OTAGO INSTITUTE

Evening Star, Issue 18627, 7 May 1924, Page 5

OTAGO INSTITUTE

ASTRONOMICAL branch.

TRANSIT OF MERCURY

To-morrow the planet Mercury will pass directly between the earth and the sun, and, with optical aid, will be visible as a small dot on the solar disc. Only two known planets traverse the region between the earth and sun—namely, Venus and Mercury’—and therefore they alone are capable of occasioning such a phenomenon. Our moon, of course, acts in an analogous and much more arresting_ manner during a solar eclipse. Transits of Mercury, although occurring in this epoch only about thirteen times per century, are not so infrequent as those of Venus, the latter planet not being due to transit again until the year 2004. For the benefit of those who will watch the sun on Thursday it may be noted that the small spot will take nearly eight hours in its passage. The following times of contact are given for Wellington, and they will be very nearly correct here, as, owing to our distance from Mercury, the shadow is exceedingly rapid in its sweep, attaining a velocity of about 2,300 miles per minute. First external contact ... 9.15 a.m. First internal contact ... 9.18 a.m. Last internal contact ... 6.5 p.m. Last external contact ... 5.8 p.m. The sun will thus have set before the planet has passed off the disc. The naked eye will scarcely perceive such a small object, for the apparent diameter of Mercury is only about l-160lh that of the sun. A small telescope or good field glass should, however, reveal the dark spot, but in every case a shaded eyepiece should be used for direct vision. In the absence of an orthodox shade a smoked glass or fogged photographic negative may act as a substitute. The ingress of the planet will occur at 58deg east of the north point on the sun’s dial, or corresponding to about 5 o’clock, if the clock-face analogy be used. At 1 o'clock the spot will be a little below the centre. The New Zealand Astronomical Society has issued a bulletin on the transit, and anyone wishing more detailed information may consult it at the Otago Institute’s observatory this evening, where, at 8 o'clock, Professor D. R. White will speak about, and by diagrams illustrate, the phenomenon.