To think we're more used to seeing Venus as a brilliant shining white star in the night sky and there it was, blacker than black.

- An h2g2 Researcher, viewing the 2004 Venus transit.

The Transit of Venus is a type of eclipse, where the planet Venus traverses the face of the Sun. In contrast to Sun-Moon eclipses, this Venus-Sun eclipse is barely noticeable, because Venus appears only as a small black dot on the Sun, and this is not enough to turn day into night. Viewable from Earth, it is possible to work out astronomical data from transits. The Earth only crosses the plane of Venus' orbit twice a year, in June and December, due to the tilt between the Earth's and Venus' orbit. The reason a transit doesn't occur every six months is because Venus will not be between the Earth and the Sun each corresponding time.

In order for a transit to occur, both planets need to be lined-up exactly. Line-ups occur because the period of Venus' orbit around the Sun (its year) is shorter than the Earth's (224.7 days compared to 365). Venus transits occur in pairs with an intervening gap of eight years, with intervals between the most recent transits of 121.5 and 105.5 years.

Sometimes only one of the transit pairs can be observed, because Venus may pass slightly above or under the solar disc on the second turn. This happened in the 14th Century and it will happen in December 3089. Overlooking these exceedingly rare events, we can confidently say that Venus transits re-occur in two pairs every 243 years (121.5 + 8 + 105.5 +8 = 243).

Eye Protection

WARNING

You should never look at the Sun directly, or through a telescope or binoculars. Looking at the Sun directly can cause permanent damage to your eyes. Looking at the Sun through the magnifying lens of a telescope or pair of binoculars will fry your retina and might cause instant, permanent blindness. Since it has been shown that most individuals who sustain sun-related eye injuries are children and young adults, it is a preventable tragedy, with better education. Therefore please teach young children to take care of their eyes to avoid any more horrific injuries. Read what the NASA website has to say about eye safety and the Sun.

To safely view the transit of Venus, you can make a simple pinhole viewer. A pinhole in a piece of card¹ will project an upside-down image on to another piece of card held behind it. Alter the distance between the two pieces of card to focus, enlarge or brighten the image. The Exploratorium website shows practical diagrams to help you choose a safe viewing method.

While you should not look at the Sun through a telescope to see a transit, eclipse or sun-spots, if it is securely mounted on a tripod you can use it to project the image onto a flat surface like a piece of card fixed behind the telescope, or even a wall.

Some of the available amateur telescopes come with appropriate filters for safe observation of the Sun. These can be installed in front of the ocular lens. However, make sure you read all the instructions through thoroughly and seek the advice of more experienced astronomers whenever possible.

Harper's Magazine printed a drawing of a man and woman observing the Venus transit in 1769. The woman is shown looking through a telescope at the Sun. Obviously there were no Health and Safety regulations then.

Why The Transit Of Venus Is So Important

The great number of observers witnessing past transits and recording the data contributed to a significant scientific finding. By exactly measuring the transit of Venus at a number of widely spaced locations on the Earth's surface, it would be possible to calculate the solar parallax. Knowing the solar parallax and the mean Earth radius allows one to calculate the distance from the Earth to the Sun, the 'Astronomical Unit' (AU). From this, the exact dimensions of our Solar System could be calculated. Scientists and astronomers can now use the data gathered from transits to help in the search for extrasolar planets.

Early Sun-watching

Since the dawn of time, mankind has been fascinated by the heavenly bodies in the sky, their movements and mysteries. Aristotle (384 - 322BC), the great thinker and philosopher, watched a partial solar eclipse while sitting under a tree thanks to the camera obscura effect.

There were astronomers in the Bible; you may recall the Magi (wise men) and a certain Christmas story. Long before that, the ancient Egyptians worshipped the Sun as a god, as did the Incas, Aztecs, and a whole bunch of other ancient civilizations. Solar eclipses, obviously, were not considered to be a good sign; indeed, that particular celestial phenomenon would have been an ill omen bordering on catastrophe.

Soon enough people started to realize the usefulness of astronomical observations (think of navigation) and to understand the mathematics that described the orbits of planets². From the planetary orbit model it is just a small mental leap that leads to the question whether it is possible to observe a planet passing in front of the solar disc.

The first drawing of a sunspot (that we know of) was made by Friar Brother John of Worcestershire, England, on 8 December, 1128. This was long before the invention of useful things like the telescope, so we can assume another method of safe Sun observation was utilised.

Venus Transits in 1518 and 1526

The Venus transits in the 16th Century were visible from Europe and could have been observed by Nicolaus Copernicus (1473 - 1543), but no record exists of anyone having witnessed them. According to the records at the Nicolaus Copernicus Museum in Frombork, Poland, Copernicus returned to observing the planets after a gap of several years, in 1518. In 1526, the year of the second Venus transit during his lifetime, Copernicus was busy mapping the kingdom of Poland and the Grand Duchy of Lithuania.

Johannes Kepler

German astronomer Johannes Kepler (1571 - 1630) calculated that Mercury would pass in front of the Sun on 7 November, 1631, and Venus would do so the following month. No record exists of anyone observing the 1631 Venus transit as a large storm obscured the Sun across most of Europe, preventing any observations. However, Kepler made an error when he stated that the next Venus transit would be in 1756. Never mind, there was a certain young Lancastrian churchman who was prepared to disagree with the great Kepler, and establish his own place in the history of astronomy.

The 1639 Transit

Jeremiah Horrocks (1619 - 41), a 20-year-old curate in Lancashire, accurately predicted the date on which the planet Venus would cross the Sun's disc. Luckily it was during his short lifetime. On 24 November³, only Horrocks and his friend William Crabtree of Manchester are known to have seen the 1639 Venus transit taking place. Horrocks would have used the telescope and tripod method and projected the inverted image onto a wall for safe viewing.

He was gratified by beholding the pleasing spectacle of Venus upon the Sun's disc. Rapt in contemplation, he stood for some time motionless, scarcely trusting his own senses, through an excess of joy.

- Horrocks' description of William Crabtree's reaction.

Neither Horrocks nor Crabtree made the kinds of measurements that would be needed to work out the solar parallax. This was due to Horrocks being unable to witness the whole event; however the half-hour of charting of the transit that he recorded turned out to be one of the most important half hours in scientific history. The data gathered⁴ increased our knowledge of the Solar System by a factor of 15, according to Dr Robert Walsh, senior Lecturer from the University of Central Lancashire's Centre for Astrophysics.

Horrocks and Crabtree were the first recorded humans to view a Venus transit, they proved that the spectacle existed, and Horrocks' detailed papers - undiscovered for years - accurately predicted future Venus transits. They would take place on 6 June, 1761 and 3 June, 1769; and Horrocks also worked out where on the Earth they could be viewed from. Jeremiah Horrocks died of unknown causes two years after viewing his wondrous transit, aged just 22 years.

Edmond Halley

Astronomer Royal Edmond Halley (1656 - 1742) had noted in 1716 that an accurate timing of the transit of Venus would aid the calculation of the distance between the Earth and the Sun. Although Halley knew he would not live to see the next anticipated Venus transit in 1761, his detailed plans and papers ensured that expeditions were sent out to the farthest reaches of the globe to make observations.

The 1761 Transit

Dozens of hopeful astonomers were watching the skies at numerous locations around the world. In most cases the results were very disappointing, due to 'cloudy weather and other unfavourable causes', namely the Seven Years' War (1756 - 1763) between England and France (known as the French and Indian War by Americans). British astronomer Charles Mason (1730 - 86) and surveyor Jeremiah Dixon⁵ had been sent to Sumatra to view and record the transit by the Royal Society. Their ship, the HMS Seahorse, was attacked by a French frigate and they had to return to Portsmouth. Their second voyage wouldn't have got them to Sumatra in time so they stopped at Cape Town, South Africa, as guests of the local governor and set up their equipment. Mason and Dixon obtained several accurate measurements of Venus' position on the solar disc, providing some of the most useful data from the 1761 event, as it was the only successful observation made from the Southern Hemisphere.

The director of the University and Gymnasium at the Petersburg Academy of Science, Mikhail Vasilevich Lomonosov (1711 - 1765), observed that the Venusian atmosphere consisted of dense gas, before it entered the disc of the Sun.

The 1769 Transit

The Venus transit in 1769 started just after 7pm on Saturday 3 June, and was witnessed by a great number of people, including, in his own garden in Selborne, Hampshire, the Rev Gilbert White (1720 - 1793), who is regarded as England's first ecologist and the author of the Natural History and Antiquities of Selborne.

Saw the planet Venus enter the disc of the Sun. Just as the Sun was setting, the spot was very visible to the naked eye.

- Eyewitness Gilbert White

Among the observers in America was astronomer and President of the American Philosophical Society David Rittenhouse (1732 - 1796), one of the leading American scientists of the 18th Century, second only to Benjamin Franklin. The sight of Venus on the solar disc affected him so much that he fainted next to his telescope. Only five hours after this transit there was a total solar eclipse, which was visible from Northern America.

Some lucky individuals managed to view both the 1761 and the 1769 transits, Charles Mason observed from Donegal, Ireland, while Jeremiah Dixon, who was in Norway, was clouded out.

Captain Cook and Charles Green

A famous voyage was undertaken by Captain James Cook (1728 - 1779), who sailed to Tahiti on the Endeavour to witness and record the transit. Also on board was Charles Green (1735 - 71), former assistant to the Astronomer Royal Nevil Maskelyn (1732 - 1811), who was instrumental in setting up the new Observatory at Armagh.

Not a Clowd was to be seen the whole day...we had every advantage we could desire in observing the whole of the passage of the Planet Venus over the Sun's disc.

- Captain Cook's description of the astronomer's dream.

Disappointingly, discrepancies in results from around the world caused the recording of the Venus transit to be declared a failure. Cook's voyages as a whole were a huge success though, due to the pioneering anthropological⁶, botanical and zoological work; and the vast improvements in navigation at sea. Cook and Green's historical drawings of the Transit of Venus are preserved for posterity at the Armagh Observatory, where there is a special Venus Transit Exhibition.

A monument marks the place at Venus Point, Tahiti, where the observations of the transit of Venus were conducted by Cook and Green in 1769.

The King's Observations

The Royal Observatory at Richmond, Kew, was designed by Sir William Chambers for King George III, who was interested in many aspects of science and particularly astronomy, specifically for the Royal Family to observe the 1769 transit of Venus. The King and the Royal Family provided detailed and accurate information on the transit. The telescopes and clocks used to observe the 1769 transit were presented to Armagh Observatory in 1840 by Queen Victoria, after the Royal Observatory closed.

Guillaume Le Gentil

The French astronomer Le Gentil⁷ had travelled to India to observe the 1761 Venus transit, but didn't reach his destination in time due to the war being in full swing. He survived a hurricane and severe illness, but remained to view the 1769 event, when, just during the transit, there were clouds blocking the Sun.

Le Gentil was so depressed that he couldn't even write his report to the French Academy of Sciences. When he eventually returned home after an absence of 10 years, he found he had been presumed dead, his wife had remarried and his job at the Academy had been given away. Murphy's Law could be renamed Gentil's Law in his honour.

Jean Chappe d'Autoroche

A French expedition led by Jean Chappe d'Autoroche (1728 - 69) went to Tobolsk, Siberia, to record the 1761 transit. The team survived a treacherous river crossing and a lengthy journey through difficult boggy conditions, eventually arriving at their destination just six days before the transit was due. Just before the transit, Jean was attacked by some of the locals, who believed he had caused unusually severe spring floods by interfering with the Sun. Cossack guards managed to save the team of astronomers, who eventually managed to make good observations of the Venus transit.

The French Academy were so pleased with the success of his previous mission that they engaged Jean Chappe d'Autoroche to record the next expected one in 1769. He specified that he would travel anywhere in the world as long as the temperature wouldn't be below freezing. Tragically, when Jean and his team arrived at Vera Cruz in Mexico, they found themselves in the middle of a plague epidemic. Instead of moving on to a safer place, the team decided to remain and help care for the sick villagers.

Even though Jean was mortally ill, he managed to record astronomical observations to establish the latitude and longitude of the site. These recordings were vital for the calculations of the astronomers back home in France, and were made with an accuracy unprecedented in the history of his peers, given the hazardous conditions in which the data was gathered. The observations they made provided some of the best data of the 1769 transit. Jean died of a fever just after the transit, aged 41 years.

The sole survivor of Jean Chappe d'Autoroche's six-man 1768 expedition, an engineer-geographer named Pauly, arrived back in France in 1770 with Jean's notebook, which he presented to the French Academy of Science. King Louis XV awarded Pauly a pension of 800 francs a year. Sadly Pauly's health was so poor he was unable to support himself, so eventually he had to petition the King for an increased pension.

Anders Planman

Swede Anders Planman (1724 – 1803) was a brilliant astronomer and the Professor of Natural Philosophy at the University of Âbo in Turku, Finland. His main ambition in recording the data from the Venus transits was to determine the solar parallax. He wrote numerous papers on the 18th Century observations of the transits, including his own. Planman observed both the 1761 and 1769 Venus transits from the city of Cajaneborg in Finland.

Co-operation and Goodwill

Finally, the problem of the transits of Venus produced an intensity and breadth of effort on the part of 18th Century scientists that was unmatched by any other single problem. It brought to a common focus men of almost every national background with an abiding concern for the advancement of knowledge. In doing so, it helped to shape the growing international community of science and to demonstrate with striking clarity what co-operation and goodwill might achieve in the peaceful pursuit of truth.

- H Woolf, Author: The Transits of Venus (1959)

The 1874 Transit

The transit on 9 December, 1874 was not considered important for the determination of the size of the Solar System. However, this transit had a distinction of its own, in that it was the first to be photographed. Charles Burton was the photographer; he had travelled from Dublin to Rodrigues Island in the Indian Ocean, accompanying an expedition organised by the Royal Observatory, Greenwich.

The Astronomer Royal from 1835 to 1881, Sir George Biddel Airy (1801 - 92), started making plans to view the 1874 transit 17 years prior to the event. One of his more important writings was the On an Inequality of Long Period in the Motions of the Earth and Venus.

The American Transit of Venus Commission engaged and equipped eight expeditions to take recordings of the transit: three were sent to the northern hemisphere and five to the south of the equator. It was observed by a large number of astronomers in Ireland; and Italian astronomer Pietro Tacchini (1838 - 1905) led an expedition to Muddapur, India, to witness and record the transit using spectroscopic instruments for the first time.

Indian astronomer Ankitam Venkata Narsinga Rao (1827 - 92), who owned a private observatory at Visakhapatnam, submitted his observations to the Royal Astronomical Society, and they were published in 1875.

The 1882 Transit

Astronomers at Dunsink and Armagh Observatories, University College Cork, Markree Observatory, County Sligo and Daramona Observatory, County Westmeath observed the transit on 6 December, 1882.

On our departure we left two iron pillars, on which our apparatus for photographing the Sun was mounted, firmly imbedded in the ground, as we had used them. Whether they will remain there until the transit of 2004, I do not know, but cannot help entertaining a sentimental wish that, when the time of that transit arrives, the phenomenon will be observed from the same station, and the pillars be found in such a condition that they can again be used.

- American expedition member Simon Newcomb, after the observation from near the town of Wellington in South Africa.

Amherst College astronomer David Peck Todd (1855 – 1939) climbed to the summit of Mount Hamilton in California to view the transit from Lick Observatory. He used a solar photographic telescope and took 147 negatives. The glass plates were securely stored and lay undiscovered for 120 years. When they were found, digital technology was used to create a movie, providing the most complete surviving record of a historical transit of Venus. Todd may have had this purpose in mind, at that time his peers were taking the first tentative steps into chronophotography (the recording of sequential motion) - the parent of cinematography.

John Philip Sousa (possibly the world's first international pop idol) composed the Transit of Venus March in honour of the celestial event.

There will be no other transits of Venus till the twenty-first century of our era has dawned upon the earth, and the June flowers are blooming in 2004. When the last transit occurred the intellectual world was awakening from the slumber of ages, and that wondrous scientific activity, which has led to our present advanced knowledge, was just beginning. What will be the state of science when the next transit season arrives God only knows. Not even our childrens' children will live to take part in the astronomy of that day.

- William Harkness (1837 - 1903), Director of the US Naval Observatory.

The 2004 Transit

On 8 June, 2004 the entire transit was visible from the UK. The Sun rose at 4:45am and Venus began its egress across the face of the Sun at 6:19am. By 9:21am it was halfway across, and it finished its transit at 12:23pm.

Sir Patrick Moore watched the transit at his home in Selsey, Sussex, with Dr Brian May⁸ for the BBC programme The Sky at Night, the longest-running programme in the history of television.

In honour of Jeremiah Horrocks (see above) more than 100 astronomers from around the globe descended on the village of Much Hoole, which is where Horrocks had lived and had made the first recording of a Venus transit.

American astronomers on the island of Crete and at Thessaloniki in Greece studied the 'black drop effect' which appears to shape Venus into a teardrop as it approaches and leaves the sun. The 'black drop effect' was explained in 1770 by brilliant French astronomer Joseph-Jérôme Lefrançais de Lalande (1732 - 1807), who proved that the 'black drop' is caused by the motions of the Earth's atmosphere.

Unfortunately the astronomical phenomenon was not visible from everywhere on Earth. This map at the NASA website shows the world visibility of the transit. Man's technological advancement nullified this slight obstacle - those who were interested but unable to be in the viewing path, were able to log on and see the transit live on the world wide web, or watch on television⁹.

It was a brilliant opportunity to know the mechanics of our solar system.

- Shereeza Feilden (14) who witnessed the transit at the Royal Observatory in Greenwich.

Think of what this meant to people who spent their lives trying to solve these ancient mysteries!

- Flavio Fusi Pecci, Director of the observatory at the 900-year-old University of Bologna.

We had a very busy day at Jodrell Bank, over 350 visitors in total! We missed first contact due to a badly placed bank of cloud, but got to see the rest quite well. For those of you in the Granada region, Fred the Weatherman came along (for everyone else: Fred is a bit of a local legend around here, he's very enthusiastic) and did a couple of reports which was fun. There are lots of images on the Jodrell website now, including some stills from our webcam which was on the H-alpha telescope.

- Megan Argo, astronomer.

It's like a fine French wine for the people who know about it and enjoy it.

- Jay Pasachoff of Williams College in Massachusetts, who watched from the Aristotle University of Thessaloniki in Greece.

If you missed it, or would like to see it again, the 2004 transit can be viewed at the Transit of Venus 2004 Expedition.

Future Transits

6 June 2012

North America will be able to see the start of the transit, while South Asia, the Middle East, and most of Europe will catch the end of it. In the UK it will only be possible to observe the last hour of the transit at sunrise on 6 June, as the main part of the transit will take place overnight on 5 - 6 of June.

The Venus transit of 2012 will be visible in its entirety from Hawaii, Australia¹⁰, the Pacific Ocean and eastern Asia, so, for those of you on the other side of the world, start saving now. This map at the NASA website shows the predicted world visibility of the transit.

Transits for future generations to watch:

• 11 Dec 2117
• 08 Dec 2125
• 11 Jun 2247
• 09 Jun 2255
• 13 Dec 2360
• 10 Dec 2368
• 12 Jun 2490
• 10 Jun 2498

Uninterested Parties

In the interest of balance, here are a few comments from those not impressed:

'Venus Transit' makes me think of something like Scooby Doo's Mystery Machine, but decorated with lots of hearts instead.

Doesn't the world end in that year [2012] though? Admittedly that might not be before December, so I guess I could see the transit, then.