The 1874 transit of Venus: New Zealand's first foray into international astronomy

Abstract

The nineteenth century transits of Venus provided astronomers with the opportunity to use these rare events to calculate the solar parallax, and New Zealand played a vital role. Apart from activities by local astronomers, international expeditions from England, France, Germany and the USA were based in New Zealand for the 1874 transit. Although many observing sites were clouded out on the vital day, New Zealand-based observations contributed to the determination of the solar parallax and led to further local observations during the 1882 transit of Venus.

Keywords:

• New Zealand
• 1874 transit of Venus
• Auckland Islands
• Burnham
• Chatham Islands
• Queenstown

Introduction

A transit of Venus occurs when Venus is seen to cross the disk of the Sun. Venus is closer to the Sun than the Earth is and orbits the Sun in 225 days compared to our 365 days, but because the orbital inclinations of the two planets are not identical Venus normally appears to pass above or below the solar disk when viewed from the Earth. Thus, transits of Venus are rare events that happen when the alignments of Venus and the Earth are just right, and they occur in pairs 8 years apart, with more than a century between successive pairs. The first known observations of a transit of Venus occurred in 1639 (Horrocks 2012), and since then they have been observed in 1761 and 1769, 1874 and 1882, and in 2004. The next transit is due on 06 June 2012. During the eighteenth and nineteenth centuries, precise observations of a transit of Venus were viewed by astronomers as the most effective means of determining the solar parallax (i.e. half the angular equatorial diameter of the Earth as viewed from the Sun), and hence pinning down that fundamental yardstick of solar system astronomy, the ‘astronomical unit’ (i.e. the distance from the Earth to the Sun).

New Zealand has had an intimate association with transits of Venus, for it was the June 1769 event that brought James Cook to the Pacific. After successfully observing the transit from Tahiti (Orchiston 2005) he went on to re-discover New Zealand and circumnavigate its two main islands (Orchiston 1998).

Despite the plethora of observations made worldwide during the 1761 and 1769 transits (Woolf 1959), the values derived for the solar parallax were too wide-ranging (from 8.43″ to 8.80″, representing a variation in the distance of the astronomical unit of c. 6.57×10⁶ km), and this led to intense interest in the 1874 transit (Clerke 1893). Elsewhere I have noted that ‘given their fortuitous location on the globe, Australia and New Zealand were collectively to contribute in a manner that was far out of proportion to their combined geographical area and population’ (Orchiston 2004:219). Since Cook's first, second and third voyage stop-overs, scientific astronomy had developed slowly in New Zealand, and overseas astronomers had shown remarkably little interest in New Zealand, so the 1874 transit was to be its first foray into international astronomy.

The international expeditions

Drawing on some of the problems encountered during the eighteenth century transit programs, it was realised that for the 1874 event improvements had to be made in instrumentation and in longitude-determination of the various transit sites. One of the notable developments in nineteenth century astronomy was the emergence of photography, which was to play a key role during the 1874 transit (Lankford 1987).

The importance of the 1874 transit and New Zealand's ideal observing situation (weather permitting, the entire transit would be visible) attracted American, British, French and German expeditions to our antipodean shores and adjacent islands.

The Americans

The USA based two transit parties in New Zealand (Newcomb 1880), one at Queenstown on the shores of Lake Wakatipu in the South Island (see Fig. 1 for localities mentioned in the text) and the other on the Chatham Islands.

Figure 1  New Zealand localities mentioned in the text.

The Queenstown transit party was led by Chief Astronomer Dr CHF Peters (Dick 2003:255). As at other US stations, their primary instruments were a horizontal photographic telescope (Fig. 2), a broken-tube transit telescope and a 5-inch Clarke refractor (Orchiston et al. 2000).

Figure 2  The top diagram is a schematic side elevation showing the layout of the horizontal photographic telescope used by the Americans at Queenstown and Whangaroa. On the right is the clock-driven heliostat that tracked the Sun and directed sunlight via a collimator to the photographic plate-holder in the prefabricated ‘Photographic House’ on the left. Extending from this building towards the heliostat is a tube, and above it a framework, designed to shield the incoming rays of the Sun from interference. The two panels below show close-ups of the photographic plate-holder (left) and the heliostat (right) (after Orchiston et al. 2000).

At 5 am on 09 December, the day of the transit, the sky was cloudy and stormy, but the Sun miraculously appeared just 2 minutes before first contact, ‘permitting observations of Equatorial and photographs to be made without interruption for 1¹/₃ hours about. After that only at intervals, up to within 16 minutes before computed time of 3d contact. The Sun then remained invisible until … after 4th contact‘ (Peters 1874). In all, 237 photographs were obtained, 178 of the ingress contacts and 59 of Venus on the Sun's disk (Newcomb 1881:439–440). Observing with the Clark refractor, Peters obtained a series of micrometric measures of Venus's position relative to the Sun's limb (Newcomb 1880).

In a bid to extend the longitudinal spread of the US southern hemisphere transit sites, the other New Zealand-based transit party was stationed at Whangaroa on the Chatham Islands (Dick et al. 1998), led by Chief Astronomer Edwin Smith from the U.S. Coast Survey (Dick 2003, p. 255). On this occasion the weather was not so kind, and although the first and second contacts were observed, only eight photographs were obtained (Dick 2003:259).

The British

The British, led by Major HS Palmer, set up their sole New Zealand transit station at Burnham, near Christchurch (Palmer 1874a). Their principal astronomical instruments were a 15.2-cm (6-in) Simms equatorial, a transit telescope (also by Simms), a Dallmeyer photoheliograph (Fig. 3), a small azimuth instrument and an astronomical clock. With the instruments came prefabricated equatorial, photoheliograph, transit and altazimuth huts (Simmons 1880).

Figure 3  The Dallmeyer photoheliograph used by the British party at Burnham (courtesy: RAS Library).

To allow for possible inclement weather at Burnham, Palmer had Lieutenant Crawford and Captain Williams from the Merope (which brought the British party out to New Zealand) set up a satellite observing station at Naseby, in dry, sunny, northern Otago (The transit of Venus, 1874a). This was furnished with a 10.2-cm (4-in) Simms refractor and a portable transit telescope and astronomical clock (Palmer 1874a, 1874b).

On the morning of 09 December, unfavourable weather at Burnham prevented observation of the first contact, but when the clouds thinned Venus ‘was then seen to have advanced apparently about three-eighths of her diameter on the Sun. Both Sun and planet could only be just made out through the clouds, without any coloured shade to the eye-piece’ (Airy 1881:493). Palmer attempted to use the Simms refractor to take a series of micrometric measurements of the transit through gaps in the clouds but was unsuccessful. Just before second contact, ‘the Sun showed again, when the cusps were about one-twentieth of a diameter apart, and connected by a dimly marked ligament, not nearly so sharp as the “black drop” of the model in full sunlight’ (Airy 1881:494), and 3.5 seconds later, ‘the ligament seemed to undergo a change in depth of colour, but clouds prevented me from seeing whether any streak of light connecting cusps played across it’ (ibid.). Just 3 seconds later clouds intervened, preventing any further observations of the ingress phase. Nearly 15 minutes after the estimated time of second contact, the clouds cleared sufficiently for Palmer to attempt further micrometric observations, but the results were ‘of little use’. Clouds and rain prevented observations of the remainder of the transit, but 10 minutes after fourth contact the Sun emerged and continued to shine brightly until near sunset. While Palmer had only limited success at Burnham, inclement weather prevented Lieutenant Crawford from making any observations at Naseby (Airy 1881:484).

The French

The French sited their New Zealand transit station on Campbell Island, but despite all their preparations the weather prevented any observations from being made (Bouquet de la Grye 1882; Filhol 1885). Given the prevalence of inclement weather one could be forgiven for wondering why the French and their German colleagues chose these subantarctic islands for their observing stations. The reason was so as to maximise the latitudinal spacing between the official northern and southern hemisphere observing sites.

The Germans

The Germans, led by astronomers Hugo Seeliger and Wilhelm Schur, set up their transit station at Port Ross, on the Auckland Islands (Auwers 1888). Their instruments were: a 7.6-cm (3-in) Fraunhofer heliometer; a 10.7-cm (4.2-in) Steilheil photoheliograph; three small refractors; a Pistor & Martin transit telescope; and time-keeping and meteorological equipment (ibid.; Duerbeck 2004).

Light rain fell before the transit started, but by the time the ingress contacts occurred this was replaced by passing clouds. Soon after second contact the clouds disappeared and the remainder of the transit was visible, permitting Seeliger to observe with the heliometer, Schur with the largest of the refractors and Wolfram, one of the photographers, with one of the smaller ones. Meanwhile photographer Hermann Krone exposed 115 plates with the photoheliograph (Auwers 1888:202–206), most of which showed Venus on the Sun's disk. Towards third contact, both Seeliger and Wolfram saw the ‘black drop’ (Auwers 1888:199, 201).

The local response

The 1874 transit of Venus promised to be the most important astronomical event in New Zealand since the European settlement of the country, and it attracted enormous public attention. Interested members of the general public were introduced to it through the booklet ‘December 9, 1874. The transit of Venus and how to observe it’, which was penned by Arthur Stock (1874), the Astronomical Observer at the Colonial Observatory in Wellington.

Government initiatives

With support from the New Zealand Government, Major Palmer arranged for a network of official subsidiary stations to be set up and manned by New Zealand observers in Auckland, Thames, Wellington and Dunedin. All were supplied with time, by telegraph, from Burnham (The transit of Venus 1874a), and all, unfortunately, were clouded out and did not see the transit.

The amateur astronomers

Amateur astronomers in Auckland, Thames, Featherston, Wellington, Nelson, Christchurch and Dunedin readied themselves for this once-in-a-lifetime event, but in most locations inclement weather conspired to dash their plans (see The transit of Venus 1874b).

To show just how unpredictable the weather could be we need only examine the situation in Auckland. Although Captain Heale was prevented from observing the transit altogether, another transit team located in a different Auckland suburb had some success. They timed the first contact, then saw the black drop, and during the ingress phase secured ‘A good many sun pictures of great nicety …’ (The transit of Venus 1874b).

Sunny Nelson was one of the few locations in New Zealand where the transit was also observed, but although both ingress and egress contacts were visible, during the intervening period clouds and rain prevented observations being made (The transit of Venus 1874c).

Discussion

The outcome of the 1874 observations

While inclement weather prevented the French party on Campbell Island from seeing the transit and this same fate befell many of those on the North and South Islands of New Zealand, some of the astronomers in Auckland, Canterbury and Otago and the overseas parties on the Chatham Islands and Auckland Islands did meet with varying degrees of success. So New Zealand was able to play a role in the solar parallax calculations.

In a long paper, Colonel Tupman (1878:456) concluded that the British 1874 transit of Venus observations ‘only enable us to determine that the solar parallax probably lies between the values of 8.82″ and 8.88″ …’, which was basically in accord with the value of 8.883±0.034″ obtained by the Americans (Todd 1881) and figures of 8.810±0.120″ and 8.8796±0.0320″ reported by the Germans (Auwers 1888), in part on the basis of observations made in New Zealand.

The 1882 transit

On 07 December 1882 successful observations were obtained by British and US parties based at Burnham and Auckland, respectively, by the Government observers stationed in New Plymouth, Martinborough, Wellington and Clyde, and by amateur astronomers in Auckland, Thames, New Plymouth, Martinborough, Wellington, Nelson, Christchurch and Dunedin.

Thus, in 1882 New Zealand played a vital role in providing observations that yielded further solar parallax values. Using data from different contacts, Stone (n.d.) obtained values of 8.827±0.051″, 8.882±0.045″ and 8.942±0.047″ for the solar parallax from all of the international ‘British’ observations (including those by New Zealand observers), while Harkness (1889) derived a final figure of 8.842±0.0118″ from the full suite of US observations. These various values should be compared with the currently accepted figure of 8.794148±0.000007″ adopted by the IAU in 1976.

The subsequent blossoming of New Zealand astronomy

The two nineteenth-century transits of Venus served as catalysts that led to the rapid development of New Zealand astronomy. Amateur astronomers increased in number; the first astronomical societies were formed; large historically significant refractors were obtained from England for Wanganui and Napier; a local amateur telescope-making tradition was established; pioneering efforts took place in astronomical photography, and important contributions were made to international solar and cometary astronomy (McIntosh 1970; Eiby 1978; Orchiston 1983, 1993, 1995, 1998, 2001, 2002a, 2002b).

Concluding remarks

The 1874 and 1882 transits of Venus ‘marked the end of one astronomical era and the birth of another. They were the last transits used in a serious attempt to resolve the solar parallax problem, before other methods gained favour. They also marked the first internationally co-ordinated assault on a major astronomical problem using the emerging technology of photography. Photography and its investigative “handmaiden”, spectroscopy, would quickly become the indispensable tools of the “new astronomy”, astrophysics’ (Orchiston 2004:300).

The 1874 transit of Venus also marked a milestone in New Zealand astronomy as it was the first time that this nation was party to a major international astronomical venture. Despite far from cooperative weather, New Zealand did play a part in pinning down the solar parallax and this in turn encouraged local and international astronomers to use New Zealand as a base for observations of the 1882 transit. In the following three decades there were important developments in New Zealand astronomy, and many of these can be traced back to the two transits of Venus.

Acknowledgements

I thank Peter Hingley (RAS Librarian) for providing Figure 3 and Professor Boonrucksar Soonthornthum for providing me with a Visiting Professorship at NARIT where I was able to complete this paper.