Preparing for the transit of Venus: Then and now

Eos,Vol. 85, No. 21, 25 May 2004 Heller, R.,R.T.Merril,and PLMcFadden (2002),The variation of Earth's magnetic field with time, Phys. Earth Planet.Int., 131,237-249. McElhinny,M.,and R.Larson (2003),Jurrasic dipole low defined from land and sea data, Eos, Trans. AGU, 84,362-366. Morales, J., A. Goguitchaichvili, and J. Urrutia-Fucugauchi (2003),An experimental evaluation of Shaw's paleo-intensity method and its modifications using Late Quaternary basalts, Phys. Earth Planet., 57,11-19. Solodovnikov, G.M. (1998),The intensity in the Eocene Geomagnetic field, Izv. Acad. Nauk, SSSR, Phys. Solid Earth, 70,865-869. Shaw, J., G. J. Sherwood, A. E. Musset,T. C. Rolph, K.V Subbarao,and PV.Sharma (1991),The strength of the geomagnetic field at the Cretaceous-Tertiary boundary: Paleo-intensity results from Deccan Traps (India) and Disco lavas (Greenland), J. Geomag. Geoelectr., 43,395-408. Tarduno, J. A., R. D. Cottrell, and A.V Smirnov (2002), The Cretaceous superchron geodynamo: Observations near the tangent cylinder, Proceedings of the National Academy of Sciences, 99,14,020-14,025. Tauxe,L.,and H.Staudigel (2004), Strength of the geomagnetic field in the Cretaceous Normal Superchron: New data from submarine basaltic glass of the Troodos Ophiolite, Geochem., Geophys., Geosyst., 5(2), doi: 10.1029/2003GC000635. Thomas, D. N., and A. J. Biggin (2003), Does the Mesozoic Dipole Low really exist?, Eosjrans.AGU, 84,97,103-104. Preparing for the Transit of Venus: Then and Now PAGES 2 0 9 , 2 1 3 On 8 J u n e 2004,Venus will pass directly between Earth and the Sun. Local weather conditions permitting, this "transit of Venus" will b e visible in its entirety over much of Europe and Asia, from London to Beijing. In London, the times of the first and last contacts will b e 05:19:52 and 11:23:16 Universal Time, respectively—just over 6 hours from start to finish. In Sydney and Tokyo, only the ingress of the planet will b e visible before sunset, and residents of Buenos Aries,Toronto, and New York will have to b e up early to s e e the final contact. Another transit will o c c u r on 6 J u n e 2012, and then there will b e no more transits until 11 December 2117 and 8 December 2125 [Moor, 2 0 0 0 ] . Venus is nearly four times as large as the Moon. It is the nearest planet to Earth, but at its closest approach, it is still a hundred times more distant than the Moon, and its angular extent, as seen from Earth, is only about onethirtieth that of the Moon.To the naked eye, the planet will appear as no more than a small black dot on the brilliant face of the Sun.Viewed through a telescope, the planet will b e seen as a black disk, which will cover about 0.1% of the solar disk (Figure 1). Edmond Halley ( 1 6 5 6 - 1 7 4 2 ) first b e c a m e interested in the transits of planets when he observed a transit of Mercury from the island of St. Helena in 1677. It occurred to him that transit observations might b e used to obtain an estimate of the distance between Earth and the Sun, referred to as the astronomical unit, if the times of the four contacts of the limbs of the Sun and Venus (two at ingress and two at egress) were recorded by widely separated observers. After analyzing the geometry of transits more carefully, Halley concluded that Mercury was too far from Earth to yield an accurate estimate of the astronomical unit and focused on the transits of Venus. There would b e no transits of Venus during Halley's lifetime, but he worked out the details of the transit that would o c c u r in 1761 and B Y WILLIAM E. CARTER AND MERRI SUE CARTER challenged astronomers to collaborate in collecting observations [Maor, 2000] .A number of parties s u c c e e d e d in observing the transit of 1761, but the resulting estimates of the solar parallax—the radius of Earth divided by the astronomical unit, which yields angular units—varied from 8.3 to 10.6 seconds of arc, a scatter greater than 25%. Though the results of the first international observing campaign were disappointing, teams were again dispatched to observe the transit of 1769. Among the British parties was o n e led by Captain J a m e s Cook ( 1 7 2 8 - 1 7 7 9 ) . He observed the transit from Papeete,Tahiti.The Fig. 1. This photographic Sun in 1882. (Courtesy Author Information Avto Goguitchaichvili, Jaime Urrutia-Fucugauchi, Luis M.Alva-Valdivia,and Juan Morales, Instituto Geofisica, UNAM.Ciudad Universitaria, Mexico; and Janna Riisager and Peter Riisager, University of California, Santa Cruz For additional information, contact Avto Goguitchaichvili, Laboratorio de Paleomagnetismoy Geofisica Nuclear,Instituto de Geofisica, UNAM, Ciudad Universitaria, 04510 Mexico D£ Mexico; E-mail: [email protected]. weather was excellent, but Cook was not altogether pleased with the observations, because he and his fellow observers could not agree on the exact times of the contacts. Still, the observations collected in 1769 were more consistent than those of 1761, yielding solar parallax values from 8.43 to 8.80 seconds of arc, a range of about 5% [Maor, 2 0 0 0 ] . By the time that the next transit of Venus was to occur, in 1874, the U.S. Congress decided that American astronomers should join in the international effort. A five-member commission was appointed to organize the American effort: Rear Admiral Sands, superintendent of the Naval Observatory; Benjamin Peirce, superintendent of the Coast Survey; Joseph Henry, president of the National Academy of Sciences; and Simon Newcomb and William Harkness, professors of mathematics, Naval Observatory. plate shows an image of Venus near the halfway of the U.S. Naval Observatory.) point on its transit of the Eos, Vol. 85, No. 2 1 , 2 5 May 2004 Simon Newcomb ( 1 8 3 5 - 1 9 0 9 ) was appointed as the secretary of the commission, and effectively b e c a m e the chief scientist for the enterprise [Newcomb, 1903]. Photography was revolutionizing astronomy, and Newcomb set out to design an instrument that would fully exploit the power of the new technology. He sought advice from Lewis M. Rutherfurd ( 1 8 1 8 - 1 8 9 2 ) and received the following reply:"The photograph of the sun will have a greater or less diameter by many s e c o n d s of arc, according to the energy of the rays which have produced the image; and this discrepancy may b e produced by a change in the aperture, in the length of the time of exposure, in the transparency of the atmosphere, in the hour of the day, or in the sensibility of the chemicals. "Perhaps you will b e tempted to say if this b e true, what reliance can b e placed upon the results of photography? I should answer, that the sun has no sharply defined outline, even to the eye, but, in its best state, is an irregular, seething, ever-restless object, utterly unfit to b e the starting point for measures of precision; and that, while the eye is confined, in its attempts at measures, to s o m e small part of the sun's limb, the photograph of the whole sun c a n b e placed upon the stage of the micrometer, and accurately centered with reference to the average of the whole contour; and thus escape the errors sure to b e the result of measures based upon local bisections. The image of Venus ought to b e sharp and capable of easy and accurate measurements. Its place should b e compared with the center of the sun, and not with the limb." [U.S. Congress, 1872]. Rutherfurd's description of the Sun explained well why observers rarely agreed on the precise times of the contacts. However, with a properly designed photographic instrument, hundreds of images could b e collected during the time it would take Venus to cross the face of the Sun, and locations of the planet could b e measured under laboratory conditions. Considering the extant stages, microscopes, and micrometers, Newcomb concluded that the image of the Sun should b e roughly 10 c m in diameter to obtain the most accurate results.That implied an effective focal length of about 12 m,but the aperture need b e only about a decimeter in diameter. Based on the work of Joseph Winlock ( 1 8 2 6 - 1 8 7 5 ) at the Harvard College Observatory, Newcomb [U.S. Congress, 1872] proposed that a heliostat and fixed telescope b e used (Figure 2 ) . To reduce the transit observations, the precise astronomic latitudes and longitudes of the observing stations had to b e determined. Transferring time accurately to remote stations remained a formidable problem.The most direct approach was to use ensembles of chronometers to directly carry time to the remote stations. Unfortunately even the best chronometers lost or gained enough time during the months-long sea voyages to the remote stations that the mean of several chronometers could still b e in error by seconds. Another approach, for sites near telegraph facilities, was to transfer time signals through the telegraph lines. George Biddle Airy (1801-1892) reported:"I have had a good deal of experience with telegraphic longitudes, and I dislike them; I would rather spend a long time for the determination of longitudes by independent methods than trust myself to all the imperfections and disappointments arising from a combination of telegraphic communications..." [Airy, 1874]. The most c o m m o n "independent methods" were observations of stellar-lunar distances and lunar occultations. In both cases, the accuracy of the longitude determined depended on the accuracy of the lunar ephemeris. A c h e c k of the best ephemeris against recent observations revealed errors far larger than could b e tolerated, and Newcomb set about trying to identify and fix the sources of the errors. Eventually, he concluded that the best he could do in the time available was to add "empirical corrections" to the ephemeris, to provide accurate values for the period of the transit [US. Congress, 1872] (Figure 3 ) . The results of the 1874 transit observations o n c e again proved disappointing.Values of the astronomical unit derived from the transit observations scattered more than 1%. Newcomb was convinced that a more accurate value could b e derived by combining the velocity of light and the aberration of light. During 1880 and 1881, Newcomb c o n d u c t e d experiments in Washington, D.C., to more accurately determine the velocity of light [Carter and Carter, 2 0 0 3 ] . When he c o m b i n e d his result (299,860 km/s) with a value of 20.942 seconds of arc for the aberration of light, determined by Magnus Nyren (1837-1921), Newcomb found a value of 149.59 million km for the astronomical unit—only 0.005% smaller than the value accepted today [Newcomb, 1883].As the transit of 1882 drew near, Newcomb argued that it would b e a waste of money for the United States to deploy teams globally and suggested that American astronomers should limit their participation to the less costly effort of collecting observations in their h o m e country However, o n c e it was a n n o u n c e d that American teams would b e deployed globally Newcomb decided to lead an observing party to South Africa. On 10 D e c e m b e r 1882, N e w c o m b wrote to his w i f e / T h e great event has c o m e off and I have seen a transit of Venus under conditions about as favorable as it is possible for a human being to have—cloudless sky and the finest definition. And yet, I am not quite happy over it, and have b e e n worried by s o m e things. First, there was a strange disagreement...to the time of contact. In the next p l a c e the photographs show the reflector to have b e e n a little warped by the sun's rays while the operation was going on. It is not probably a defect of real importance, and yet I never felt so strongly the want of having another c h a n c e . Not till the year 2004, when our children's children will have passed away, will another c h a n c e b e available." [Library of Congress, 1921]. Ten American teams collected 1380 "measurable" photographic plates at six stations in the United States—Washington D.C.; Cedar Key Florida; San Antonio,Texas; Cerro Roblero, New Mexico; Lick Observatory, California; and Princeton, New Jersey—and four stations in the southern hemisphere—Wellington, South Africa; Santa Cruz, Patagonia; Santiago, Chile; and Auckland, New Zealand. Reducing and analyzing the transit observations, as well as the latitude and longitude observations collected at each station, took William Harkness ( 1 8 3 7 - 1 9 0 3 ) and his assistants more than 5 years to complete. Eos, Vol. 85, No. 2 1 , 2 5 May 2004 On 11 O c t o b e r 1888,Harkness a n n o u n c e d that they had found a value for the solar parallax of 8.847 s e c o n d s of arc, corresponding to a distance from the Earth to the Sun of 148.672 million km. During the following year, Harkness and his assistants refined their analysis and revised their estimate of the solar parallax downward to 8.842 s e c o n d s of arc—still too large by more than four hundreds of a second of arc [D/c*, 2 0 0 3 ] . The U.S. Congress had appropriated $275,000 specifically for the transit of Venus observing program—a huge investment in science during an era in which there was no federal i n c o m e tax. In contrast, only $5,000 was appropriated for Newcomb's velocity of light experiments. Ironically, the value of the astronomical unit derived from the velocity of light experiments was nearly two orders of magnitude more accurate than that derived from the transit of Venus observations. References Airy, G. B. (1874), Preparations for the Observation of the Transit of Venus, Monthly Notices of the Royal Astronomical Society, Vol. XXXV, No. 1,1-10, Royal Astronomical Society, London. Carter,WE.,and M.S.Carter (2002),The NewcombMichelsonVelocity of Light Experiments,Eos,Trans. AGU, 83(37) ,405,410. Dick,S.J. (2003),*% and Ocean Joined, The U.S. Naval Observatory 1830-2000, Cambridge University Press, New York. Library of Congress, The Simon Newcomb papers (1921), Manuscript Division, LOC, 101 Independence Ave. S.E., Washington, D.C. Maor,E. (2000), June 8,2004:Venus in Transit, Princeton University Press, New Jersey F/g. 3. The U.S. transit of Venus party prepares the heliostat Indian Ocean. (Courtesy of the U.S. Naval Observatory.) Newcomb,S. (1883), Measurements of the Velocity of Light, vol. II, pt. III., p. 4512, U. S. Naval Observatory, Washington, D.C. Newcomb,S. (1903), The Reminiscences of an Astronomer, Houghton, Mifflin and Company, Riverside Press, Cambridge, Massachusetts. U.S. Congress, Papers relating to the transit of Venus in 1874 (Part l),from a commission authorized by Congress in 1872 to organize the U.S. effort to track the planet's 1874 transit, Government Printing Office, Washington, D.C. Widespread, Low-Level Contaminants Present in Many U.S. Streams and Rivers PAGE 2 1 0 A federal water quality monitoring program has found that widespread levels of contaminants at very low concentrations are present in many U.S. river basins and aquifer systems. Mixtures of chemical compounds, degradation or "breakdown" products, and seasonal pulses of chemicals are also present in these water bodies, according to scientists with the U.S. Geological Survey The scientists provided this information during a 14 May briefing on Capitol Hill, which USGS used to announce the release of a program report, "Water Quality in the Nation's Streams and Aquifers - Overview of Selected Findings, 1991-2001." The detection of low levels of contaminants— including pesticides, nutrients, metals, and gasoline-related compounds—does not automatically translate into human or aquatic health impacts, according to the USGS' National Water Quality Assessment Program, known as NAWQA. Additionally, the program has found that U.S. rivers and streams generally are suitable for irrigation, supplying drinking water, and other uses. However, USGS scientists acknowledged that the lack of drinking water standards and guidelines for many contaminants makes it difficult to assess their potential impacts. Timothy Miller, chief of the USGS Office of Water Quality said,"Based on what [chemicals are] currently being used, we know a fair amount in terms of what is present in the environment. We don't often know what the impact is on human health or on aquatic health." He said there is also a "vast unknown world" concerning potential impacts from mixtures of compounds, breakdown compounds, and seasonal spikes, which developed criteria often do not take into account. Miller said that of an estimated 60,000 chemicals in use in the U.S., about 5,000 to 10,000 are evaluated and regulated to s o m e degree, and that industry probably is developing thousands of new chemicals annually He said USGS looks at the amount of active ingredients in chemicals and how widespread the chemicals may b e used in determining whether to evaluate and regulate them. NAWQA has found that at least one pesticide was present in about 94% of water samples taken, 90% of fish samples from streams, and Author at a station on Kerguelen Island, Information William E. Carter and Merri Sue Carter For additional information, contact William E. Carter, Department of Civil and Coastal Engineering, University of Florida, Gainesville; E-mail: [email protected]; Merri Sue Carter, U.S. Naval Observatory, Earth Orientation Department, Washington, D.C; E-mail: msc@ maia.usno.navy.mil about 55% of shallow wells sampled in agricultural and urban areas, according to the USGS report. The report notes that at least o n e pesticide guideline established to protect aquatic life was e x c e e d e d in about 93% of urban streams sampled.Volatile organic compounds—such as trichloroethene and methyl tert-butyl ether (MTBE)—were detected in about 90% of monitoring wells sampled beneath urban areas, though only in about 20% of wells beneath agricultural areas. Individual compounds seldom o c c u r alone, and breakdown components frequently are as c o m m o n in the environment as parent compounds, according to the report. Three general factors can contribute to the degradation of ecological health: contaminated water, toxics in sediment, and physical alterations to streams that c a n result in changes to water flow, temperature, and other key indicators, the report noted. Also, natural features—including geology, climate, and soils—can affect the transport of chemicals and can "result in different contaminant concentrations among basins that have similar land-use settings and c h e m i c a l use." The report noted the benefits of urban planning, watershed protection, and land management in minimizing c h e m i c a l concentrations