Scientific and Useful.
The Transit of Mercury. SUCCESSFUL OBSERVATIONS AT BRISBANE.
An event of very great importance to practi- cal astronomers and to students of astrono- mical science occurred on Sunday morning. This was a transit of the planet Mercury across
the solar disc . The previous transit was observed in Australia on 8th November, 1881, when in New South Wales alone there were no fewer that six telescopes employed at specially selected stations within the colony under the direction of the Government As- tronomer. The next transit of Mercury will not occur until 14th November, 1907, when the conditions will not be so favourable for scientific purposes as those associated with the present occurrence. It will, moreover, not be visible in Australia. It will thus be seen that the observations of yesterday morning possess a value distinctly national in character, besides the mere local interest associated with rare celestial phenomena, such as our astronomers are privileged to study and observe from time to time. For the information of lay readers it may be as well to state that the present observations of the transit of Mercury will be of very great utility in settling such vexed questions in astronomical science as the secular acceleration of the moon's mean motion and changes in the axial rotation of our own planet. Owing to the comparatively slow motion of Venus, which favours accurate observations, when in transit across the solar disc, and its great distance from that luminary as compared with the inner planet, astronomers employ the observations obtained at its ingress and egress in determining the solar parallax and hence the distance of the earth from that light and heat-giving body, that "rules the day." The motion of Mercury being more rapid than that of Venus renders accurate observations difficult, except under favourable conditions, and this even in itself is a powerful reason why the measures of its transit are not more generally used in determining the solar parallax. About a year ago that indefatigable and eminently successful astronomer, Mr. John Tebbutt, of Windsor, New South Wales, com- municated with the President of the Queens- land branch of the Royal Geographical Society of Australasia (Mr. J. P. Thomson) regarding the transit of Mercury. As the latter gentle man had conducted the observations of the transit of Venus in the Western Pacific in 1882, it was thought that the necessary steps ought to be taken for the fulfilment of a similar duty by him in connection with the planet Mercury. A short time ago Mr. Thomson, on the sug- gestion of Mr. A. M'Dowall, Surveyor-General, sought the cooperation of Mr. F. D. G. Stanley, who not only readily consented to take an active part in the undertaking, but also cheer- fully granted the use of his fine powerful telescope and observatory. THK OBSERVATORY. The instrument used was an equatorial, with object glass 6in. in diameter, built by Sir Howell Grubb, at his works at Dublin, in 1884. The fooal length of the telescope is 6ft., and it is mounted on a hollow cast-iron column 5ft. 9in. in height and 18in. in diameter at the base, in which is placed the driving clock necessary to keep the tube pointed continuously on any object under observation. The whole is mounted on a stone and con- crete foundation, which is carried down 6ft. into the ground and rests on a rock foundation, perfect freedom from vibration and rigidity being a first essential in the use of an instrument of this kind. The telescope is fitted with the usual right ascension and declination circles, with clamps, slow motion fittings, &c. The observatory in which the instrument stands at Ardencraig, Toowong, is a wooden building 12ft. square, the roof of which is arranged so as to roll entirely off on a railway and framing built to receive it, this mode of working having many advantages, especially in a fine climate. The magnifying power of the eye-piece used in the observations of the transit was one hundred diameters, this having been found to give the best and clearest definition. In order to reduce the sun's heat as much as pos- sible, the object glass of the telescope was stopped down to about 4in. In the observatory is also placed a transit instrument by Carl Bamberg, of Berlin (1879), which is set up in the usual manner on a stone pedestal, the observations for transit being taken through the hollow central axis of the instrument, the eye-piece being fitted with micometer, &;. For several days and nights previous to the date of the transit the party were busily engaged in testing the telescope and rating the chrono- meters. The party consisted of Mr. J. P. Thomson, who observed the transit, his Excel- lency the Governor, who had charge of mean time chronometer No. 542, by Frodsham ; Mr. F. D. G. Stanley, in oharge of mean time chronometer No. 2139, by Arnold; and Mr. A. Cleminson, of the Surveyor-General's staff, in oharge of sidereal chronometer No. 4230, by Kullberg. The Observations. At five minutes past 5 o'clock yesterday morning the sun appeared on the eastern horizon in a cloudless sky. The atmosphere was almost phenomenally steady, and a calm, quiet mantle overspread the whole face of the country. The preceding days of fine weather had freed the atmosphere of a great deal of the moisture assooiated with the recent showery weather, leaving typical conditions for astronomical observations. Rarely indeed has the transit of a planet been observed under more favourable auspices. At ten minutes past 5 o'clock the equatorial was directed to the sun, when the planet was observed as a small, sharp, round disc, having advanced across about two-thirds of the solar surface. The whole periphery of Mercury was re- markably clear and well-defined. There was no trace of haze or vaporous aureola around the disc of the planet, but a bright spot was distinctly seen near the oentre. In applying for a short time an eye-piece with a power of
450 diameters it was thought that two bright spots were visible, but the appearance of the second spot was too doubtful to render its observation of any practical value. At the instant of internal contact at egress there was a slight tremor in the limb of the planet, and a faint phenomenon resembling ligament. This, however, was only momentary. When the external contact occurred the planet's limb tangential with that of the sun was remarkably clear and sharp. There was not a trace of disturbance, and the phase may be regarded as a pure geometrical contact— contact geometrique a ine deformation. No trace of the planet's periphery could be seen when it left the solar disc, although it was carefully looked for. A combination of neutral tint and other dark glasses was used in the eye-pieoe to modify the glare of the sun. The accuracy of the time records was remarkably satisfactory, the greatest difference between the three observers being less than four-tenths of a second of time. The following are the recorded times of the transit:— Internal contact at egress 19h. 24min. 01.5sec. External contact.. .. 19h. 25min. 47sec These times are liable to a plus or minus correction when the necessary reductions are made. After the observations the chronometers were transported to the city observatory for comparison. The information respecting the observations was obtained from Mr. J. P. Thomson. Sydney, November 11. The Government Astronomer, Mr. H. C. Russell, states that fifteen photographs were taken of the transit of Mercury this morning. Although the morning was fine and clear, and the planet Mercury was visible on the sun soon after sunrise, still the atmospheric conditions were not good for observation. As the planet crossed the sun it presented the appearance of a round and intensely black disc without any fringe such as has been noticed in former transits, and owing to the unsteady state of the air towards the close of contact, the " black drop" phenomenon took place, preventing clear definition.