TRANSIT OF MERCURY.

Press, Volume LXIII, Issue 19154, 10 November 1927, Page 9

TRANSIT OF MERCURY.

-—•♦ [By £. Q. HOGG, M.A., T.E.A.S.] There will be a transit of the planet Mercury across the sun's diso this afternoon, which will be partly visible m New Zealand. Exterior contact of the planet with the sun's disc at ingress will occur about 3.32 p.m., interior contact at ingress taking place one minute il.-i seconds later. Mercury will be at its leas,t 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. total time occupied by the transit will be about 5 hours 27 minutes.

The transit may he seen either by using a telescope, or by allowing the sun's light to pass through a small hole into a darkened room and catching the image on a sheet of white paper. The shadow of the planet will be seen as a minute black speck which moves very slowly across the paper; if a circle of convenient size be drawn on the paper and the un's image be kept on this circle it is possible, by marking the position of Mercury's shadow from time to time, to obtain an interesting record of this somewhat rare, astronomical event. Much importance is attached just now to the accurate determination or the times of contact at ingress and egress of the planet with the solar disc, as there is cumulative evidence that our earth is not the perfect timekeeper it was once supposed to be and a transit of Mercury affords an excellent meaus of throwing light on a very difficult matter. It would seem that for the past half-century or so the earth has been running slow, so that certain astronomical events are observed to occur a little earlier than they should according to theory. Tho last transit of Mercury—that of May 7th. 1923, which was visible in New Zealand—was carefully observed at many stations in Europe, and on comparing results furnished by the different observatories, it was deduced that the external contact at egress took place no less than 37.7 seconds earlier than the predicted time. It is to be hoped that when the observations at ingress made in New Zealand and Australia, and those made at egress in Europe have been reduced a very definite adition will have been jaada to our knowledge of the variability of tho earth's rotation. The next transit of Mercury will occur in May. 1037. and the following in November, 1940.