ASTRONOMICAL NOTES.
FOR JULY, 1925.
(specially vraiTTXs for "ths press.")
(By E. G. Hogg, M.A., F.R.A.S.)
The planet Mercury sets on July Ist a,t 5.22 p.m., and on July loth at 6.37 p.m.; its apparent magnitudes on these dates are —l.O and 0.1 respectively. Venus sets on July Ist at 5.52 p.m., and on July 15th at 6.27 p.m.; its apparent magnitude during the month will bt, —3.3; Mars sets on these dates at 6.2" p.m. and 6.15 p.m. respectively. Jupiter rises on July Ist at 5.15 p.m., and on July 15th at 4.7 p.m.; Saturn rises on these dates at 1.1 p.m. and 0.6 p.m. respectively and sets on July 2nd and July 16th at 2.33 a.m. and 1.3S a.m. respectively. All the planets mentioned above will be visiblo at the same time during the earlier part of the month —a rather unusual occurrence. On July 11th n very interesting and rare configuration of the planets—Mercury, Venus, and Mars —may be observed. At 1.27 p.m., Venus and Mercury approach within six minutes of are of each other—an apparent distance equal to one-fifth of the moon's diameter. About an hour later Mercury will be about lomin of arc from Mars, and three-quarters of an hour after this Venus will be within 22min of arc •ot Mars. These planets will set together about an hour and a half after the sun. Venus should be looked for during the preceding evenings so as to make sure of its position on the evening of the 11th. The earth will be in aphelion on July 3rd at 4.30 p.m., when its distance from the sun will be about 94,550,000 miles. There will be an annular eclipse of the sun on July 21st, which will be partly visible ?,n Christchurch as a partial eclipse; it begins at 6.33 a.m., but the moon will not rise here until 7.18 a.m. The path of the annular stage of the eclipso passes through the extreme north-west of New Zealand. The maximum obscuration of the eclipse will be reached in Christchurch about 8.10 a.m., when nearly four-fifths of tho sun's disc will be hidden: the eclipse will end for Christchurch about 9.40 a.m.
One of the most important additions to our knowledge made during the past twenty years has been the exact determination of the amount of heat radiated from the sun; this has been ascertained by measuring the heat received per second at the earth's surface, .When the work was begun there was no idea that the amount of heat so received varied: it was believed to be a constant quantity, and was therefore termed the solar constant, and this name has heen kept though later investigations showed that the sun's radiation did not always have the same intensity. Without going into details, we may say that the mean value was found to be 1.94, i.e., the amount of radiant heat from the. sun falling perpendicularly on a square centimetre, if the earth's atmosphere were removed, was sufficient to raise the temperature o? a grain of water in one second by 1.94 degrees Centigrade.
The foremost worker in this field has been Dr. C. G. Abbott, of the Smithsonian Institute, and the observations have been mainly carried on at stations at Mount Harqua Hala, in Arizona, and Mount Montezuma, in Chile, with a check station in Algeria. The principal difficultv encountered has been in making the proper allowance for the amount of heat absorbed by the earth's atmosphere before it has reached the recording instrument, but the skill and patience of Dr. Abbott and his co-workers have overcome this, and now the observations at the different stations show such a high degree of concordance that the results obtained may be received with confidence. ( Dr. Abbott has just published in a graphical form the' results of the observations taken at the mountain station from 1918 to 1924, and from his graphs certain conclusions of some interest may be gathered. In the first place, the solar radiation is subject to rapid variations, so that within a comparatively short space of time its measure has changed from 1.93 to 1.97, an alteration of about two per cent, in its value, a variation which cannot have occurred without affecting considerably the meteorological conditions" prevailing on the earth's surface. Again, w;e note that the average value of the.solar radiation is much higher during the years when sun-spots have been prevalent on the sun's disc, than during the years when the surface has been relatively quiescent. The years 19181921 marked the close of a sun-spot period, when much activity was manifested in the solar atmosphere, and during these years we'enjoyed relatively hot summers and mild winters; in 1922 the average value, of the solar constant fell, reaching—according to the graph—a reading of only-1.88 in the September of that year, and since that date, though it has fluctuated considerably, it has never reached the mean value of 1.94. These later years have been characterised by a lower average temperature on the earth's surface and more humid conditions generally. The material accumulated is now being studied with the object of deters mining the relation between changes in the amount of the solar radiation, and climatic variations, and we may hopefully expect that some important conclusions will result? from this study. For some time past the meteorologists of the Argentine have been supplied with information as to the changes in the solar radiation, and have been able to apply this information with a fair measure of success to the difficult prob-' lem of weather-forecasting. As all our meteorological activities are dependent on the sun's heat, it is only reasonable to expect that as our knowledge of solar conditions increases, we shall he able to trace with greater surety their effects orf terrestrial climate, and thus prepare the way for far more efficient forecasts of weather than we now posAt the meeting in November last, of the Tloval Astronomical Society, Dr. Joans, F.R^S. —the newly elected president of the society—put forward an extremely interesting application of our knowledge of the solar constant. It is | generally held now, that the energy I expended by our sun and other stars in the form of radiant heat is, maintained by the conversion of matter into energy, though the exact nature of the mechanism by which this change is effected is not understood. From the known value of the solar constant, Dr. Jeans has calculated that the sun must be losing mass at the rate of no less than 4,200,000 tons a second, while the mass of a giant star of absolute magnitude —o must be decreasing at 10,000 times this rate! Such a loss of mass each second as i over four million tons strikes us as stupendous, but we must view it in relation to the sun's mass, which is enormous compared with that of the earth. It will help us perhaps to visualise what is taking place when we find that a cube of silver whose edge is 81 yards long, would weigh almost exactly 4,200,000 tons and that the mass lost by the sun during a. whole year would only weight as much as a plate of silver one inch thick covering the entire surface of the earth. It would I take 133 million years for the solar
(Continued at foot of next column.)
mass to diminish by an amount equal to the earth's mass.
It is, however, necessary to bear in mind that the rat© at which a star loses mass by conversion of its matted into radiant energy depends on the mass of the star, the rate of loss being faster when the mass is large than when it is small. If the solar mass is being lessened, it must hav|& been greater in the past than it is now; there may have been a time when the sun's mass was double its present value and there is even the possibility that it once had four times as much mass as it now has. If this ever happened our sun would then have been a B-type star —at its maximum brilliance and surface temperature—and Dr. Jeans shows that if-our sun in its evolution* ever passed through this stage, it did so about 7.1 million, million years ago. This continuous loss of mass which our sun is undergoing has another interesting aspect. The earth and other planets of the solar system are held in their orbits by the sun's attraction, and the -force with which the sun attracts a planet depends on the solar mass; if then the sun's mass is slowly decreasing, the attractive force it exerts on the planets is also slowly decreasing and their orbits are therefore being slowly enlarged. Looking back then we see that there must have been a time in the distant past when the planets circled round the sun in closer attendance than now. We cannot, however, learn anything very definite about the primitive orbits of the different planets as we do not know how long they have been ini existence as separate entities. There seems good reason to believe that some of the oldest known rocks on the earth's surface are over 1000 million years old, and even if the material composing the earth had been collected together for a similar priod of time before it had cooled down "sufficiently" for its rocks to crystallise, 2000 million years is too short an interval in the sun's evolution tor any very marked change in the orbits of the planets to have taken place.
When he comes to deal with the stellar universe as a whole, Dr. Jeans writes: "The mass of the galactic system, of which our sun is a member, must be decreasing with a time-scale which is comparable with that of a typical star. If, as Poincare suggested, the .Milky Way may be regarded as an encircling belt of stars held in equilibrium by slow rotation, then the radius of this ring must be continually expanding. ... If, as is more natural, we regard the galaxy as a system of indeoendently moving stars, ... the galaxy must *» j slowly expanding. When our sun was a B-'type star, and so of mass eqvM to four times its present maw, t«e mass of the whole galaxy must have beeu at least four times what it now is. At this time the .radius of the galaxy would be less than * garter of its present, radius and go stars racked together at least e»"y; r °j£ time*
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Press, Volume LXI, Issue 18425, 4 July 1925, Page 11
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1,753ASTRONOMICAL NOTES. Press, Volume LXI, Issue 18425, 4 July 1925, Page 11
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