by Michael Holmes, Patrick Downey and Michael Potton

Nelson College

Introduction
The Transit of Venus, which occurs when the planet Venus passes in front of the sun, is one of the rarest astronomical events known to mankind.It has only been observed 5 times by astronomers. Its present pattern of recurrence, which changes over time, is at intervals of 121.5 years, 8 years, 105.5 years, 8 years, 121.5 years, 8 years, 105.5 years….. The first time the Transit was recorded as having been seen was in 1639 by Jeremiah Horrocks and William Crabtree. It has since been observed in 1761, 1769, 1874 and 1882 and the next Transit will be on June 8^th 2004, this year. The Transit of Venus first became sought after by astronomers when Edmund Halley, Astronomer Royal of England, suggested that the Transit could be used to deduce the distance from the Earth to the Sun, otherwise known as the Astronomical Unit or AU. Knowing this distance and using Kepler’s third law (which relates the orbital period of a planet to its distance from the Sun) the size of the Solar System could be calculated and also such things as the density and size of the Sun. This would greatly help astronomers in their work deciphering the mysteries of space. Measuring the distance between the Earth and Sun is a concept found in the legends of Polynesia. Legend tells about the demi-god Maui had to make a net long enough to reach and trap the Sun to make the days long enough for planting and fishing.

Calculating the Astronomical Unit
To use the Transit of Venus to work out the distance from the Earth to the Sun, you must have at least two observers at widely spaced longitudes and at accurately known positions. These observers must have synchronised clocks with which they record the time when the start and finish of the Transit. The difference between the times from the two observers is used to work out the distance. For example;
• Assume that the two astronomers are on the Earth’s equator and that they differ in longitude by 60º.
• Astronomer B sees the planet Venus first touch the Sun 5 minutes and 37 seconds after Astronomer A
• Assume that Earth and Venus are in orbits of periods 365.24 days (P_e) and 224.7 days (P_v) respectively and that the radius of the Earth is 6360km.
• We know that Earth is 1 AU from the Sun and that Venus is 0.723AU
• We now work out the baseline between the observers – in this case 6360km.
• Multiply this by the distance from Venus to the Sun – 0.723AU and by using the rule of similar triangles we can show that it takes Venus 337 seconds to travel 4598km
• Since the Earth is moving as well, we must factor this into our equations.
• To do this, use this equation; true interval=
• The answer is 130 seconds which is the time that Venus takes to travel 4598 km and this gives Venus a velocity of 35.2kms^-1.
• However the velocity is orbit circumference divided by the period; V= or 35,2kms^-1 =
• This gives the distance from the sun to Venus as 1.088 x 10⁸km. This value is 0.723 of the Earth-Sun distance. Therefore the length of 1 AU is 1.5 x 10⁸ (2s.f.)

Parallax of Mars
In 1672 Giovanne Cassini, Head of the Paris Observatory, with another astronomer, Jean Richter, used observations of the Parallax of Mars to find the length of the Astronomical Unit. The method was about 6% less than the modern accepted value of 149 million kilometers. It was likely that the mehtod was not universally accepted by astronomers and the transit was seen as more accurate way of finding of finding the length of the AU.

Centre of the Universe
There is no evidence from ancient records of the Transit of Venus ever being observed. Part of this is due to the fact that detailed charts of the planets had not been made and the world believed in the Ptolemaic system of thought, which held that the Earth was at the centre of the universe and the planets and sun travel around it. Another reason was that the telescope had not been invented and although the Transit is visible to the naked eye, it is difficult to see and you cannot look directly at the sun. For many years the Transit of Venus may not have been considered significant because using the Ptolemaic system of thought the only thing discovered would be the distance from the Earth to the Sun. However there was a rival theory put forward by Aristarchus in Greece in the 2^nd century BC. He proposed that the sun was in the middle of the universe and that the planets and stars traveled around it in circular orbits. This was revived again in the 16^th century by Copernicus and became the most accepted theory. Once the Copernican system had been accepted and Kepler’s Third Law had been discovered, then knowing the Astronomical Unit would allow size of the solar system to be calculated.

Father of the Space Age Johannes Kepler was a German astronomer who is famous for his three laws of planetary motion and his tables predicting the movements of the planets. He was the first to predict the Transit of Venus in 1631 and the Transit of Mercury the same year. His three laws of planetary motion transformed astronomy. His Third Law was that R3=T2. Here R stands for the distance from a planet to the Sun in AU and T is the time that that planet takes to travel around the sun in years. For example Jupiter which is 5.2 AU from the Sun, takes 11.86 years to orbit the Sun. Using the equation R3=T2 you come up with 140=140. Now that was need was the length of the AU to calculate the distances to all the planets. Unfortunately Kepler died in 1629 never having observed the Transit of Venus.

Gassendi
In 1631 the French astronomer Gassendi made the first recorded observations of the Transit of Mercury. One month later he attempted to observe the Transit of Venus. Unfortunately in 1631 the Transit of Venus was not visible in Europe as it happened at night. This would have been a great disappointment to Gassendi as Kepler had predicted erroneously that the next Transit would not take place until 1761. This was one of the few errors found in Kepler’s tables.

The First Recorded Observations of the Transit of Venus The 1639 Transit of Venus was known to be observed by only two people – Jeremiah Horrocks and William Crabtree. Horrocks was a priest at a church in the small town of Hoole near Liverpool. He discovered the errors in Kepler’s tables only a few months before the event and only had time to alert his friend, Crabtree. He was unable to observe the whole Transit because it was in December when the days were short so the sun set before the Transit finished. Horrocks did not think of using the observations to calculate the length of the Astronomical Unit – instead he wanted to work out the size of Venus. Horrocks died two years later before he and Crabtree had an oppurtunity to discuss the transit.

Halley’s Insight Edmund Halley, Astronomer Royal of England, was the first to propose the use of the Transit of Venus for determining the length of the Astronomical Unit, around the turn of the 18th century. He drew up detailed plans which he presented to the Royal Society on how to go about the observations even though he knew he would be dead before the next Transit occurred. He got the idea from watching a Transit of Mercury. Mercury is unsuitable for that purpose because it is too close to the Sun and it is too small for accurate measurements.

The First International Scientific Study The 1671 Transit can be said to be the first truly international scientific survey. Over 150 observers saw the Transit worldwide but weather, poor clocks and the ‘black drop’ effect (discussed below) conspired to make the efforts almost useless. In 1769 there wasn’t the great push for observations but after the urging of the Royal Society, the British Admiralty sent out three different groups of observers to different places on the globe. One of these was Captain James Cook who was sent out to the island of Otaheite (Tahiti). He successfully observed the Transit, but, as all the other observers found the results did not match up. Cook then went on to make many major geographical discoveries in the Pacific region. He was eventually killed in Hawaii. Joseph Banks accompanied Cook on his first voyage and made many important discoveries in botany. Cook recognised the navigational skills of the Polynesians and took a Tahitian navigator, Tupaia on his voyages around the Pacific.

So Unfortunate
One unlucky person observing this Transit was Father Maximilian Hell. He observed the Transit from Vienna but waited a long time before publishing his results. This lead to suspicion that he waited to see other results published and then edited his to fit. After his death this was supposedly ‘proved’ by the astronomer Littrow whose evidence showed corrections that were done in different coloured ink. In 1890 however, Father Hell was proved innocent when it was found that Littrow was actually colour-blind and that he had got the ink colours wrong. Another unfortunate person was the French astronomer, Le Gentil. He traveled to India to observe the 1761 Transit but as France and Britain were at war, he was denied entry and could not carry out his observations. He decided to stay in Asia to observe the next Transit from his vantage point in Manila in the Philippines, but the day of the Transit was clouded over and he couldn’t observe anything. On the way home to France he suffered a couple of shipwrecks and upon his arrival he found his family dividing up his estate as he was presumed dead.

19th Century
The two Transits in 1874 and 1882 were still considered important by scientists around the world. The US Navy sent eight expeditions around the globe to each Transit. They still encountered the same problems, but by combining all of the sets of data they gained a fairly accurate idea of the length of the Astronomical Unit. One American expedition in 1882 even came to Queenstown here in New Zealand.

Coming Soon
The next Transit of Venus occurs on June 8, 2004 and the next one in 2012. While these rare events will be watched by many astronomers, they are no longer of such great scientific interest. The distance from the Earth to the Sun has been found with greater accuracy by firing a radar beam to the sun and then measuring the time it takes to come back to Earth. In the future, Transits of Earth may be watched from Mars and other planets. Transits of Earth as seen from Mars normally repeat after 79 years but after the last pair (1905-1984) there is a period of 100 years before the next Transit.

‘Black Drop’ Effect
The Transit of Venus is a fairly inaccurate way of determining the Astronomical Unit. Clocks have to be accurate and the timing precise. The ‘black drop’ effect where Venus appears to becomes slightly bulbous in shape and seems to connect to the Sun, which can be fatal to the timing. Weather is prone to disrupting observations and many Transits are not visible from some parts of the world. However by using all of the data gathered from the Transit observations, a rough idea of the distance can be worked out.

Conclusion
The Transit of Venus, while it is no longer of major scientific interest, is still a rare and fascinating astronomical event. It is special because there have been so few recorded observations throughout history. The background is rich and varied and many people will watch the Transit this year. The Transit of Venus, while in its purest scientific form did not achieve much, indirectly initiated many explorations including Cook’s explorations of the Pacific and gave the Western world important information about the other cultures that existed at that time.