THE TRANSIT OF VENUS.

Otago Witness, Issue 1160, 21 February 1874, Page 4

THE TRANSIT OF VENUS.

We were last week favoured with a view of diagrams prepared by Mr A. H. Ross, Bond street, showing the transit of Venus across the Sun, as it will be seen on December 9th, 1874, in this locality — that is, provided there is nothing in the shape of masses of clouds to impede the vision. The diagrams will be on view in the Athenaeum from this evening, and for the convenience of those who would like to refer to them, we furnish the following particulars : —

Diagram No. 1 represents the sun's place in the heavens. As to the letters on the diagram, Z C N H, represents the meridian place of observation; H O represents the horizon; S P, thepolaraxis, the South Pole being elevated ; EE, the equinoctial ; ZN, the prime vertical ; Z being the zenith, and N the nadir of the observer ; aaa a a represent circles of altitude ; d circle of declination ; hour circles are represented by the curved lines converging in S and P ; azimuthal circles are represented by curved lines converging in Z and N. A ruler laid from S to where the hour circle intersects the equinoctial will cut the meridian at a point, the distance from which, to E will give on the scale of chords the hour anale at each phase of the phenomena, which being converted into time, will be the time past noon. A ruler laid from Zto a point where the azimuthal circles intersect the horizon will cut the meridian at a point, the distance from which to O or H will, on the scale of chords, give the sun's azimuth or distance from the south or north points of the horizon.

Diagram No. 2 shows the path of Venus across the sun's disc, and the appearance of which will not differ appreciably to ordinary observers in any part of the Colony. The hours and minutes are given in scale, and-the diagram sufficiently explains itself.

Ihe diagrams have been constructed for the position of the Dunedin Meteorological Observatory, the longitude of which is lldeg. 22min. and 4-ec. East; the latitude 45deg. 52rnin. llsec. South. British stations for the observations of the transit are proposed at Woahoo, Kerguelen Island, Rodriguez, Canterbury, New Zealand, and Alexandria. Lord Lindsay intends to occupy a station at Mauritius. Russia occupies a station at Nertchinsk, North Siberia ; the Germans at Tchefoo, China ; the French at Campbell Island ; the Americans at Auckland Island. Many other stations are suggested as being suitable, seveial being withiu the Autarctic Circle. Two of these (says Mr Proctor, hon. secretary of the Royal Astronomical Society) ought to be occupied, by Great Britain. The elements of the transit, with the times of external and internal contact of Venus at ingress and egress, as referred to the centre of the Earth, with equations and reduction to the surface, are given at page 434 of the Nautical Almanac for the current year. Particular-) of the transit at forty-six stations, selected for observation by Great Britain, France, the North German Confederation, and Russia, are given in an appendix to the same Almanac. The last three transits of Venus took place on the following dates — December 4, 1639 ; June 5, 1761 ; June 3, 1769. The next transit will take place on December 9 of this year, and the one after that on December 7, 1882. The transit of Venus in 1882 is the last that anyone now or then alive on earth may expect to see in the mortal state, as the next will not occur till 2004, being 122 years afterwards, so that many great astronomers yet to come, like many in the past, may not have the opportunity during a long life of beholdiaga transit of Venus. After the transit in 2004, the next is to take place in 2012. The last transit of Venus took place in 1769, being 105 years ago, so that it may be pretty safely said to be boyond the memory of the oldest inhabitant on the face of the earth. If no living person in this day has seen the transit of Venus, at least for purposes of observation, the same is also to be said of the greatest astromers'of the past. Copernicus, Tycho Brahe, Kepler, Sir I. Newton, with Flamstead, Halley, and his other groat contemporaries, had not in their time an opportunity of observing the transit of that planet. The ohief objeot in observing these transits is

to determine ihti distance of the earth from the sun, as they furnish the most precise mode of ascertaining the same. Halley was the first to point otrt tfce utility of Observing these transits, and preparations ■were made' to observe' that of 1761. While the transit of 17GI was- observed but by five parties, and most of tbe observations were injured or hindered by #1* weather, ther transit of 1769 was observed in its completeduration at seven stations ; also, in its itigressJ at some stations, and in its egresß at other*/ This time, however, observations will bo taken in at least fifty different places, and the importance of having so many different observations may be seen from the fact that the larger the number, by competent persons, the more accurate the mean of all the results would likely be. No steps for observing the transit have as yet been taken in Otago. The probable expense would be from £200 to £250. Probably the Provincial Council will be asked at its next session to do something in the matter.