THE TRANSIT OF VENUS.

New Zealand Herald, Volume XIX, Issue 6569, 6 December 1882, Page 6

THE TRANSIT OF VENUS.

To-morrow morning, at an early hour, will occur an event in which New Zealanders should take more than an ardinary interest, as it was largely due to a previous event of a like nature that New Zealand was discovered by Captain Cook, and became a British colony. We allude to what is callod the transit of Venus. This phenomenon is due to the fact of the planet \ enus passing between the earth and the sun. The Earth and Venus do not complete their yearly circuit round the sun in the same time, nor do they move in their orbit or course round the sun on the same plane as each other. The orbit of Venus is, as compared with that of the Earth, somewhat tilted up at one end and depressed at the other. In consequence of the differences in the length of the year and the plane of their orbits, the Earth and Venus at varying periods come into such relative position that Venus—being Dearer the snn than the Earth—comes into a position which enables the Earth's inhabitants to see the solid body of Venus as a black spot on the bright surface of the sun. When this position is occupied, Venus continues on her way in her annual course round the snn, and the Earth also pursues the even tenor of her way heedless of the interest which the relative positions of these three globes cause to the Earth's inhabitants. This movement produces the appearance, to those who occupy favourable positions on the Earth for seeing the phenomenon, of a black ball passing from one side of the sun's surface to another. There are ofeher places again not so favourably situated where only but a portion of the phenomenon we have described can be seen. At some places only the beginning of the transit—that is, that portion where the dark body of the planet is seen t» encroach upon the illuminated edf»e of the sun—occurs a short time before sun-set, and when the sun sinks below the horizon the further progress of the planet's course across the sun's disc is hid from view. At other places, again, the beginning of the transit will be invisible, and only some latter portion of the event will be seen. Auckland is one of those places. The transit will begin about 1 45 a.m., but as the sun to-morrow does not come above the horizon here until 4h. 41m., the planet will have necessarily traversed a considerable portion of the sun's disc before the event can be seen here. The egress will be the valuable portion of the transit as seen at The two principal portions will be what is called the "internal contact," the dark body of the planet touching the edge of the sun, but the body of the planet still visible in the sun's disc; and what is known as the "external contact," the dark edge of the planet toueh'ng the illuminated edge of the sun, but the body of the planet beyond the sun's surface. If the weather be favourable, these should be seen hero without difficulty, as the former occurs at 7h. 2Sm. 345., and the latter at 7h 4Srr>. 17a., a.m. When these are clearly seen, and the time carefully recorded, and the position of the place where the observation is made accurately known, valuable data are furnished for calculating the scale upon which _ the solar system is built.

THE SUN AND THE ANCIENTS. The event will be regarded from different points of view by various people. By many who will look upon the progress of Venus passing between the observer on the Earth and the sun's disc, it will be regarded as a kind of astronomical luxury on account of its rarity, but a larger number will view it for the sake of the important information which is likely to be derived from it concerning the dimensions of the solar system. The size of the system to which we belong is known very accurately relatively, but the exact scale upon which the solar system is built has yet to be determined. The estimated distance of the earth to the sun, and consequently the exact size of the sun itself, have been subjects of dispute through all the periods of known history. Herodotus, the father of history, says the first dawn of thought as regards the sun's distance made its appearance in his day. He gives us to gather what these thoughts were from the opinions of the Earth and of the universe then generally held by the learned Greeks. These were that the Earth was the chief body in the universe, that it was but a flat plane, whose centre was at Delphi; that the sun was merely a secondary, uncertain, and easily influenced body, so much so as to be blown south during the winter by the Etesian winds. These opinions Herodotus regarded as most probable. The learned Greeks looked upon the sun as being within a few miles of the earth. Not long after this Auaxagoras affirmed that tlie size of the sun was about as large as the Peloponnesus ; such a size, considered in reference to the sun's now known aimle. would indicate a distance of about 14,000 miles. A century later, about B.C. 2SO, Aristarchus, the astronomer of Sainos, concluded by measurement of the moon's distance from the sun, that .the sun was at least 5,300,000 miles off. His plan was ingenious, but his measurement was inaccurate ; yet the great Ptolemy, four centuries later, adopted his determination, and, combining it with the observations of Hipparchus, computed the sun's parallax to be three minutes — twenty times its true value. Then further down the stream of Time, Kepler made the sun's distincc to be 20,400,000 miles. The transits of 1761 and 1709 put it up further to our late school-book astronomy of 95,000,000 miles. A few years before "the transit of IS7I, the old observations of 17G9 had been overhauled and rediscussed by Stone, Neweome, Proctor, and others, and the distance of 91,200,000 was agreed upon as an approximation of the truth. The observation of the transit of Venus in 1874 have confirmed thisconolusion to a large extent, and at present there appears to be a difference of some 300.000 miles in dispute. The difference is not great, considering the enormous distance under discussion, but even 300,000 would produce a sensible dilTercnoe of error in all calculations upon which the earth's distance from the sun form an important element. The nearer we approach the absolute truth in this respcct the more correctly we will be able to weigh the sun's mass, and thence to calculate his power of attraction upon our near neighbour —the moon. As the truth iu this respect is reached, so will we be able to give the absolute longitudes in the table of placcs, etc., in the Nautical Almanac, and thus aid our navigators not only to keep the property of their employers from danger, but to preserva the lives of seamen and others from loph by shipwreck. A correct estimate of the distance from the earth to the sun is to the astronomer and navigator like a correct yard stick to the merchant, or an accurate base line to the surveyor on which to compute the value of his triangles. Ail error of large extent in this initial measurement throws out of joint all the calculations respecting the other parts of the starry world. It has been calculated that an error of a thousand miles iu the distance of the earth from the sun, would produce an error of 200,000.000 miles in calculating the distance of the nearest fixed star from the earth. If such a vast error would arise in estimating the distance of the nearest star, how much more would such errors be magnified and multiplied in plotting out tlio more distant but still visible central suns, the fixed stars. The subject is one of great practical and theoretical importance, and hence tho interest excitcd, as the transit of Venus is one of the known means by which icrror in this respect can be eliminated, or at least considerably reduced.

THE SUN'S PARALLAX. What is sought to be ascertained by the present transit of Venus across tho sun's disc is what £a called tho parallactic angle, the "parallax of the sun," or the apparent change of a heavenly body in its position as seen by two persons widely distant—for

instance, one stationed i» the southern and i the other in the northern hemisphere—from < which angle or its subtended chord the distance from us may be learned. This accurately known, the distance which separates the earth from the sun could be easily computed. There is but one direct method by which the distance of any object we are unable to reach can be ascertained. This i.-i the ordinary'method of trigonometrical survey. A base line being accurately measure, the distant object is viewed by suitable instruments from each end of the base line, and the angles of the lines along whi .-h the object is seen from being determined, tlia science o; trigonometry enables us to construct the whole triangle, and thus to measure its sides, whose lengths are the distances of the objoct from the respective points of view. This being the principle upon which such calculations are made, it will be found in practice that when the measured base line i 3 small as compared with the sides of the triangle, greater difficulty is experienced in computing accurately the area of the triangle. In such a case as the one under notice —ascertaining the distance of the sun from the earth, the greatest possible length of baseline to be obtained from which two observers canactsimultaneously, istheaverage leugth of the diameter of the earth. This cannot be measured on the surface, as a quarter of a mile or a mile would be measured on a plain, and consequently the difference in the local time or longitude of the two observing stations is used instead. A little reflection will show that this practically comes to the same thing, as will be seen by the following extract. Professor Forbes, of the Andersonian University, Glasgow, in attempting to explain in an elementary manner the method of ascertaining the value of the sun's parallax from observation of contact supplies the following, accompanied by the two following cu ts : —" 1. It must be thoroughly understood from what has already been said, that if we know the amount of the sun s parallax, in other words, if we know the angle subtended by any known distance on the .Earth s surface, at the sun, we know the sun's diameter. 2. We know that the relative positions of the Earth, Venus, and the sun are given by supposing the Earth to ao round the sun in 365 days, and Venu3 in 224 days. Or, it we please, we may take no accoujit of the Earth's revolution, but suppose it fixed, in which case the revolution of Venus relatively to the Earth (i.e., the synodical revolution) is 584 days. 3. If, then, V enus moves round the sun through SCO relatively to the Earth in 584 days, she moves 1 through of that in one day, and 584 360 of a degree in one hour, which 584 x 24 is at the rate of about 1 l-ssec. of arc in a minute of time. Now, we are ready to understand Halley's reasoning. The cuts are as follows":—|

LetAbcthe position of an observer on the earth at the time of the first internal contact, S is the sun, and V is now the position of Venus. This observer see 3 the coutact earlier than a hypothetical observer at the earth's centre would see it, by the time Venus takes to move over VI, V 2. If we knew by calculation the instant when an observer at E would see it. and the observer at A sayr it eight minutes sooner, then, Venus moves over 1 l-.Osecs. in a minute, she h?,9 moved over S x 1 1-5 or 9 3-asees. of arc in this time, and hence we learn that the angle ASE is equal to 9 3-ssecs. Suppose that by the time of the last contact the point A on the earth's surface has been carried by her rotation to B, the time of the last contact will now be too late by eight minutes ; since the whole duration of the transit, as seen by this observer is 16 minutes too long, and the angle moved over by Venus in 16 minutes is the sum of the sun's parallax, as seen from A and B. But we cannot calculate with absolute accuracy the duration a transit would have when seen from E, because we should require to know more accurately than we do the values of Venus' and the sun's diameter. Halley got rid of this by taking another station which should be in the position of A at the beginning of the transit. In the case we have been considering the time of the first contact would here be too late by eight minutes ; and if this place had reached B' by the end of the transit, the time of contact would be too soon by eight minutes Hence in this case the whole duration would be sliorteued by sixteen minutes ; but in the former case it was lengthened by sixteen minutes. Hence thirty-two minutes is the time taken by Venus to pass over an angle equal to the sum of the parallaxes of the four cases considered. This difference of duration, whether it be thirty - two minutes or anything else, is a quantity which can be observed. Now Venus moves over about 1 l-ssccs. of arc in a minute, or 3S 2-ssecs. in these thirty-two minutes. Hence, one-fourth of 3S 2-ssees., or 9 3-ssecs., would appear from the above hypothetical observation, to be the value of Venus' parallax. It must be noticed that we have here supposed that the transit takes exactly twelve hours, whereas the longest transit cannot exceed eight hours. We havo also supposed that two stations had been selected which were exactly situated so as to bring out the full effect of parallax at the time of each observation. These suppositions have been introduced only to simplify the explanation of the method. Anyone who has followed the above explanation will see how the method may be applied to actual cases that may occur."

Such ia briefly the outline of the method proposed by Halley. Other methods have since been devised, and there is now far less importance than formerly attached to a transit of Venui for giving information to calculate the sun's horizontal parallax, but the above will be sufficient to the non-mathe-matical reader to indicate the careful character of the work which requires to be performed

THE BLACK DROP. What has been designated the "black drop" was part of the phenomena of the earlier transit of Venus as recorded by different observers. It is strange that the records of these transits are so scanty, though transits must have been visible to the Earth's inhabitants four times every 243 years since the Creation. Had the world been spared that outburst of wild aud misguided religious enthusiasm which destroyed the library of Alexandria in the early centuries of the Christian dispensation, we would certainly have had much more complete ancient astronomical records than we now do. Among the clay records from Assyria and Babylonia, now in the British Museum, is a clay vessel in which are carved the movements of the planet Venus during a period extending back to the sixth century b. e. There is no transit, however pictured thereon, but had the more ample literary materials at one time stored iu the library of Alexandria been in existence to-day, another story might have been told. The first transit of Venus which we have any distinct record was in 1631, predicted by Kepler but not observed. The next was in 1639, predicted, and, so far as is known, observed only by young Horror. The next were in 1761 and 1769, and in these the " black drop " has always been observed in some form or other. The following are some of the shapes which the appearance

assumed iu 17G9, as seen by observers at different stations :—

The cause of this has never been satisfactorily explained. A number of astronomers h:\ve attributed it to as many different causes. But whatever may be the true emanation, it is somewhat singular that this appearance was scarcely noticed at all in 1574. At une of the stations in Nev.- South \\ ales it was reported to have been seen, md also at the Cape of Good Hope. Whether its nonobservance was due to clouds obscuring the vision at the particular moment when it would have been visible, or whether it was really wholly absent, due to any optical illusion on the part of the old observers, or to defects in the instruments then employed, it is impossible now to say, but the general record of the observations in 1574 were as remarkable for the absence of all notice of this peculiarity, as the previous observations wtrii to the contrary. Whether this appearance will be present or absent on the occurrence of the transit wiiich takes place to-morrow morning is a circumstance of little moment to observers in Auckland, as the first stages of the transit will not be seen here, the sun only rising after the transit has been for some time in progress. The " black drop" was usually seen just after the planet had made what appeared to lie a full entrance upon tile sun's disc, and the dark body of the planet appeared oil the bright surface of the sun suspended to the blue vault of heaven by a thin band or lieament, which assumed different shapes at diliercnt stations, as will be seen on relereuce to the above engravings. It wa3 an instruction to all the observing parties in 1574 to note carefully the exat time when the ligament should be seen to break.

HABIT ABILITY OF VENUS. These transits aflforrl a gnnd opportunity of noticing the physical peculiarities of \enus Many of the observers in lSi-t insisted that the body of the planet showed much diversity of surface, indicating a considerable resemblance to the appearance of the Earth. From the early observations of tlje transits of Venus it was concluded that she was without an atmosphere, and consequently unfitted for the support of human life. But this notion has now been dispelled. The observations of 1 ST-t showed that she is possessed of an atmosphere of considerable density in some places, which will afford shelter to her inhabitants from the intense rays of the orb of day. Mr. C. H. Russell, of Sydney, also noticed certain bright spots towards the poles of the planet, which he convinced himself were collections of snow or ice. If this conjecture be true, then it is evident that abundance of water exists upon the planet's surface, and that in consequence clouds must necessarily exist. These are conditions necessary to the support of animal life such as we know it, and the canopy of clouds would afford a shelter from the direct rays of the sun just as tropical countries on the earth have a greater degree of cloud than places nearer the poles. Upon these and other questions relating to the physical constitution of our nearest planetary neighbour the observations made to-morrow morning by the hundreds of telescopes and other instruments which will be directed to it, may be expected to throw some additional light. Some have thought that the bright spots which Mr. Russell saw and attributed to collections of snow or ice, may be explained by the gleams of bright sunlight which would be seen through rifts in dense masses of storm clouds. But be the cause what it may Mr. Russell is quite of opinion that the circumstance is due to the cause which he has assigned.

PECULIARITIES OBSERVED. There were also other peculiarities observed in 1574 which were uot placed on record before. This may be due to a large extent to the improvement in tiio instruments then used as compared vith what were formerly employed. One of these features was the projection of the dark body of Venus against the sky just outside the outer liinb of the Sun. T'lis feature was not only observed at many stations, but was registered !>y the camera r.t several stations. According to the late Professor C. 11. Young, iu his woik on the "Sun,'' this phenomenon implies behind the planet a background of sensible brightness in comparison with the illumination of our atmosphere, and is probably due to the existence of the solar corona. Whatever is the cause the present transit will afford another opportunity of examining it. Soinc observers also asserted, as alluded to above, that bright patches of light were seen 011 different par:s of the planet's surface all thrnuch the course o; the transit. By many these were attributed to defects iu the telescopes used, but to-morrow will afford the la-jt opportunity of testing this point during the remainder of the present and following century. THE AMERICAN OBSERVERS' EQUIPMENTS.

The American party of observers are the only one in Auckland amply furnished with varied appliances to record the phenomena that may be observed. \\ e learn that Colonel Tupman, at Buriiham, is only furnished with a live inch equatorial telescope. The static:: occupied at }*u-n----h.un, a short distance from Christchureh. is the one occupied by Major Palmer, in (Iu that occasion its longitude v.as well dtteimined, and the work then ('one will he >o much help now. TheCo!"!U'l has frequently exchanged telegraphic signals with the American party at Auckland, tor the purpose of reducing longitudinal error* to the smallest possible dimensions. The equipment of the American party of observers at Auckland under the direction of Mr. Edwin Smith appears to be very complete. The instruments used are those employed in 1574. The casual visitor to the Domain will see a small space near to the old 1-iloek House enclosed with an iron wire fence. Inside this fence have been erected several buildings. The one nearest the Block House contains the transit instrument. Another one adjoining, with a revolving roof, contains the liveinch equatorial telescope. Iu front of the one containing the transit instrument is placed the stand for the heliostat, from which runs the telescopic tube connecting with the dark chamber of the photographers, at a distance of some ,'!S to ,'ii) feet. I!ut inside the Block House are many appliances ot the greatest use. There is titled up the telephonic apparatus, and the telegraphic instruments, by which Mr. Smith has been enabled to exchange signals with Colonel Tuptnau at Burnham. This has been done almost every night for some time past, so as to tix as exactly as possible the longitude of the place.

A uuiform plan of observation has been adopted at all the American observing stations, and the instrumental equipment has been nearly uniform. The instruments provided for this station are the photographic apparatus complete: a transit instrument, with a clock, two chronometers, ami a chronograph, a five-inch aperture equatorial telescope for observing contacts and occupations of stars by the moon. This instrument is adjustable to any latitude, and is furnished with divided circles, clock motion, and double image micrometer. The clock motion is regulated by a Bond spring governor, with an auxiliary "tly," in the event ot the governor getting out of order. The transit instrument is placed in the same meridian as the photographic telescope, in order that the central vertical line of the photographic plate-holder could be set very near the meridian, and its small deviations be accurately determined. The transit instrument is of the broken tube construction, a prism being placed in the centre of the cube, by interior reflection from which the pencil rays is thrown along the axis, and the image thus formed at the end of the latter. The detailed plans of the instrument were all devised by Professor William

Harkness, U.S.N., and the construction wa. carried out under his personal direction This form of instrument has thus great ad vantages of convenience in observing and o rapid and easy manipulation, but is still sub ject to the disadvantage of a collimatioi varying with the zenith distance of thi abject observed. One of the chronometer; furnished to this station is a sidereal oae creaking the galvanic circuit at every seconi ;xcept at the sixtieth of each minute, whili ,he other chronometer is regulated to keej nean time. By means of the chronograpl my peculiar feature can be noticed, as thi >en of this instrument is in continued motion Chus the exact instant can be recordee nerely by touching a button. The sidcrea look was made by the Howard Clocl Company, of Boston, after a plan fur lished by Professor William Harkness Che chronograph was made by Alvai }lark and Sons, and is regulated by th< Hipp spring. This construction i 3 some vliat inconvenient, on account of the greal vcight required to run the instrument, anc he whirring noise caused by the spring, .t was, however, preferred to the Bond ipring governor, on account of a supposed lability of the latter to get out of ordei vhen used in the field. The photographic ibjectives are each five inches in diameter n clear aperture, and, as above stated, be ;weeu 38 and 39 feet in fecal length. Th«y ire corrected for the photographic rays. Che heliostat, iroin the mirror of which tin ays of the sun are thrown into them, is lurned by clockwork on a single fixed axis vhich. is so adjusted that the rays of the :uu will be nearly thrown in a constan: lirection during the whole of the transit L'his plan was adopted as a compromise be ;ween having no clock motion at all in thi ielio3tat, the adjustment being made by ai i-sistant for each photograph, and the ex tensive apparatus necessary to throw thi iolar rays in a direction mathematically :onstant. The mirror is seven inches ii liameter, of unsilveretl glass, and slightly ;hicker on one side than on thi >ther, in order that the reflection fron .he second surface may be thrown awaj 'roin the photographic plate. It is lef insilvered in order to prevent any unequa ibsorption of heat by the two surfaces of the ;lass. So far as could be detected, no disortion of the unsilvered glass was produced >y the direct action of the sun's rays in the ibservations made in 1574.

The plate-holder in the focus of the photographic objective consists of a brass frame, about eight inches square, turning on an axis passing vertically through its centre, mounted on a hollow iron pillar, and having a spirit-level attached to the top of the frame. A vertical cylinder hole passes through the axis from top to bottom, is the ceutre of which passes a fine silver plumb-line, the bob of which hangs in a beaker of water below. A square disc of plate-glass, about three-tenths of an inch thick, is set in one siile of the brass frame, so that the plumbline passes very near its surface. The surface nearest to the plumb-line is ruled with a system or horizontal and vertical lines one half-inch apart, by Professor \V. A. Rogers, of the Observatory of Harvard College. In taking the photograph the ruled plate is in the side of the plateholder nearest thephotographicobjective, the sensitized plate being inserted from the other side. Between the ruled surface of the one and the sensitized surface of the other is a ! space of about o*l6 inches, through the middle of which hangs the plumb-line. The images of the plumb-line and of the ruled lines aro ; . thus impressed on the plate with each photograph. The plates are dry, re eeive the impression instantaneously, and are the result of a preparation of collodion, the bromide-gelatine process being objected to on the ground that the gelatine film on the plate is liable to shrink, and thus have an important effect upou the micrometerie measurements to which the different stages of the transit recorded on the plate are exposed. This fear, we understand, however, experience has shown to be groundless, as the only sensible shrinkage of the film which has been observed on these plates lias been in the thickness of the film. While the film firmly adheres to the gla&s plate, the room for lateral shrinkage is almost, nil, the thickness of the film being exceedingly minute. Should the weather prove favourable, Mr. Smith anticipates very good results from the use of photography, and regards this method of work as of more value in ascertaining the true horizontal parallax of the sun than observation of contact by the human eye. From the time that the sun rises sufficiently above the bank of haze and clouds which aro always seen along the horizon in the early morning, so as to be clearly seen, it is intended to take two photographs every minute, except during the time of the inner and external contacts, when photographs will be taken more frequently at regulated intervals. In addition to the five-inch equatorial telescope, a smaller one will be used at the time of the contacts, so as to increase as much as possible the number of observations sit these particular phases of the phenomena. -

LOCAL OB3ERYKKS. In addition to the American party of observers there will be several local observers. Mr. Tlieo. Heale has obtained the use of the equatorially mounted telescope imported some time ago by Messrs. Iv "or r r anil Co. of this city, and had it removed v> bis residence at Point Resolution, I'lirnell. Mr. Heale intends only to note the times of the inner ami external contacts, anil any peculiar phenomena which may be connected therewith. Professor Lambert, it is believed, will also be employed in the same direction, as will Mr. Stevenson of Newton-road. The arrangements of these private observers are necessarily on a small scaie as compared wuli those beinc carried out under the direction of Mr. Smith. THE PATH OF YEXUS. To enable our readers to enjoy the observation, if the weather be favourable, we append an engraving of the estimated path of the planet across the sun's disc: —

As previously stated, the sun .isc-s at Auckland to-morrow at 4.41 a.m., and as the transit begins at 1.45 a.m., it is calculated that the engraving herewith will nearly represent the position of Venus when fi'it seen here. Steadily the planet will pursue its course to the western limb of the sud, and about Thrs. 4Smins. ITsecs. it will part company with the outer edge of the sun's disc until June 2004. iThc internal contact at egress is calculated at 7hrs. 2Smins. 34secs. It should oe remembered that the calculations made respecting the actual times of the internal and external contact are made from imperfect elements, which it is hoped the present transit will help to perfect, and should the contacts occur a few seconds either earlier or later than the calculated times, it will be the duty of observers to note the difference.