ICE AGES.

Hawera Star, Volume XLVI, 13 November 1926, Page 6

ICE AGES.

ASTRONOMICAL CAUSES. PAPER BY DR. W. M. THOMSON. “The astronomical cause of an me ao-e” was the subject of an interesting piper delivered by Dr. W. M. Thomson - Italic weekly meeting °f tte Hawcra Astronomical Society on Thursday. “Geologists seem to be fairly generally agreed that what is known as an :cc age occupied the greater part of tho pleistocene period,” said the lecture*. * ‘ They even go so far as to state that there were live or six glacial periods in - succession, with intervening gen.al periods. Both European and American geologists recognise such features of the ice ages in the deposits of their respective countries. Ice action is to be evident also m some of the much older rocks, e.g., eambian, m Central China and South Australia; carboniferous in South Africa, Australia and Central India. Geologically the sedimentary rocks are divided into three mam epochs: Primary, Palaeozoic; secondary, Mezozoic, Tertiary, Cainozoie. < “The great ice age is of considerable interest to us as immediately following it man appeared, and if science can evei furnish a date for this period, the actual duration of man’s existence will be approximately fixed. “The theories advanced to account for the ice age include (1) a general cooling of the earth due to unknown causes (this we will dismiss at once as there is no data available to give anv help at all); (2) a distribution of land and water causing alteration m climate through land barriers, elevation or lands, influence of wind, etc.; (3) astionomical theories: (a) variation m the sun’s output of heat, (b) passage of the solar system through a part of the universe that was colder, (c) excessive variations in the earth’s orbit, (d) (the present theory) known variations in the earth’s orbit combined with variations in the line of equinoxes. “Sir Robert Ball takes it as proved that the earth’s orbit can only vary slightly: no major purturbations are possible. It varies from an almost perfect circle to an ellipse. This variation is caused mainly by the pull of the planets Venus and Jupiter, the latter having only half the influence of the otheT. The* influence of the other planets is almost negligible. Venus exercises an influence of one twenty - seven thousandth of that of tlie sun. The eccentricity of the orbit varies at a very slow rate, hundreds of thousands of years being required to pass from ellipse to circle and vice versa. “When the orbit is circular, and the line of the equinoxes is parallel to the major axis, the seasons are of equal length, and the climates of the two hemispheres will be very similar, but each receives 63 per cent, of heat in summer and 37 per cent, in winter. Every 21,000 years, the line of the equinoxes is so placed with reference 1> the elliptic path of the earth that the difference in duration between the two -seasons attains a maximum. Given .all the masses and other necessary data connected with the solar system, it required to determine the greatest eccentricity that the earth’s orbit can assume under the disturbance arising from the other planets. This problem has been solved and the mean between the different determinations of its maximum value may be adopted. This shows that -when all circumstances combine to accentuate as much as possible the difference in the length of the seasons, one of them may be 196 days long and the other 166 —33 days difference. As the rotation of the line of the equinoxes takes place every 21,000 years, '•t is probable that the earth can pass through this period of sharply accentuated seasons at least two and perhaps several times; because the orbit changes from its state of high eccentricity with marked slowness. “In the case of the earth’s orbit neing nearly circular, and the line of the equinoxes so placed that the seasons are of equal length, each hemisphere in turn passes through the same relative position to the sun and receives exactly l the same amount of radiation. Each, pole is glaciated and the glaciation advances and recedes approximately to the same extent in every year. When the reverse condition is present and the

earth’s orbit is in its maximum posi/tion of eccentricity, each hemisphere by the precession of the equinoxes, passes through a state in which the winter is 33 days longer than the summer, but it takes 10,500 years for the change of seasons to pass from one hemisphere to the other. During the short summer, say of the Northern Hemisphere, when the earth is near the sun there is likely to be intense heat while the corresponding winter of the Southern Hemisphere is likely to be mild because of the proximity to the sun, and it is also a short winter. The Northern Hemisphere then has a long winter far from the sun and the south would have a mild ■summer. In these conditions glaciation at the North Pole may be expected to extend its limits each winter, and this extension does not completely melt during the summer. As the chance of seasons is slow this recurring increase of

glaciation may go on for a thousand years, and a very considerable area may be glaciated. The conditions for each hemisphere are reversed every 10,50(1 years, and the Northern Hemisphere Vould then have the long summer and ,»nort winter while glaciation would be transferred to the South Polar region. ‘Such a theory would demand that an ice age would be simultaneous in Eurasia and America, that it would gradually disappear in the North Hemisphere and appear in the south, and would reappear perhaps several times. Geology appears to support, this.” The lecturer concluded by mentioning certain eminent geologists who had disagreed as to the number of glacial periods experienced in Europe. Appreciation of the paper was goner-, ally expressed, and a good discussion followed, after which members observed the moon and several planets visible with the aid of the telescope.

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