REV. MR. HABENS’ LECTURE.

Globe, Volume I, Issue 78, 31 August 1874, Page 3

REV. MR. HABENS’ LECTURE.

As considerable interest is taken in the forthcoming transit of Venus, we append a condensed summary of the/lecture pn the subject, delivered by the Rev \V. J. Habeas, at the Oddfellows’ Hall, on Friday evening last:—

The lecturer, at the outset, announced his intention of making the lecture, a strictly popular one, and of avoiding, as far as possible, mathematical terms, especially one term (parallax, we suppose), which it was somewhat difficult to explain and to fix in the mind. ITis object would be to array facts commonly known in the order which, is necessary to a clear Conception of the phenomenon, of the use it is to serve, and of the methods of utilising it. In effecting this object he would be, at the same time, setting the facts before any who did not know them. He referred to the necessarily imperfect character of all graphic or moving illustrations of the relations of the heavenly bodies, owing to the impossibility of representing the objects and the distances in their true proportions. We have first to get clear conceptions of 'the motionS of the earth' itself. /[Here the lecturer produced a globe of about twenty inches in diameter for the earth, taking a table lamp for the sun]._ The globe was marked with an arrow, showing the direction of its daily rotation, and it Was seen that when the South Pole was placed uppermost, and the position of an observer assumed to be at the South Pole, the rotation was in the direction of the movements of the hands of a clock—from left, to right. When the North Pole was chosen as the point of observation, the same rotation appeared to be from right-to left. Keeping the South Pole up, because our place is three times as near the South Pole as the North, and the part of the heavens over our heads is only a little more than forty-five degrees from the part which is over the South Pole, we- see; tha,t the daily rotation carrying us past successive points of the heavens from left to right, causes all celestial objects to move apparently from right to. left —from east to west. The rotation of the earth having been shown, the next thing was '•to i 'kriow : ‘ the direction of its revolution. This was shown by carrying the globe round •thedatap', the (South Poleffieing kept always at the top, with the axes perpendicular, the globe was placed leyel with the lamp, hiding it from'the audience. This represented the earth’s place in our autumn. It was then carried round the lamp in a circle from left to right, as seen from the lamp, the circle not being level, but inclined about 23 degrees. The’ highest;ppVats was reached at: the first quarter, on the lecturer’s right, and it was seen that the South r Pole was in that position out of sight of the : lamp, as at our winter. Passing through another quadrant the globe ’"as , brought level behind ; the lamp, as at our spring time, and reached its lowest point on the extreme left (from the speaker’s point of view), when it was seen that the South Pole and its neighborhood was illuminated as during our summer. This elucidated by a diagram in which the earth represented in four positions, corresponding to the four seasons, with the axis always perpendicular, and the path inclined to the Equator. The revolution then of the earth, as seen from the southern parjtp jof: the sun, ’and the rotation of H e earth, as’ seen from its own southern parts, are both in the s »me direction, from left to right, as in the case of the hands of a clock. The relative motions of the earth and Venus were.jjext explained by the aid of a model. A red disc 49 inches'wide, placed perpendicularly, and supported by a stand behind it, represented the piano of the earth’s orbit, and had placed on its edge a ball for the earth. The middle part of the red disc was hidden by a blue one 35 inches wide, for the plane of the orbit of Venus, having also a ball attached, A large ball fixed in the common centre of the discs stood for the sun. The discs were caused to revolve, as the hands of a clock, ; but, at unequal rates, the outer performing'eight revolutions while the inner performed thirteen, to show that Venus goes round the sun thirteen tiines in eight of our years, and so is always gaining on us. The three balls being placed in a line, it was found that after 1 3-sth revolutions of the outer discs they were all in line again ; but, of course, not in the same place as before. After eight revolutions of the outer ball, and thirteen of the inner, conjunction occurred at the place where the movements began. The discs were then removed from the stand, the larger was shown edgewise, and the smaller was passed through a slit in the larger one, so that part of the blue disc was below and part above the red one. It was thus seen that except where the two discs cut each other Venus is either too high up or too low down to come between the earth and the sun. If the earth and Venus are in conjunction where the discs cross there will be a transit, or if they are within 1| degrees (or days approximately) of the crossing, a transit having taken place at that part of the orbits, it will be eight years before a conjunction occurs again at the same part, and then it will rot be at the same point but 2.) days earlier. If the first of the two occurred very near the crossing the days will bring the next one too far back from the crossing for a transit to occur. If however the first was within two-thirds of a day of being as far forward as it could be, the first and second conjunctions would both be transits. After two transits (or one,

as the case might be,) there would be no other until the line of conjunctions 1 3-sth years later had worked back 2.) days in every eight years to the part of the orbits opposite to the first transits, and this would take 105) years or 121.), according as the space traversed by the line of conjunctions was in the slower or quicker half of the orbit. The transit of 187-1 would take place just after crossing at that point where the orbits arc lower than the sun, as explained by the lamp and globe, and about a fortnight before the earth has reached its lowest point Venus will have just entered that half of its orbit which is lower than the earth’s orbit. Therefore the place of Venus as seen in the sun will be below the centre of the sun (for ns in the south) and her path will be descending, partly because we are descending, partly because she is descending, and partly because, travelling more quickly than we do, her rate of descent will be apparently increased ; altogether, she will descend on a path apparently inclined 14;] to a horizontal line. Since our motion is to the right of an observer, supposed placed in the south of the sun, and hers in the same direction but more rapid, she will appear to us to cross the sun, moving towards the left. This was shown by a diagram representing the sun with a radius of twenty-two inches, and the disc of Venus drawn to scale, about one thirtieth of the sun’s diameter. The time of the crossing as seen from the earth’s centre, is about 3h. 41m. Ssecs. Two maps were exhibited representing the enlightened hemisphere of the'earth at the time of commencement of the transit, and at the time of its ending, respectively. The phenomenon is an insignificant one in its mere appearances. It owes its chief interest and all its value to the fact that it will be seen differently by observers in different places. The lecturer explained how observers far south would look as it were over the top of Venus (as we may regard the south as the top), and see her low down on the sun’s disc, where, of course, her path would appear shorter ; while an observer far north would look as it were under her lower edge, and see her disc higher up where her path would be longer. He also showed that an observer in the extreme east would see the transit commence and end sooner than one on the extreme west. This part of the subject was illustrated by elaborate diagrams, showing that the distance by which the paths of jVenus’s centre as seen from extreme north and extreme south would be separated was about one-forty-fifth of the sun’s diameter. One of these diagrams, drawn with a radius of fifteen feet to illustrate the different apparent positions of the disc at last internal contact of the edges of the discs of Venus and the sun, showed how far to east or west an observer must be to see the contact at the first and last moments possible. It was seen that those places on the earth’s surface through which a line was drawn exactly facing the edge of the sun at the place of contact would see the contact at the same moment, but not exactly in the same place on”the sun’s edge ; while, speaking generally, those who were on a line exactly opposite the radius of the sun passing through the point of contact, would see the contact earlier or later as they were nearer to one or other end of the line. These lines were marked on the two maps for ingress and egress. The lecturer .stated that in Canterbury,- the New Zealand mean time of the first internal contact would be about December 9th, Ih. 45miu. past noon, and the last internal contact about sh. 1 Grain., giving a duration of about 3h. Blmin. The problem to be solved was the sun’s distance from the earth. Halley’s method is based upon the difference of the north and south paths of Venus across the gun. As stated in Kepler’s third law, the distances of Venus and the earth respectively are as 5 to 7, or more nearly as 723 to 1000. It is easy to calculate by similar triangles that if two observers are separated by 7000 miles, and their lines of vision cross at Venus, those lines will widen out again to 18.000 miles, when they reach the sun. If then the apparent separation of lines 18.000 miles apart on the sun’s disc can be measured in minutes and seconds of arc, and so compared with the sun’s diameter in minutes and seconds, we shall know the sun’s diameter in miles, and knowing the size of the sun, we shall know how far off he is to look as large as he docs. But it is difficult to determine precisely the place occupied on the sun’s disc by a moving spot at any moment, its distance from the centre, and its bearing from the centre. Halley’s method obviates this difficulty by observing the length of the lines traversed by the spot. It is known exactly how long the spot would take to traverse a diameter of the sun. By comparing with this time the actual time occupied in crossing in any line distant from the diameter, the place of this line on the disc is known. The time for a northern observer is nearly four hours, and the probable error of observation not much more than four seconds, about one three-thousandth of the time. A north and south observation of duration will give the place of two lines on the disc, and therefore their separation, and so the sun’s diameter, and so his distance. Dclislc’s method is based upon the fact that eastern observers see the contact earlier than western. A. diagram exhibited the principle of the method. The earth was represented in such a position that from its eastern side a line through the centre of Venus reached the centre of a disc, supposed to be the disc of Venus, as seen on the sun from that place at the moment of internal contact. Parts of the orbits of Venus and the earth and their relative distances were shown, of such a length that, Venus travelling more rapidly than the earth, a line from the earth’s western side passing through the centre of Venus reached the centre of the same disc. It was explained that if twenty-five minutes elapsed between the two observations, the earth would have travelled 29,000 miles, while the places of observation being, say 8000 miles apart, would increase the line to 37,000 miles. In that time of course Venus must have travelled five-sevenths of 37,000 miles ; and if Venus travels 37.000 x 5-7 in twenty-five minutes, it is easy by the aid of Kepler’s second law to know how far she travels in 228 days, that is what, is the length of her orbit. The length of her orbit being known, her distance is known, and so the earth’s distance, which is known to be toilers as 7to 5. The lecturer stated that the most hopeful method of ascertaining the position of the lines of transit by direct observation was by the use of an astronomical telescope fitted at the eye end with a photographic camera for taking pictures of the sun at frequent intervals with very brief exposure. The references to places chosen for observations, and to the instruments required, were very slight, owing to the lateness of the hour at which the lecture ended.