Fig. 551.—Solar prominences.
There are valid observations against two items in the support of the old theory—viz., the affirmed connection of the evening and morning cones seen on the same night (if the corresponding sides be prolonged), and the participation of the cones in the daily motion of the heavens. The zodiacal light is sometimes seen when daylight has not yet disappeared; and, on the other hand, it sometimes fails to appear, though there is complete darkness. There would seem to be a real lengthening and shortening. It has been observed by Schiaparelli, that the light is much more difficult to make out when it passes through the meridian, than when it is only 30° above the horizon, and is less easily seen when the air is clearest than when a sort of mist is present. Indeed, the bright parts of the Milky Way may be seen to be weakened by mist, while the zodiacal light at the same height is unaffected. The zodiacal light has temporary variations of light intensity, and it shows from time to time remarkable changes of form and position, so sudden and short as to be hard to explain on the planetary hypothesis. The elongations of the cones show a half-yearly period, which is independent of the transparency of the air. The cone follows the observer northwards or southwards, so that there is no parallactic action; and this peculiarity (so adverse to the extra-terrestrial hypothesis) cannot be explained by reflection or absorption of light. As to spectroscopic observations, the author finds (1) that the zodiacal light consists partly of proper light; (2) that its connection with polar light is but secondary, temporary, and accidental; (3) that the cause of the second phenomenon is such that it may strengthen the zodiacal light and modify its spectrum; and (4) that the results of spectrum analysis rank with other arguments tending to find the source of the zodiacal light in the neighbourhood of the earth (like the polar light). Herr Gronemann, then, thinks the zodiacal light a terrestrial phenomenon, though he will not say that it cannot be influenced by cosmic action. He throws out the suggestion that the cone may be a kind of optical illusion, arising from some fine matter—gas or dust—being more accumulated near the observer in one direction than another. The apparent length of the cone might be conditioned by the conical shadow of the earth, and the changes of length be due to cosmic and electric influences. In any case, there is need of a more scientific theory than the old one.
Fig. 552.—On the sun’s disc.
We may conclude our brief notice of the great luminary to which we are indebted for everything, by a resumé of his distance from us, his diameter, and a few other plain facts and figures. In the first place the actual distance of the sun from the earth has never accurately been determined, but perhaps the next transit of Venus will assist the observers to a nearer estimate. It is quite sufficient for our purpose, however, to state that the sun’s distance from the earth is 92,000,000 of miles. The distance varies in winter and summer. In the former period the sun is nearer than in summer, and yet as the rays strike over us, and pass us, as it were, we feel less heat. When, as in summer, we are more in the focus of the rays, we feel the greater heat.
We have already spoken of parallax, and it is by finding the solar parallax that the distance of the sun from us is found. This parallax has not been exactly ascertained, or rather authorities differ, and as difference of 0·01″ in the solar parallax means something over 100,000 miles of distance, it is evident that exactness is almost impossible. If 8″·80 be settled as the solar parallax, 92,880,000 miles is the distance of the sun from the earth. If 8″·88 be taken we have nearly 92,000,000 exactly.
The volume of the sun is 1,253,000 times that of the earth, and yet the density of the former is only about one-fourth of the latter, so the attraction of gravitation at the sun must be more than that of the earth’s surface twenty-seven times. A body dropped near the surface of the sun would fall 436 feet in the first second, and have attained a velocity of ten miles a minute at the end of the first second. The diameter of the sun depends in our calculations upon its distance from the earth. If we suppose that to be 92,880,000 miles, the diameter is 866,000 miles. If we take 91,000,000 of miles as the distance from the earth the diameter is 850,467 miles. The sun makes (apparently) the circuit of the heavens in 365 days, 6 hours, 9 minutes, and 9·6 seconds; the transit from one vernal equinox to the next being only 365 days, 5 hours, 48 minutes, 48·6 seconds, owing to the precession of the equinoxes already mentioned.
When we consider the power and grandeur of the sun we may well feel lost in the contemplation. The sun balances the planets and keeps them in their orbits. He gives us the light and heat we enjoy, and coal-gas is merely “bottled-up sunlight.” In darkness nothing will come to maturity. We obtain rain and dew owing to the sun’s evaporative power; and no action could go on upon earth without the sun; and yet we receive only about 1/2070650000 part of its heat and light.
As to the colour of the sun, Professor Langley states that it is really blue, and not the white disc we see. The whiteness is due to the effect of absorption exerted by the vapourous metallic atmosphere surrounding our luminary; and if that atmosphere were removed, his colour would change.
