Transits of planets across the disc of the Sun are among the most fascinating phenomena in the Solar System astronomy. As seen from the Earth, transits of only Mercury and Venus are possible. These have held great scientific importance in early telescopic astronomy when the transits enabled astronomers to determine, using triangulation, the solar parallax and thence mean Sun-Earth distance with an unprecedented accuracy. The AU, the Astronomical Unit, one may note, is one of the most basic units in astronomy, used to denote distances, particularly in the Solar System. It is the mean Sun-Earth distance, fixed today from direct ranging measurements and an elaborate planetary ephemeris-fitting, with an accuracy to within a few metres; the value used by the Jet Propulsion Laboratory in the computation of the ephemerides is 1.49597870691 x 108 km.
On an average, there are 13 transits of the planet Mercury in a century. As the orbit of Venus is much bigger than that of Mercury, a transit of Venus is even rarer.
In its course, Venus passes in between us and the Sun and a line-up takes place every 584 days. But, the transit does not happen since the orbit of Venus is inclined at 3.39 degrees to that of the Earth and each time a line-up takes place. However, when the line-up occurs at a place where the orbits cross each other the transit will happen. A transit is difficult to observe since the planets are much smaller than the Sun in angular dimensions. Venus subtends an angle of about 1 arc min compared to 31.5 arc min of the Sun (mean radius 695950 km) at the time of inferior conjunction and so will appear as a small dot against the bright disc of the Sun. That is also the limit of detection by normal human eyes (20/20 vision). Venus in transit can be distinguished from a sunspot only because of its regular shape and the movement across, much faster than the Sun’s rotation. As the smaller disk of Venus moves in front of the Sun’s, the transit is marked by four contacts, namely, at Ingress – the exterior contact (Contact I) and the interior contact (Contact II), and at Egress – the interior contact (Contact III) and the exterior contact (Contact IV). Just around the second and the third contact, it is possible to notice what has been termed as the ‘black drop effect’. This mysterious effect happens at those crucial times when the disk of Venus appears a bit deformed, clinging to the limb of the Sun by a thin column or thread. The breaking of this thread marks the Contact II. The Contact III happens in exactly the reverse order. This effect is a result of seeing effects due to the turbulent atmosphere of the Earth, the equipment optics and solar limb darkening.
The transits of Venus number about 12 in a millennium. These have interestingly a 243-year repetition, with two transits in December 8 years apart, followed 121.5 years later by two June transits 8 years apart. There have been seven transits of Venus since the invention of the telescope. There was no transit in the 20th century after the last transit of Venus that took place a good 130 years ago, on December 06, 1882. In the 21st Century, the first of such lines-up happened on June 8, 2004 followed by the one on June 5– June 6, 2012 and visible over India.
After the telescope, the first of the transits of Venus as predicted by Johannes Kepler (1571-1630 CE) happened on Dec 7, 1631 CE. The French astronomer Pierre Gassendi (1592-1655 CE), who had already watched the transit of Mercury on Nov 7, 1631, tried to observe the transit of Venus the following month from Paris but did not succeed. The accuracy of the prediction in Kepler’s Tabulae Rudolphinae of 1627 on the one hand, the Contact IV was already over at 06:47 UT before the Sun rose at Paris (sunrise 07:34 UT). Looking through Kepler’s tables, the British astronomer Jeremiah Horrocks (1618-41 CE) deduced that there was yet another transit situation, due to fall on Dec 4, 1639 CE. He was able to observe it in Liverpool, as also his friend William Crabtree based inSalford. It was James Gregory in 1663, and later Edmund Halley in 1691, who proposed that one should be able to determine solar trigonometric parallax and deduce an unprecedentedly precise measure of distance to the Sun from the timings of ingress and egress of the transit of an inner planet when observed from different locations on the Earth. Halley died in 1742 CE.
The next transits of Venus were to happen in 1761 and 1769. As the time drew near, the forthcoming transits evoked great scientific interest in Europe to observe these from different parts of the globe. The transits were observed from many observatories in Europe and in addition, many expeditions were mounted to diverse places with a view to gain long baselines. What then followed is history, and well documented.
Guillaume Le Gentil a French astronomer originally set sail in 1760 to come over toIndiato follow the transit of Venus of June 6, 1761 from Pondicherry then in French possession. He was in the French expedition that was part of an international collaboration for the purpose. Caught in the Seven Years’ War between two European superpowers the French and the British, he could not make it for he came to know that the British had taken Pondicherry. Disheartened but not giving up having ventured so far fromFrance, Le Gentil knew that the next transit was due on June 4, 1769 and so determined to stay around and observe it. The intervening period spent in drifting and sails was sufficiently trying but he eventually returned to Pondicherry on March 27, 1768, well in time to be able to observe the transit that to his misfortune was clouded out. His eleven-year voyage to theIndian Ocean to observe the transits of Venus of 1761 and 1769 has been called as the longest lasting astronomical expeditions in history.
On the other side, the British too had prepared for and had observed the two transits from various locations in India. Of particular interest is the 1761 transit, observed from top of the Governor’s house, Fort St George, Madras by the Rev. William Hirst who made a significant observation – of having seen at the moments of ingress a nebulosity about the planet. That in fact is the discovery of atmosphere of Venus, duly recorded in his communication as presented in the Vol. 52 of the Philosophical Transactions of the Royal Society of London. The discovery of atmosphere of Venus has been attributed to Mikhail Lomonosov alone that he made during the same transit observed from the St. Petersburg Observatory.
In India, the transit of 1769 was observed from Dinapoor (25º 27′ N) by Capt Luis Degloss, Captain of Engineers, and from ‘Phesabad, lat. 25º 30′ north’ by Capt Alexander Rose of the 52d Regiment. Capt Degloss had made his observations with three Quadrants and a reflecting telescope. At sunrise it was cloudy but at 5h 20m 32s, ‘the Sun disengaged from the clouds when Venus appeared on the ☉’s disk’; he timed the beginning of the egress at 7 5 22 hrs, end of the egress 7 23 36 hrs. With a telescope and a stop-watch, Capt Rose observed the transit when it was in an advanced stage. He saw the egress and measured at the moment of egress, the timings of the first (at 6 52 25 hrs) and the last contacts (7 10 47 hrs) (the Contacts III and IV respectively), their interval being 18 min 22 sec.
The 1761 transit produced solar parallax values that ranged from 8″.28 to 10″.6. In the 1769 transit observations, the value improved but ranged from 8″.43 to 8″.80. The last figure came close to the modern value (8.”794148). The divergence in results implied large differences in the value of the distance to the Sun. That did not satisfy the astronomical world. The astronomers looked forward to the next transit pair, falling in 1874 and 1882. By this time, the techniques for angular measurements and geo-positions and instrumentation had improved while the Solar System itself had grown bigger with the discovery by Sir William Herschel of Uranus on March 13, 1781 and of Neptune on September 23, 1846 by Johann Galle and Heinrich d’Arrest. The coming transits of Venus gave rise to an excitement and scientific activity of even greater magnitude, all over. Simon Newcomb worked on the four transits results and came up with a value of 8″.794 for the parallax.
In India, the transit of Dec 9, 1874 was observed from several places. As for the following one, the Sun had already set when the Dec 6, 1882 transit commenced. The Madras Observatory made elaborate arrangements to observe the transit of 1874. To recall, it was a private facility erected at Egmore in Madras(now Chennai) in 1786 by William Petrie (d. 1816), an officer with the East India Company. Soon, it was taken over by the Company. The Madras Observatory eventually evolved to the present day Indian Institute of Astrophysics. When Venus transited the Sun, for most part of the event, clouds frustrated the preparations. Well before the transit was to take place, the Government Astronomer Norman R Pogson was concerned that it be observed from more stations inIndia. Failure in observations due to bad weather notwithstanding, Pogson still provided valuable help to other observers stationed elsewhere through telegraphic determination of the respective longitudes. In this connection, C Ragoonatha Chary (1828-80) who was the First Assistant to Pogson deserves an honourable mention. He came from almanac makers family and had joined the Observatory in 1847 when just around eighteen during the time T G Taylor was the Astronomer at the Observatory. As the transit date drew near, Ragoonatha Chary brought out a booklet entitled ‘Transit of Venus’ in English, Kannada and Urdu. With several figures, the booklet beautifully explains the transit to the lay public. The English version was presented as dialogue but the style differed when he presented the versions in the local languages.
Observations of the 1874 transit were made from various stations in India. Capt J F Tennant (1829-1915) who had headed the Madras Observatory during the brief span of 1859-60 took charge of observing the transit from two stations, Roorkee and Lahorewhere Capt George Strahan was sent. From Masauri (Mussoorie) in the Shivalik hills, 6765 ft above the mean sea, the transit was observed by J B N Hennessey (1829-1910) of The Great Trigonometrical Survey. The Rev. H D James observed the transit from Chakrata, a place in the Shivaliks at an elevation of 7300 feet with a telescope of his own – by Smith and Beck, an object-glass 3 1/2 inches and focal length 4 feet. The Belgian Father Eugene Lafont (1837-1908) who taught at the St. Xavier’s College in Calcuttahad participated in Italian astronomer Pietro Tacchini’s expedition to observe the transit of Venus on Dec 9, 1874 from Muddapore (Madhupur) in Bihar, discovering in the process presence of water vapour in the atmosphere of the planet. Capt. A C Bigg-Wither (1844-1913), an Engineer with the Indus Valley Railway, observed the transit of Venus on December 9, 1874 from his observatory in Mooltan in Punjab (Multan, now in Pakistan) where many gathered to watch the event.
Another highlight of the 1874 transit event belongs to A V Narsinga Rao (1827-92) who had inherited a private astronomical observatory from his father-in-law Jugga Row and carried out observations lifelong apart from initiating telescopic photographic work and maintaining the time service. Earlier at this observatory, Narsinga Rao (Anglicized as Nursing Row) had observed the transit of Mercury on Nov 5, 1868 with a telescope having a 4.8 inch object glass. He observed the transit of Venus on Dec 9, 1874 with a 6 inch clock driven Cooke equatorial, 7 ½ feet focal length; clouds allowed only the last thirty minutes of the transit including the egress of Venus to be observed. He noted the observed sidereal times and just after the egress was over noticed a slight indentation in the solar limb that he said was perhaps because of the atmosphere of the planet.
The transit of Venus 2004
The first of the two transits of the planet Venus in the 21st Century took place on June 8, 2004. The planet’s passage in front of the Sun was well observed from a large part of the globe and extensively covered in the media. In India, the transit was witnessed live by thousands who visited the various observatories, institutions and planetaria where arrangements had been made for public viewing. Not just the scientists and students, the visitors included people of all ages, from all walks of life and from near and far.
At the Kodaikanal Observatory of the Indian Institute of Astrophysics, the ingress and egress were both clouded out on the day. Still, 10 broadband images and over 40 H-alpha images were taken at the Observatory that could be used to measure the effect of the atmosphere of Venus on the background chromospheric radiation. At the Solar Tower Tunnel Telescope, 35 high-resolution digital images of the full Sun were taken where a 34 cm image of the Sun is formed. At the Koramangala Campus of IIA, an arrangement was made to view a 30 cm image of the Sun on a screen in a tent formed with a 20 cm coelostat system. The event began at 10:45:31 IST. The sky was generally cloudy throughout the day but in between, the event was possible to be watched whenever the cloud cover got thinner or cleared up for a while. A fuller description of the event, as observed from Bangalore by R C Kapoor and from Kodaikanal by S P Bagare and S S Gupta, is available in the IIA Annual Report 2004-05.
The Transit of Venus 2004 in the news (clockwise from top left: Vijay Times May 28, The Times of India Jun 9, Hindustan, Deccan Herald Jun 8, Vijay Times Jun 9; in the middle – The Times of India Jun 8, 2004; all papers from Bangalore except for Hindustan from Delhi).
The transit of Venus 2012
The next transit of Venus falls on June 5– June 6, 2012. As an event of great scientific, educational and historical significance, its value is immense. The whole event can be witnessed from north-west parts of Canada and the US, eastern and northern parts of Asia, the eastern half of Australia and the Western Pacific Ocean. It shall be visible over India, with the transit in progress when the Sun rises. The 2004 transit lasted 6;13 hours. The 2012 transit shall last 6:40 hours. In the 2004 transit pictures, we do not find any noticeable sunspots. However, in 2012, with the Sun getting increasingly active, the planet might have some for company.
An observation of the Sun should be carried out with due caution and preparations only. Use right kind of filter to watch the transit of Venus naked eye. To watch with binoculars or a telescope, mount these properly on the objective.
Venus will have a date with the Sun again in 2117 on December 11 and in 2125 on December 8, and so on.
*For details, see http://prints.iiap.res.in/handle/2248/5757
R.C. Kapoor 