ECLIPSE OF THE SUN.

Otago Daily Times, Issue 21163, 22 October 1930, Page 7

ECLIPSE OF THE SUN.

LECTURE TO OTAGO INSTITUTE. PHENOMENON VISIBLE TO-DAY. The eclipse of the sun, which, should weather conditions prove favourable, will be visible in Dunedin this morning, was the subject of a lecture by Mr ’A. W. Tucker, M.A., BiSc., before the aistro’npmical branch of the Otago Institute at the Qtpgq Museum last evening. Dr R. Jack presided and briefly' introduced the speaker. Mr Tucker dealt first with the causes of ellipse in general, and slipped that solar eclipses occurred when ‘ the ' moon in ifs orbit came exactly between tlie earth and the sun, while a lunar eclipse occurred when the earth came exactly in line ' between the sun and the trioon and cast a shadow oh the latter, this being possible "only at full moon. A model of- the sun-earth-moop system showed that the moon’s orbit was inclined at an angle of about five degrees to 'that of the earth. The line of intersection of the two' orbits' was not but rotated in a period of about 19 years'. Unless this line, called the line of nodes, waf pointing towards the sun, or nearly so, the moon, when new, was 'often so far above or below the plane of the. earth’s orbit that its shadow was thrown above" or below the earth, and thus no solar eclipse could occur! Correspondingly, when the mopn wps fuff, if was clearly too far below or above the shadow of the earth to allow a lunar eclipse to occur. * ‘

If the line of the nodes were stationary one pnd or the other would point to the earth every six months, but im- the line of nodes retrogressed by 1-19 of a rotation in a year, a particular node would point to the sun in a period of 18-19 of a year, or about 346 days. The other node would have pointed to the sun in half this period (173 days)'. Thus every 173 days one podp or tjie other -vyould be in line with the and sun. If a full or new moon happened lo‘ occur* at these particular times there would be. a lunar or solar eclipse respectively. As the plane of the moon’s orbit was so close to that of the earth it was possible |qr a solar pc]ipse to qepur eyen if the new mpqn Vere hot actually at the node itself, but within 18‘degrees of it, as up to that limit the two orbital planes W§re epmejently ngar |'o each other for gqjar eclipses to" occqr. Ear h lupar eclipse tp occur," hpryeyer, the ipoon the full must be within 12 degrees of the node. By means of the model the lecturer showed eclipses occurring near the nodal points, and failing to occur when the line of nodes was turned sufficiently far away fl'om the pun, The fact that solar eclipses had a larger limit than lunar was due to two tilings. The moon was nearer the sun at wqr eclipsea : than at Ifliiat, and'also the e ai'ib -prqyidcd ft relatively larger screen tp receiye the mqdn’s shadow than the moon did tp receive the shadow of the earth. Contrary, therefore, to what most people would expect, solar' eclipses were mqre frednent than lunar in the proportion p| about three to two, but the area in which a solar eclipse; wap visible was very small compared with a whole hemisphere for a lunar eclipse and thus at any particular plpce a solar eclipse waft ft very rare occurrence. By means of-an eclipse gauge Mr Tucker showed that in any calendar year the number might vary from a maximum of live solar and two lunar eclipses, or four solar and three lunar eclipses, down to a tnini«UU!P of two eclipses, both solar, in the one 1 year. Regarding the prediction of eclipses, the lecturer referred to the'periqd of 18 years 10 L 3 days in which all eclipses repeated themselves almost exactly! This period was discovered by the Chaldeans and was known as the Saroa. A lunation, or period between ope pew moon and the Wfkt, occupied 29.53 gays* Sun, nipon, apd earth came in line —a nodal half-period— every 173,31 days.’ Suppose, therefore, that a solar'cclipse, at new , moon, occurred at a node, 223 lunation? of 28.53 days each would equal 6585,32 days, apd 38. nodal half-periods pf 173.31 days each would equal 6685.78 days. The coincidence of these periods meant that every 6585.32 days the sun, moon, and earth would come into almost exactly the same relative positions, again, pud there would be the same eclipse, only very slightly modified by the small difference in the periods. The odd one-third pf a day would mean that the earth wopld have made qnethird of a rotation beyond the even nay, sq that in 54 years’ time to-day’s eclipse would appear again in New Zealand on November 24.

A map o£ the path of to-day’s eclipse was shown. The times p£ the beginning and end of the eclipse were given for Dunedin, namely, 8.15 a.m. and 9.58 a.m., New Zealand standard time. Daylight saving time would be 30 minutes later by the clock. It was pointed out by the lecturer that a partial eclipse was a very disap* pointing spectacle compared with a total eclipse Lantern slides were used to show the corona and solar prominences visible in total eclipses, and reference was made to the scientific aims of observations of such phenomena. Great credit was due to the people of New Zealand, whose interest in science was great enough to cause them to subscribe so liberally as to enable a New Zealand astronomical expedition to proceed to Nioufoou, said Mr Tucker in conclusion, even though the utility to man of such astronomical research might not at present be apparent. Mr Tucker was accorded a hearty vote of thanks for his address.

ARRANGEMENTS FOR OBSERVAHON. MOVING PICTURE TO BE TAKEN. (Pee United Pbess Association.) . WELLINGTON. October 21. The eclipse of the sun to-morrow, which will be total at the island of Niua Foou, in the South Pacific, will be visible as a partial eclipse throughout New Zealand. The circumstances of the eclipse are: It begins (first contact) at 8.33 a.m. (Few Zealand summer time), and its greatest phase is at 9.31 a.m. The magnitude of the eclipse is 0.42, which means that at the greatest phase slightly less than half the suns diameter' will be obscured by the moon. Observations will be made at Kel'burn by the staff of the Dominion Observatory, assisted by members of the New Zealand Astronomical Society. A series of photographs of the eclipsed sun will be taken with the nine-inch equatorial telescope of the city observatory, the camera shutter automatically making a record of the chronograph. A moving picture of the eclipse will also be taken. The camera will ge attached to the five-inch equatorial telescope.