Named for the roman goddess of love, beauty and chastity, Venus is the brightest object in the night sky apart from the Moon. Ancients who gazed at the stars in wonder were apt to mistake Venus for two separate astral bodies, a morning and an evening star (Phosphorus and Hesperus), until Pythagorus recognized that it was a single object. The Mayans called Venus Chak ek, “theGreat Star,” a representation of Quetzalcoatl, their principle deity, while the Akkadians (pre‑ Babylonian Mesopotamians) made it the special star of Ishtar, the mother god. To the Chinese, the brightest of shimmering stars was the god Sin xing.

In keeping with the goddess theme, almost all the features on the planet bear feminine names, yet despite this overture to the gentler sex, it is hard to imagine a more brutal place to spend your holiday. The entire planet is blanketed in dense high altitude clouds of sulfur dioxide laced with sulfuric acid, and, as you descend through the upper atmosphere, winds can reach 350 km per hour. The atmosphere is 96 per cent carbon dioxide, 3.5 per cent nitrogen, and the greenhouse effect from those dense clouds ensures that temperatures on the surface rise to a sweltering 480°C. The atmospheric pressure at ground level is 90 times higher than on the surface of Earth, roughly equal to 1km under the sea.

Yet, despite an impressive list of disagreeable physical charms, Venus is still the most earth-like planet that we have encountered. It is only slightly smaller than Earth and is about 30 per cent closer to the sun. Water may once have existed there although there is now only a trace in the atmosphere. What we have gleaned about conditions beneath the clouds is fascinating enough, but that knowledge is secondary to a far greater insight that this neighbouring chunk of rock has revealed to us about the solar system, the galaxy, and even—yes—the Universe.

Due to periodic transits—all but invisible eclipses which visually resemble a small black hole gliding across the face of the sun—Venus now holds a very special place in the history of modern astronomy, not to mention a foundational role in our nation’s history.

“We took our leave of Europe for heaven alone knows how long, perhaps for Ever,” begins the ship’s log of a young naturalist, Joseph Banks, as Endeavour set sail from Plymouth on August 12, 1768. The instruction given to her captain, Lt James Cook, was to locate Tahiti, a 20 km wide speck in the vast Pacific Ocean which had been encountered for the first time by Europeans just a year earlier. On arrival, Cook was to construct an astronomical observatory specifically for the purpose of observing the next transit of Venus, expected on June 4, 1769 (local time). The reason this seemingly quixotic mission mattered so much to the Royal Society, who sponsored the voyage, was that it would help to solve an enormous puzzle that had vexed 18th-century science ever since Edmund Halley had suggested a way of answering it in 1691. The question—what is the size of our solar system and what does that reveal about the scale of the cosmos?—could be answeredby observing Venus in transit from a series of widely spaced locations around the globe. Halley had observed a transit of Mercury but had quickly realized that Venus, being closer to Earth, would yield more useful data, saying: “This will be the only kind of observation that in the next century with the highest precision will disclose the distance from the Sun to the Earth.” Comparing the start and stop times of the transit and calculating according to the principles of trigonometry—known as parallax—from a variety of locations on Earth could yield an answer, but the further apart the observatories were, the better.

The only problem with this solution to the puzzle was that transits of Venus are relatively rare events, occurring in a pattern that repeats every 243 years, with pairs of transits eight years apart separated by long gaps of 121.6 years and 105.5 years. Halley realized that he could never put his solution to the test, being born out of phase with the transits—he died in 1742—so the mission passed to his successors.

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In 1761 a team of international scientists attempted to time the first transit, but due to a variety of factors including bad weather and Anglo-French hostilities, their data was all but useless. If the 1769 transit did not furnish good data, then every living astronomer would be dead long before the next transit, due in 1874.

Cook had to round Cape Horn to get to the Pacific, a passage famous for inclement conditions, although this time it claimed only five lives. He also had to battle scurvy, the perennial scourge of long-distance sea voyages (see New Zealand Geographic, issue 37). As an experiment devised by the admiralty, he ordered his men to eat foods such as sauerkraut and malt wort. Crewmen who sought to avoid this diet were flogged—no fewer than one in five, an indication of how unpalatable the servings were.

On arrival in Tahiti, Cook and his men were warmly received by the Tahitians and found the island and its inhabitants very much to their liking. The mission was accomplished, with excellent weather for viewing the transit. Cook noted in his log:

“..the Air was perfectly clear, so that we had every advantage we could desire in Observing the whole passage of the Planet Venus over the Suns disk..”

The start and stop times of the transit were blurred by the “black drop effect”, and the timing was therefore not quite as accurate as had been hoped. Following the completion of his primary mission, Cook then acted on secret orders from the navy to search for the great southern continent, Terra Australis incognita, supposed to exist somewhere between Tahiti and New Zealand. Geographers of the day tended to believe there had to be a massive southern continent to balance the great land masses of the Northern hemisphere. Cook spent months at the fruitless task before he made his way to New Zealand, where he extensively charted the coastline before continuing to the east coast of Australia.

By the time he returned to England from this epic voyage, he had circumnavigated the globe, catalogued thousands of new species, documented a transit of Venus, dismissed the notion of a great mid-Pacific continent, and made contact with Tahitians, Maori and Aborigines. It was an astonishing accomplishment.

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This year on June 8, another transit of Venus took place. Sadly in New Zealand it began right at sunset, but in many parts of the world it was clearly visible and well chronicled. Three parties of New Zealand schoolchildren travelled to Britain and one to Tahiti under the auspices of the Royal Society of New Zealand to observe the transit and report back.

Professional interest in this transit had less to do with refining the Astronomical Unit (which is accurately known from space-probe telemetry and radar observations), than with refining techniques for seeking out extra-solar planets. Most of the planets found orbiting distant stars (in mid 2004 there were 123 confirmed, although a further 100 located by the Hubble Telescope are still to be confirmed), have been pin-pointed through gravitational anomolies in the stars they orbit.

In 1999, however, the first planets were detected due to observed diminuition of light caused by transiting bodies. This method of detection is more informative as it allows astronomers to accurately determine the orbital plane, size and density of a planet.

The next and last opportunity for current subscribers to New Zealand Geographic to witness a transit will be on June 6, 2012, as another isn’t due until December 11, 2117. Should longevity be extended by medical break­throughs in the next century, further transits can be viewed in 2125, 2247, 2255, 2360, 2368, 2490 and 2498.

Black Drops and Parallax

Observations made by James Cook (left) and Charles Green from the temporary observatory constructed on Tahiti (listed at the time as King George’s Island) of the 1769 transit of Venus. It is worth noting that the recorded timing of the transit by these two men, who stood next to each other, differed by as much as 42 seconds, a fact attributed to atmospheric blurring at the edge of the disk. A significant phenomenon effecting observations is the “black drop effect”. As Venus approaches the edge—or limb—of the Sun, the black of space appears to reach in and touch the planet. This is mainly— although not entirely—due to an occurrence known as irradiation, whereby light from a cosmic source is slightly scattered as it passes though Earth’s atmosphere, so that the luminescent object appears a little enlarged.

This spreading of light against the black of space distends the Sun’s apparent diameter while diminishing the size of a dark object in front of the Sun, such as a transiting planet. The apparent circumference of the Sun appears larger than the true circumference, whereas the apparent circumference of the planet appears smaller. Just before these apparent limbs make contact, the true limb of Venus reaches the true limb of the Sun, obscuring the background luminescence and eliminating irradiation. For a brief interlude, the black drop you see is an overlap of the actual boundaries of both objects.

While irradiation explains the bulk of the effect, a subtle black drop was recorded from space in a Mercury transit in 1999. As Mercury has no atmosphere, a portion of the underlying cause of this effect is still unexplained.

EARLY IMAGING

Although wet bromo-iodide plates were used to record the 1874 transit, by 1882 dry collodion emulsion plates provided images like this, one of only 11 surviving plates from the US Naval expeditions.

PARALLAX

These images of the 2004 Venus transit are composites by Mogens Winther, in Denmark, who matched his own photos with some from Australia, captured by a GONG network telescope at the same time. The different positions of Venus clearly demonstrate parallax. More of these images can be viewed at the EUC Syd and Amtsgymnasiet in Sonderborg website: www.amtsgym-sdbg.dk/as/vt-2004

Andrew Caldwell

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