TRANSIT OF MERCURY.

New Zealand Herald, Volume LXI, Issue 18702, 7 May 1924, Page 11

TRANSIT OF MERCURY.

PHENOMENON TO-MORROW.

PASSAGE ACROSS THE SUN

OBSERVATION CONDITIONS.

(By the Hon. Director, Wanganui • 5 ; Observatory.)

t The interesting phenomenon ,of a transit of Mercury may be observed to-morrow morning,:. and throughout the day. This is the longest transit since that of May 5, 1907. New Zealand is rather much, toward the cast for tlie most favourable conditions of viewing this transit, as the sun will have set before the planet passes off its disc. Japan is the most favourably placed, as from Tokio the whole transit from beginning to end may be observed. '•

The planet will be in external contact at-9h 15m 31.5s a.m., and will have entered upon the sun's disc at 9h 18m 50.55. Although very easily seen in a telescope of fair power, the planet cannot be seen without . optical assistance. That is, a piece of smoked glass is not sufficient to reveal that interesting happening. A pair of good field glasses or, better still, a good hand telescope, held steadily, will suffice. In any case one must on no account attempt to look at the sun without having a piece of darkened or smoked glass between the eye and telescope or eye and sun.

Point of First Contact. The planet may bo seen making its transit throughout the whole ot the time the sun is visible, after the times given above. The observer should .took at the eastern side of the sun, or. that part- nearest the horizon, a minute or so before 9.15, when presently he will see a minute notch in the limb, or edge, of , the sun, 'as the ; planet comes before our luminary, or between it and ourselves. The point of contact is a little north of his centre (towards the left) and it will be of interest to take a look from time to time during the day atj'd watch the progress of the tiny disc across the sun's face. Unless • the observer has a very powerful telescope he will not perceive the phenomenon until the planet is some seconds past its first contact, or until it has made a perceptable notch, in the side of the sun. The most interesting phase is : when in-, ternal contact is being accomplished. The observed should watch the bright horns of light from the sun, approaching each other, as they encircle the planet. As - they, reach ; out and : the planet is nearly entirely onthe sun, the _ moment of contact arrives, and a fine bright filament of light, which is the continuation of the sun's limb, will appear to: shoot across and join up the two points, : or horns, encircling the planet on its side nearest the edge of the sun. V'-v Mercury, the first planet, so .. far observed, from the sun is 3030 miles in diameter, and is situated lat an average • distance from our luminary of 36,000,000 miles. , Its period of revolution is 88 days. : Its orbit is less circular than any of the two planets being at times less than i 19.000,000 ana at others over 43,000,000 miles . from the sun. It may approach to within less than 50.0u0,000 v miles of the earth and again, when it is on the opposite side of the sun, it may be as much as 136,000,000 miles away. At the nearest its ' apparent diameter 'is very small, 13 ; seconds of arc, which is as if one • viewed a rod 13ft long from a distance of 40 miles. The ': best time to view Mercury are when it makes its eastern elongations, which is 1 when its apparent, distance from the sun is 4 greatest ! after sunset. This separation is never more than 28 deg. That which has just occurred, April 17, was only 19.9 dee,, but the next morning one, toward the west, will be 24.25 deg.. •!j Characteristics of the Planet. - . ' si. Owing to its small departure from the sun, j:I . and the . consequent difficulty >of observation, very little is known of this ' planet's I' physical condition. Compared with the pearly whiteness of Venus, vhe has a marked ; reddish hue; like Venus, he passes through: all the phases with which we are familiar in the moon, being a ' body without light of its own, as well as moving relatively into all corresponding positions in which it offers now a fully illuminated disc, and again the most !"- slender of ' illuminated crescents.

■ An interesting feature seen during some past transits of Mercury is that of a bright spot slightly west of the planet's centre. It has been seen on several occasions, and should be looked for bv those using powerful telescopes. Another, is the appearance of a - bright areola, or encircling halo of light which is, apparently,? brighter than the , photosphere of the sun, as reported on former transits. ; Ithas a violet tint. .. • .' • The bright spot has been thought to" be an- active volcano, but this is an opinion shared by few tstronomefs of to-day. The areola was thought', iq' be due to 'an immense atmosphere, ; bull as the measures made of this areola gave it dimensions' altogether in excess of'possible atmospheric conditions known to ys the idea has been abandoned, and this rather singular / appearance is now considered to be due to certain optical conditions of a refractive nature. As the heat of the sun •on Mercury . would be about 4Jt times as great 'as on. .the earth, i when farthest, and times when nearest the sun, the -areola may' be affected by these conditions as itis seen to change slightly at v different transits. . : " ' ' • The next favourable transit- of ■ Mercury will ;be on November 9. • 1927. / .'