Twice in a lifetime Venus

For the first time in the Century, Venus will transit the Sun's disk on June 8, 2004. Transits are nothing but eclipses, that is, the planet's orbit happens to lie across the Sun at the time of its passage, yet the feeble 58 arcseconds wide (58/3600 of a degree) spot will not dim our star's light in any noticeable way. Asia, Africa and Europe will be privileged with ingress start at 5:13 UT to egress end at 11:26 UT, while Australia will catch the show opening before sunset and South America (except the southern tip) and North America from the Rockies eastward, will peek the last phases after sunrise.

Paired chances

Venus transits come in pairs 8 years apart with an about 105 year interval. Don't count on another chance after this and the June 6, 2012 one, next comes December 11, 2117. Venus's orbit is tilted 3.39 deg. with respect to the Ecliptic - the plane of Earth's orbit - while the orbital ratios of 13 turns for Venus in 8 years minus 2 days makes for the peculiar rhythm of 8 - 105 . You will find the pattern of Venus sightings as morning or evening star repeating the same months each 8 years. When the favorable position or node as it is technically known drifts once in place, it is still about right next turn 8 years latter, but wanders for 105 years until the same small drift brings about another opportunity.

Venus 8 year cycle was known as far back as the times of ancient Babylonia, where a tablet found in 1920 and dated to about 1500 BC during the reign of Ammisaduqa registered this astronomic observation, important on religious grounds as were all significant sky related events. Incidentally the Venus Tablet was for long used to controversially date the fall of Babylon in 1531, 1595 and 1651 BC until work by astronomer Vahe Gurzadyan using Moon eclipse records settled it to a more widely accepted 1499 BC (not to be confused with the final 539 BC fall).

The first observed transit

It is likely Venus transits had been observed for long though unknowingly. Chinese astronomers knew about solar spots, so it is entirely possible for them to have witnessed transits yet not associating the observed blemish with the planet.

German astronomer Johannes Kepler was the first to predict such an event for December 6, 1631. Unfortunately he died several years before and no one else was aware of his work in this respect. Neither had he predicted the next in the cycle due on December 4, 1639, which was discovered barely one month before taking place by young Jeremiah Horrocks of Lancashire, England. He and friend William Crabtree were the only humans to know of, and observe the transit. Horrocks died at 22 in 1641, but his accurate calculations confirmed by observation earned a name in the annals of astronomy.

Sir Edmond Halley of cometary fame was mapping the southern skies in St. Helena when he happened to observe a much more frequent Mercury transit. It dawned on him this kind of events could be put to good use to determine a long sought value, the Earth - Sun distance. Venus should be more useful than Mercury being farther out from the Sun, thus affording a more favorable geometry for the sake of accuracy. Tragically, he was well aware the upcoming 1761 and 1769 events were beyond his life span, yet laid the foundations for the experiment further perfected by Joseph-Nicolas Delisle.
The 1761 event was attacked with a full fledged campaign involving expeditions to Siberia, India, Madagascar, South Africa and St. Helena. As politics should have it, the Seven Years' War raged then, precluding for example nobleman Guillaume Le Gentil to land in his intended observation post of Pondicherry, Mauritius under British siege. Le Gentil waited out overseas for the next opportunity in 1769 at last in again French controlled Pondicherry but June 3 was ... clouded. Destiny had it when Le Gentil returned to France after a voluntary 10 years scientific exile, to have been long assumed dead and his possessions in process of division. Eventually he was able to marry and rebuild his life.

1769 found Great Britain transiting the climbing side of the road to the Empire where the Sun never sets, so it dispatched expeditions to Hudson Bay in Canada and the better known Endeavour captained by James Cook to Tahiti, while king George III arranged for an observatory to be built in his gardens for that purpose. Fair weather rewarded observers with excellent data except for unexpected disturbances as we'll see.

Transits gone, German astronomer Johann Encke determined in 1824 after careful analysis of 1761 and 1769 timing data, the Earth - Sun distance to be 153,34 million km (95.28 million miles), 2.5% higher than the correct 149,59 million km.

A movie shot before the invention of cinema

The 17th Century observations unveiled problems that marred the otherwise excellent precision the method could provide for the Astronomical Unit determination. For one, precision rested on the accuracy of timing contacts between the Sun's limb and Venus limb - a fact that was found to be subject to individual response times (remember, it was then clockwork era) - and then there was atmospheric disturbances, Venus' atmospheric bright halo, and the so called "black drop" effect masking the contact point at the most critical instant.

To address the timing and disturbance phenomena, the December 9, 1874 transit was attacked with leading edge technology in the form of improved optics and, most importantly, with the newly developed invention of photography. Photography should dispose of the human reaction time inconsistencies and - supposedly - visual observation shortcomings blamed for effects like the "black drop". France, Britain, and the US dispatched teams from Vladivostok to Nagasaki, Mauritius and elsewhere.

Disappointingly enough, efforts did not pay off with better quality observations that those of the 17th Century, reasons that delayed to 1881 a Paris international conference for evaluation of results and planning for the upcoming 1882 campaign, to be addressed with much lower expectations.

In the meantime, David Gill obtained from observations of Mars from Ascension Island in 1877 and with a different method, a new estimate for the Astronomical Unit that was just 0.2% in error with respect to the modern value.

Interestingly, astronomer David Peck Todd photographed the December 6, 1882 transit from the top of Mount Hamilton where the renowned Lick Observatory was under construction. 147 high quality negative plates were exposed and stored after analysis, laying forgotten for over 100 years. Current Lick Observatory astronomers Bill Sheenan and Anthony Misch got notice of the plates and used them to assemble a time lapse sequence - a movie - for the 1882 transit. When Peck took the photographs in 1882, cinema was not yet invented, so he was unknowingly the first movie director of all times.

Don't miss the upcoming June 8 transit if you are within the observing region. The sight of that small black dot (be careful, use approved filters or projection techniques) will connect you with Kepler, Halley, Cook and countless others from centuries past, and with a rare natural phenomenon of significant scientific importance in its time.