THE TIDES.

Otago Daily Times, Issue 18851, 2 May 1923, Page 4

THE TIDES.

THEIR ASTRONOMICAL EXPLANA TION. *

The Astronomical branch of tho Otago Institute hold its first general meeting of tho year at (lie Museum last evening. Professor D. R. White, the chairman of tiio branch, presided, and delivered an address on “The Astronomical Explanation of Tides.” He announced that the observatory on tho Town Belt was now open to the public. Popular subjects dealt with at their meetings would bo illustrated and explained to visitors at the observatory. At the outset of his address Professor White expressed his thanks to Mr J. M. Wilkie, engineer of the Otago Harbour Board, and to Dr Adams, Government Astronomer, for information and assistance afforded. Ho devoted attention to the tides of Otago Harbour, and said that the tables compiled fay the Otago Harbour Beard gave ample evidence of groat car© and accuracy. Such calculations could not bo made without considerable theoretical and practical knowledge of astronomy. He explained some characteristic features of the resemblance between the movements of the tide and th© movement of the moon. Tho intervals between tho tides were not regular, as might bo supposed, but varied from, day to day. On the average the tides were 50 minutes later every day, and on the average the moon rose 49.8 minutes later every day. Th© shortest time from moonrise to moonrise was 24 hours 35 minutes, and- the largest 25 hours 3 minutes. These and other uniform variations were not mere coincidences, but the moon exercised a controlling. influence over the vast masses of the ocean. During the month he had examined from new moon on- February 16 last- to new moon on March 18, the highest tide in Otago Harbour was 86 inches, and the lowest 59 inches. ■ He proceeded to explain how the gravitational pull of the moon generated the tides, making use pf several instructive diagrams. He mentioned that at the Poles there are no tides at all. The cause of the tides was due to the inequality of the pull exercised bv the moon on.,tho opposite, sides of the earth The sun also produced'tides, and if there were no moon there would •still be solar tides. If seven represented the total tidal force, then five would represent the power exercised by the moon and two that exercised by the sun. The crest or peak of the solar tide coincided with the crest of tho lunar tide only at new moon and full moon. The sun’s tide-raising influence was greatest at the equinoxes, when it was three million miles nearer th© earth than it was at mid-summer or mid-winter. The spring tides were the highest, because when they occurred the sun and the moon were both pulling in the same direction. At neap tides the tide-raising forces of the sun ana moon were pulling at right angles. He explained why tho tides at full moon were always a little less than those at new moon. He showed that the origin of all tidal movements was the great Southern Ocean, of which New Zealand was nearly the centre. Hence New Zealand was favourably situated for tho study of ■ theories of tho tjdes. A tidal wave was not a horizontal motion of the water, but a vertical motion —“a hunching up of tho water” from its lowest depths. The earth rotated from west to cast, and the tidal wave from east to west, but it was slower in its rotation than the earth. The height of the tide in mid-ocean was not more than 2jft or 3ft. He referred to subsidiary meteorological causes of the tides, such as the influence of varying atmospheric pressures; He illustrated th® principles lie had been expounding front a diagram of recent tide records of the Otago Harbour The greatest height of tide ever recorded here, 99in, occurred at new moon in perigee on August 4, 1921. Subsidiary causes were also favourable to high tides at that time. The barometer was falling steadily for three days, and a strong northeast wind was blowing up the harbour. The slowing down of the earth’s rotation was being caused by the motion of tho tides. The tides had also their effect upon the moon

Mr J.. W. Milnes said that Professor While had given them all a grander conception of the meaning and movement of the tides. He had dealt with the matter in a comprehensive and thoroughly instructive way, and they were all most grateful. On his motion a vole of thanks was Carried by acclamation.