshire was caught in early morning’s mists. We spent the best part of 6 hours watching the transit from ingress to egress (start to finish), while listening to live radio and seeing TV broadcasts from as far away as Egypt. When it was over we vowed to travel to New Zealand (the only country that the entire transit will be visible from) to see our second and last view of this event. Interestingly, the image in my telescope was clear and precise and we did not observe any of the historical problems reported with timing the event. In fact, when we analysed the timings we had made, our calculation of the distance of the Sun was accurate to half of a percent. Within a couple of days, the scientific community concluded that with the advances in optics Venus was no longer suffering blurring caused by telescopes, which prevented previous accurate calculations.
Future transits and technology – your chance to play a part
And now, it is our turn to host the next transit of Mercury in November, and Venus in several years’ time. With technological advancements in the last decade using specialised timekeeping and GPS satellite links, an accurate calculation of the distance to the Sun can be made by any observer with a simple device. We will have a GPS specially link to a video recorder to time stamp the start and end of the transit and we should be able to replay it, test our eyes and make our own, accurate, estimates of the distance to the Sun. So I encourage you to enjoy a rare view of the Sun, watch history in the making and gain a deeper understanding of our size and place in the Universe!
Stellar Parallax
The idea was to calculate Solar parallax – or simply put, seeing the Sun from different locations on earth, to calculate how far away it is. This can be illustrated in basic terms by holding a finger at arms length and observing it first through one eye and then the other. As you close one eye and open the other, your finger appears to move, though in reality it is because the angle is different from each of your eyes. Each eye is like an observer stationed at a different place on the earth.
The French astronomer Joseph-Nicolas Delisle (1688-1768) proposed, in 1722, another method related to the observation of a single phase of a transit (first or last interior contact); this method increased the number of possible observing sites by adding all the places where only one phase was observable. However, it required a very good knowledge of the longitudes of the observing sites and that was difficult to obtain in the middle of the 18th century.