TRANSIT OF VENUS.

Globe, Volume II, Issue 147, 23 November 1874, Page 3

TRANSIT OF VENUS.

(From the Press.) We are indebted to Major Palmer for a pamphlet containing instructions for the sub-stations for observing the of Venus on the 9th December. The stations are as follows :—Grahamstown, in charge of Mr H. A. Severn ; Auckland, in charge of Mr Theophilus Heale; Wellington (?) of Archdeacon Stock and Dr Hector (probably); Caversham (Dunedin), of Messrs Thomson aud McKerrow ; Naseby, of Lieutenant Crawford, R.N,, and Captain E. Williams, of the Merope. Major Palmer informs us that. Mr Severn has an eleven-inch reflecting telescope, which lie has made himself, for the e xpress purpose of observing the transit. The c ither stations have reflecting telescopes of four inches aperture, the minimum size \ vhich is considered of real use for scientific observations. The stations at Aucklaud, Wellington, Caversham, and Naseby have all t ransit instruments of some kind for determining local time, so that their difference of longitude from Murnham may be accurately found if required. Operations for the purpose will shortly be begun with the stations at Caversham and Naseby, and Major Palmer is about to connect with the same object his observatory with that of Professor Peters at Queenstown. The first part of the pamphlet consists of full instructions for conducting time signals, which will be sent from Burnham to the outstations on December flth, before and after the transit, so that the observations may be made in Burnham time. The second part contains instructions for eye observations of the transit, from which we select the following information:—The two phases to which attention needs to be given are (1) interior contact at Ingress, (2) interior contact at. Bgres3. The first of these is important, but not very important ; the second is the critical and valuable observation, but the sajne care should be bestowed upon both, and it is probable that the experience gained at the observatory of Ingress will prepare the observer to enter with confidence aud extiertne ss up m that of Egress. The Nautical Alma: aacfor 1874, appendix page 21, gives approximately the times at which for Auckland, New Zealand, the internal con-

tacts at Ingress and Egress "mny be cxDected. The time for other New Zealand stations will differ slightly from these. (At other stations tin; local mean times of the several phases will be approximately 4min less for every degree of longitudo west of Auckland, and in the same proportion for points east of Auckland.) But, inasmuch as tin; planet will bogiu to cut out a notch upon the sun's nearly half au hour before tiie internal contact at Ingress, observers will have ample warning iof this phenomenon. Obviously ilso, the planet's approach to the sun's limb for Kerens may be easily watched. The Nautical Almanac also gives (for Auckland) the angles, measured fiom the sun's north point and ve.rtey, at which the contacts at [ogress and Rgresa will take place.

For the benefit of amateur observers, we condense the following particulars, which should be attended to before hand. Arrange' merits should be made that the observer's body and head may be in easy and unrestrained positions at the two observations having regard to the altitudes (see Nautical Almanac) which the sun will have at these times. A horizontal bar, supported at any required heights between two uprights, on which the observer, in a standing posture, rest his elbows while his eye is at the telescope is recommended. The piincipal observer should practice himself beforehand in the use and manipulation of his telescope, learning to fix his eye steadily on one point of the sun for several minutes without moving, till he comes to observe numerous peculiarities. Perfect firmness of the telescope stand is important ; if out of doors, lateral shelter from the wind is desirable. If indoors it may be necessary to isolate the legs of the stai.d from the floor. It is pointed out. that ihepcrformanecof thechronometersshould be carefully tested for for a few nights before and after December oth, though the main immediate obj et is to observe the transit, wit h the help or a steady goingchronometer, referred by galvanic signals to the siderial clock at liunham. The. instructions to be observed on December 9th are as follows : However bad the weather may appear beforehand, every arrangement should be made just as if it were fine. The observer should assure himself of the correspondence of the minutes' and seconds' hands of the chronometer.

The following phenomena, in the order named, are those which may be expected ; and to which, if witnessed, chief attention should be given. But it is possible that they may not present themselves so clearly as is anticipated, and that other definite appearances may be exhibited. In this case, reliance is placed on the observer's skill and judgment for noting accurately th'i times and circumstances of such definite occurrences as he may notice.

At Ingress (1. - ) the planet seems to have exactly arrived within the sun's limb, or at such a position that the circular disc of the planet, if continued without interruption, would be tangent internally to the suu's rim, it will probably be noticed that the two cusps or horns of the sun's light intervening between his edge and that of the planet, instead of immediately meeting one another, remain separated for several seconds by a dark ligament or ' ; drop," which decreases in breadth as the planet advances upon the sun. The ligament or "black drop," which at first is as deep in color as the planet itself, will probably be seeu shortly to undergo a change in intensity. A flush of light will be seen to spread over it, reducing its depth of color, but not actually dispersing it, so that the cusps still remain separated by a dusky film This phenomenon is usually instantaneous and well marked, and the time should be accurately noted. (2) A few (from three to six) seconds later, it will probably be noticed that the ligament is finally dispersed : the cusps actually meet one another, and a thin thread of bright light is for the first time to separate the limbs cf the Sun and Venus. This is also a distinct and well-marked phenomenon, and is probably the most important of any. The above are the two cardinal phenomena at Ingress to which an observer's attention should primarily be given. Possibly, however, as above remarked, other distinct appearances may be witnessed. It will be the observer's business to notice all the expected or unexpected appearances, and to call out "Now," or some other sudden signal for time to his assistant, who will previously have been stationed in front of the clock or chronometer in such a position that he can see its face and hear its beats with perfect ease. The assistant will bock instantly the time of each signal. If time admits, a word may be given to the assistant, indicative of the appearance to which any particular time signal refers ; and, if possible, the assistant may book any such notification ; but he must regard it as his principal duty to keep up constantly the count of the chronometer beats, and to register the instant of each signal. When the Ingress is compb tc, the chief observer may give from memory the details of the appearances to which each time-signal refers.

At E'jress. The above phenomena (1 and 2) may be lookid for in reversed order, viz: —l. The first appearance of the " black drop," or the first petition of the thread of light by a dusky hnze connecting the limb of Venus with that o" the sun. 2. The sudden deepening of the dusky haze to an intensity equal to that of the planet itself, so that the cusps become very decidedly separated by an opaque ligament. It cannot be too strongly impressed upon observers that the two appearances above described, at Ingress and Egress respectively, arc those to which chief attention must be givcu. Possibly it may be noticed, amongst other phenomena, that, after the cusps have met at Ingress, a dusky haze may linger for a few seconds between the two limbs; if so, the instant of final disappearance of this haze (and, vice versa, its first appearance before separation info cusps at Egress) inny be noted if practicable. But this observation must be regarded as subordinate to the other two in value; and of those two it may be remarked that, at Ingress certainly, and hardly less certainly at Egress, the observation of cusps (i.e., their meeting at Ingress, and their first formation at Egress) is the easier, and should be undoubtedly secured even if the sudden change in the color of the ligament escape notice. It may, lastly, be a useful warning to observers to caution them to disregard altogether at the critical moments the planet aud the sun's limb, and to concentrate their attention—At Ingress—First; upon the ligament, to detect its change of color. Second; upon the cusps, to detect the instant of their junction. At Egress—First; upon the thread of light, to detect its first partition into cusps. Second; upon the ligament, to detect its sudden deepening in intensity.