ASTRONOMICAL NOTES.

Press, Volume LXIII, Issue 19146, 1 November 1927, Page 11

ASTRONOMICAL NOTES.

FOR NOVEMBER, 1927. (srECIALLT WRITTEN TOR TEE PRESS.) [By E. G. Hogg, M.A., F.R.A.S.] The planet Mercury sets on November Ist at 8.17 p.m.; it will .be seen well to the south of west and as its apparent magnitude is 0.9 it should be easily picked up. Mercury is rapidly approaching the sun, with which it Trill be in inferior conjunction on November 10th; it will be seen in the eastern sky towards the close of the month. Venus rises on November Ist at 3.4 a.m.) and on November loth at. 3.43 a.m. It is still a conspicuous object in the eastern sky, but its brightness is slowly declining;'by the end of the year it will . be about half a magnitude fainter than it is at present. Mars is too close to the sun to be seen at the beginning of the month, but may be visible in the eastern sky towards its .cJose. Jupiter sets 'on November Ist at' 3.14 a.m. and on November loth at 3.18 a.m.; Saturn sets on these dates at 9 p.m. and 9.13 p.m. respectively. Transit of Mercury. There will be a transit of the planet Mercury across the sun's dise on November 10th, which will be partly visible in New Zealand. Exterior contact at ingress will occur at 3:32 p.m., and the interior contact at ingress lmin 41.4 seconds later. Mercury will be at its least distance from the centre of the sun at 6.16 p.m.; the sun will set at 7.55 p.m. before the transit is completed. The transit cannot be seen by the naked eye, but in a small telescope it is readily visible. The sun should not be observed directly, but its rays' after passing through the telescope should be focused on a sheet of white paper; an image of the sun of any convenient size can thus be obtained and the motion of the dark speck which represents Mercury can be followed with little fatigue to the eye. Intending observers are recommended to practise this method of observation a clay or two before that of the actual transit so as to acquaint themselves with any difficulties it may present. ' Using a telescope, the planet will appear to enter the sun's disc near, its vertex or highest point, and to move across the disc in a direction making a moderate angle with the vertical. Much importance attaches to the exact instant at which the planet makes contact with the sun at ingress and egress, and to secure correct time use should be made of the signals sent out daily from the Dominion .Observatory at 10.30 a.m., 3.30 p.m., and 8.30 p.m. (During .the operation of New Zealand Summer Time for the abovo times we must reid one hour later in each case).

The planet Mercury traverses an orbit which is inclined at an angle of about 7 degrees to that of the earth, and the planes of the two orbits intersect in a line, which meets tho earth's orbit in two points which the earth passes through about May 9th and November. 9th in each year. If Mercury is in conjunction 'with" the sun on those dates it will be actually in the . line between the earth arid the sun and a transit will occur; at no other time, in the year can a transit take place.-

Previous Transits of Mercury. In 1627, Kepler completed his famous Kudolphine Tables, : which-' speedily superseded, those of. Lunsbcrj;,. ahil gave the predicted places of-the planets' to a far higher order of precision: than any previous tables. ' He was now in' a position to foretell the dates at which transits of Mercury and Venus across tho sun's disc would occur, and in 1629 he published a small tract in which he stated that Mercury would pass across the sun on November 7th, 1(531. This, the rirst observed transit of Mercury, was actually witnessed by Gassehdi at Paris, who announced his success to Shiekhard, Professor of Mathematics in the University of Pubingen, in abetter which now makes curious reading., ' The crafty god. had sought to deceive astronomers by passing over the sun a little earlier than was-suspected, and had drawn a veil of dark clouds over tho earth in order to make his escape more effectual. But Apollo, acquainted with, his knavish, trjeks from his infancy, would not allow him to pass altogether unnoticed. To be brief, I have been more fortunate than those hunters after Mercury who have sought the ounning-god in tho snn. I found him out and saw him,'when no one else had hitherto seen him."

Gassendi's: methods ..' were simple, thorough, and effective He.prepared to observe the transit by admitting the sun's light into a dark room through a small .opening.in the window .and. catching the image of the sun upon a white screen on which was described a circle of the same size as the imago. In order to learn the time of the observation, he had a person stationed in the room above him, whose duty it was to observe with a quadrant the altitude of the sun whenever he should, hear Gassendi stamp his foot. Our observer, lacking that faith in Kepler's tables which we now repose in the Nautical Almanac, decided to begin his search on November sth, but continuous rain thwarted his purpose. The following day the sky was overcast with clouds, and November 7th, the day appointed for the transit, did not open very promisingly. Towards nine o'clock, however, the sun became distinctly visible and upon its image Gassendi perceived a black spot; he roughly marked its position, but, on account of its extreme sntallness, had not the remotest suspicion that it was Mercury; he was inclined to believe that it was an ordinary .sun-spot. He, however, took the precaution of measuring the distance of the dark spot from the centre of the sun's image, and when, on the next emergence of the sun from the clouds, he measured the distance anew and found that in the interval it had considerably increased, he had his first suspicion of the real nature of the black spot and gave the preconcerted signal to his assistant in the room above, only to find that he had abandoned his post. Fortunately he returned before the planet had gone off the sun and made the necessary observations, which enabled Gassendi to calculate that the transit had lasted five hours and had occurred ohrs 49min 30sec before the predicted time. The second observed transit of Mercury happened on November 3rd, 1651, being seen at Surat in India by a young Englishman named Shakerley, who, having found by calculation that it would be visible only in Asia, journeyed to India for the express-purpose of witnessing its occurrence. In!a previous, issue of these notes attention has been drawn to the fact that Mercury Bay, a- small inlet- on the east coast of the North Island of New Zealand, owes its name to the observation there. on November 9th, 1769, of a transit of Mercury by Captain Cook, who, in. his barque, the Endeavour, was then exploring that locality.

The Eotation cf the, Earth. In these notes for November'last, the question of the reliability- of the earth as a time-keeper was briefly discussed and reasons \rere given for believing that the time occupied by the earth in completing a "rotation ; on its axis is not an exactly constant quantity, but is subject to slight fluctuations so that the earth is sometimes rotating a little faster and sometinres a little

nlower'than'the average. The"se variations in rotational speed are, of.course,, quite distinct from -the slow secular shortening of the day, which is taking place as an effect of the .tidal friction arising from the attraction of the moon and sun on the waters of the' earth.

It is very important that this variability should be clearly proved and' as this cannot be done by reference to any earthly time-keeper, we must have recourse to the motion of some heavenly body, and the ones best suited for this purpose arc the moon Jupiter's satellites I and 11, and Mercury. The.- motion of the mono is extraordinarily complicated, and.it seems almost impossible to prepare tables in which there is perfect agreement between the calculated and observed positions of our companion. Professor E. W. Brown's tables of the moon's motion, founded on the best observational material and. as far as can be judged on an exhaustive gravitational theory, have only been in use since 1923, but they already fail to'represent precisely enough the moon s longitude, and Brown has been driven to the conclusion that the divergence between theory and observation arises from a fluctuation in the earth's period of rotation. ''.-"' • The planet Mercury transits the sun-s disc about thirteen times in a century, and trustworthy observations of the time of the beginning and. ending,-of transits vt Mercury go back to the., end of the seventeenth century. When the differences between the observed and calculated times aie plotted on. a, curve and compared with Ncwcomb's. curve of the moon's fluctuations, the similarity of the curves is at once apparent and' the suspicion that the length of. the day is not absolutely constant is thereby strengthened. The whole question is dealt with in an interesting manner by Mr E. T. A. Innes, of the Union Observatory, Johannesburg, in a recent issue of "Scientia" and from his paper we learn that in the period 1700 to 1920 the earth has lost, on .the whole, 44 seconds of time—there has been a grader lengthening of the day, interrupt; ed by irregular variations. Mr. Innes also puts; his conclusion in the formthe length of the day is now increasing at the rate of oiie second in. 250 years. As to the cause of these irregular changes in the length of the day, opinion is divided. Brown first' suggested that they may be due to vertical oscillations of the whole crust of the earth, but he offered no explanation of the causes of such change of'level; he is now inclined to attribute the whole trouble to the mutual action of the magnetic fields which, we know exist in the case of the earth and sun, and may reasonably expect to exist in the tnoon, planets,' and satellites, whereby some kind of surge spreads through thj solar system, affecting the planets and satellites in the same way but in different degrees. As far as the earth is concerned there are- two opposing tendencies in play which may affect the speed of rotation, viz., tidal effects which may increase the length of the ,day and secular cooling which would 'shorten it. Jeffreys has sought the origin of the observed fluctuations in : variations of the extent and thickness of the polar ice-caps with-the consequent changes in sea-level-The whole question of the length of the day and its variations is exciting great interest in the astronomical world nt the present time. What « r as only a suspicion in IS6O i\> now, simaking generally, an admitted fact. The explanation is yet to come, and it may be that#vre shall have to wait perhaps for centuries before we find the law which will express exactly- the changes in the rate of the. earth's rotation. In the meantime; we. must receive with greater • reservations-.-,than ever the dates assigned, to historic events'"based on the testimonv afforded by ancient eclipses; the evidential value of these phenomena is'wholly dependent on the accuracy of the earth: as a timekeeper.