They occur at intervals of eight years, a small section of individual life; but alternating with intervals of more than a century, during which whole generations pass away, thrones crumble, and dynasties change.
Thomas Milner
On June 8, 2004, a natural phenomenon that no living human being had ever seen occurred. The planet Venus passed across the face of the Sun. Apart from some comets and asteroids, (usually invisible), only the two interior planets, Mercury and Venus, can do this. Transits of Mercury are fairly frequent, but are only visible in a telescope. Venus is much closer and larger than Mercury, and a transit of Venus is the only occasion when the disc of a planet is visible to the naked eye.
Venus again transited the Sun on June 5-6, 2012. The next transit of Venus will be on 10–11 December, 2117.
2004 transit of Venus (Photo by Leslie Chatfield)
1631 December 6-7
Kepler predicted in 1627 that a transit of Venus would occur in the year 1631, on December 6. Gassendi watched the Sun from the 4th to the 7th of December, but saw no sign of the transit.
The transit actually took place during the night of December 6-7th, and was therefore unseen by European observers, or, as far as we know, anyone.
Gassendi did become the first person to see a planet transiting the Sun, when, on November 7, 1631, he saw Mercury do so, 5 hours after the time predicted by Kepler.
1639 November 24
The first drawing of a transit of Venus, by Jeremiah Horrocks, 1639. The original drawing is 6 inches in diameter.
Kepler supposed that the next transit of Venus would not be until 1761, but the young English astronomer Jeremiah Horrocks, a curate in the village of Hoole, Lancashire, calculated in October of 1639 that there would actually be another on November 24th of that year.
He watched the Sun as often as possible on the 23rd and the 24th (being called away now and then on church business). At 3.15 pm on the 24th, he saw on the Sun (as a projected telescopic image) "a spot of unusual magnitude and of a perfectly circular shape, which had already fully entered upon the Sun's disc on the left". This was the planet Venus. He measured briskly for half an hour, when the Sun set.
Horrock's friend William Crabtree, in Manchester, was thwarted by an overcast sky for most of the day. Just before sunset the clouds broke, and Crabtree, with 'unspeakable delight', saw Venus upon the Sun. He was so awestruck that he gazed on the sight without making any measurements until the clouds returned. These two were, as far as is known, the first persons to see a transit of Venus and the only people who saw this one.
Horrocks devoted some thought to the question of whether the spot he observed was actually Venus, and not a ‘common sunspot’. Sunspots take two weeks to pass from one limb of the Sun to the other, whereas the ‘perfectly circular and uniformly opaque’ spot whose motion he followed traversed a twentieth part of the solar diameter in half an hour; “this fact of itself proves most clearly and incontestably that (the spot) was a very unusual one, and occasioned by Venus alone.”
Jeremiah Horrocks observing the transit of Venus, 1639
From a painting by Eyre Crowe, 1891
1761 June 6
Halley explained his method of finding the distance of the Sun by means of the transit of Venus to the Royal Society in 1716. Once this was known accurately (and observing a transit of Venus seemed the best method of ascertaining it), it would form a yardstick for the precise measurement of the solar system.
William Chapple and a friend observed the transit from Powderham Castle, near Exeter. They were hoping to catch a glimpse of the rumoured satellite of Venus, Neith, which Chapple's friend thought that he had once seen. However, they saw only Venus, so, if Venus had a moon, it was either between the planet and the Sun, or itself transiting its parent planet. Two sunspots, or 'solar maculae' were seen in a line with Venus, but they "did not follow her motion; and, therefore, could not be Satellites."
Le Gentil observing the transit aboard ship, 1761
The French astronomer Le Gentil was compelled by circumstance to observe this transit at sea near the Isle de France (Mauritius) from the deck of the frigate Sylphide, with a telescope of 15 feet focus lashed to a small mast. This was an unsatisfactory arrangement, but he hoped to time the various contacts, when Venus entered and left the Sun's limb. His watch was 'none of the best', and for more precise timing all he had was a sailor turning a sand-glass. Le Gentil made several measurements and timings, but was not happy with them. He decided to remain in the southern hemisphere for eight years until the next transit, in 1769.
Le Gentil had resolved to observe the transit from Manila, and reached the city in 1766, but was ordered to return to Pondicherry. There, transit day was overcast, although the weather had been cloudless for many days previously. Had he remained in Manila he would have had an excellent view, as the skies there were also clear.
The 1761 transit was observed by 176 astronomers at 117 locations in various parts of the Earth, but observational results were disappointing, due to "cloudy weather and other unfavourable causes". Astronomers resolved on more strenuous efforts in 1769.
1769 June 3
The ‘Black Drop’ in 1769
James Woodforde, at Castle Carey, in Somerset, saw the transit of Venus during the evening. He says it happened between 7 and 8 pm. "It appeared as a black patch upon a fair Lady's face. It will not happen again they say, until the year 1874…During the transit it was remarkably cold indeed".
Gilbert White of Selborne also saw it. "Just as the sun was setting the spot was very visible to the naked eye."
Venus passing onto the Sun in 1769, as seen by Captain Cook and Charles Green
Captain Cook observed the transit from the South Sea island of Otaheite (Tahiti), on a 'splendid day’. He wrote, “We very distinctly saw an Atmosphere or Dusky shade round the body of the planet, which very much disturbed the times of the Contact, particularly the two internal ones.” His expedition, combined with observations from Europe, resulted in the first (fairly) accurate calculation of the Sun's distance, 95,300,000 miles. This figure became a 'household word' in astronomy for nearly 40 years.
1874 December 8
Astronomers were wary as to the utility of Venus transits in calculating the distance of the Sun as they prepared for this one. The method needed an accurate timing of first internal contact, the moment when the planet crept completely onto the Sun, but this had proved almost impossible.
The problem was Venus' dense atmosphere, which produced the 'Black Drop'. The planet seemed to be attached to the limb of the Sun by a black ligament until it was well onto the solar disc. In one 1769 observation, a combination of the atmospheres of Earth and Venus turned the planet into a shapeless black smear attached to the edge of a wavering Sun.
The ‘Black Drop’ as Venus passes onto the Sun
Numerous expeditions, instructed in the problems of the black drop, were dispatched all over the world, and most saw at least some portion of the transit. After years of calculation a mean distance from the Earth to the Sun of 92,885,000 miles was announced.
Venus passing onto the Sun in 1874, as seen from Eden, New South Wales, Australia. (Drawing by W.J. MacDonnell). The planet’s atmosphere appeared as a luminous ring of a ’shadowy whitish colour’. When fully on the Sun’s disc, the luminous ring disappeared, and Venus seemed intensely black, like “an auger hole bored through the Sun”, contrasting with some small dusky brown sunspots.
1882 December 6
U.S. Naval Observatory photograph of the 1882 transit
R. S. Ball observed this transit from Dunsink Observatory, near Dublin. The morning of transit day was "about as unfavourable for a grand astronomical spectacle as could be imagined". A north wind was blowing and snow lay several inches deep. The snow continued to fall as the dome was opened and the telescope pointed at the position in the overcast sky where the Sun should be.
The transit had been calculated to begin at Dunsink at 1h. 35m. 48s, Dublin mean time. Just after 1 pm the sky lightened. In the north the snow-clouds began to break up. As the predicted moment arrived, there was a sun-glow in the clouds, though the disc was not yet visible.
Several minutes later, the sun broke through the clouds, and the planet Venus was seen edging onto the solar disc. First external contact had been missed, but the observers eagerly anticipated the first internal contact. But snowflakes, as well as Venus, began to pass before the sun, and this contact, 21 minutes after first external contact, was clouded over. Then a snowstorm set in, and it seemed that would be the last sight of the transit. Then, at 2.30 pm, the clouds began to part again.
Now Venus was completely upon the Sun, and although Ball and his assistant made as many measurements as possible, the Sun and the planet were beginning to quiver and 'boil' through the thick atmosphere near the horizon. Then the clouds gathered once more, and Ball took a last look through the telescope.
"The sun was already beginning to put on the ruddy hues of sunset, and there, far on its face, was the sharp, round, black disc of Venus…(a) pleasing picture, the like of which will not occur again until the flowers are blooming in the June of A.D. 2004."
Don't look at the Sun with the naked eye, through smoked glass, binoculars, telescopes (except by projection), old film negatives, or sunglasses.
