MOTION OF THE SUN—THE SEASONS—CHARACTER OF THE SUN—SUNSPOTS—ZODIACAL LIGHT.

Suppose that we rise early in the morning we shall, as the reader will say see the sun rise—that is, he appears to us to rise as the earth rotates. By the accompanying diagram (fig. 544) we can understand how Sol makes his appearance, and how he comes up again; not, it will be observed, after the manner stated by the Irishman, who declared that the sun “went down, and ran round during the night when nobody was looking.” The earth rotates from west to east, and so the sun appears to move from east to west. If we look at the diagram we shall see that after rising at O, the sun advances towards the meridian in an oblique arc to A, the highest or culminating point—midday. He then returns, descending to W; this path is the diurnal arc. At Q similarly, during his passage in the nocturnal arc, he reaches the lowest or inferior culmination. HH´ is the meridian.

On the 21st of March, this path brings the sun on the “ equinoctial” line mentioned at the close of the last chapter. Day and night are then of equal duration as the arcs are equal. So this is the Vernal (or spring) Equinox. Some weeks after the sun is at midday higher up at S´, so the diurnal arc being longer, the day is longer, (Z is the zenith, Z´ is the nadir, P P´ is the celestial axis). From that time he descends again towards the equinoctial to the autumnal equinox, and so on, the diurnal arc becoming smaller and smaller until the winter solstice is reached (S).

From what has been previously said, it is evident that the sun has a twofold apparent motion—viz., a circular motion obliquely ascending from the horizon, which is explained by the rotation of the earth, and by our position, o, to the earth’s axis, p p´, and also by a rising and setting motion between the solstitial points, S and S´, which causes the inequality of the days and nights. Independently of the daily motion of the sun, we observe that at the summer solstice, on the 21st of June, at midday, the sun is at S´, and one half year later—viz., on the 21st of December, at midnight, the sun is at s, from which he arrives again in the space of half a year at S´; so we are able to represent this annual motion of the sun by a circle, the diameter of which is the line, S´ s. This circle is called the Ecliptic.

The plane of the ecliptic, S´ s, cuts the plane of the equinoctial, A Q, at an angle of 23½°, and the axis of the ecliptic, S´ s, makes the same angle with the axis of the heavens, P P. The two parallel circles, S´ s´ and S s, include a zone extending to both sides of the equinoctial, and beyond which the sun never passes. These circles are called the Tropics, from t??p?, I turn, because the sun turns back at these points, and again approaches the equinoctial. The parallel circles, S s, and S´ s´, described by the poles of the ecliptic, S´ s, about the celestial poles, P P, are called the arctic and antarctic circles.

Whenever the sun crosses the equinoctial, there is the equinox; but the points of intersection are not invariably the same every year. There is a gradual westerly movement, so it is a little behind its former crossing place every year. (See diagram, fig. 547.)

[This is the “Precession of the Equinoxes,” because the time of the equinoxes is hastened, but it is really a retrograde movement. Hipparchus discovered this motion, which amounts to about fifty seconds in a year. So the whole revolution will be completed in about 28,000 years.]

It is obvious, then, that the sun is the most important star in the universe; and when we come to speak about the earth we shall consider the seasons, etc., more fully. Now we must endeavour to explain what the sun is like, and this can only be done with specially darkened glasses, for a look at the sun through an ordinary telescope may result in great, if not permanent, injury to the eye.

The sun is not solid so far as we can tell. It is a mass of “white-hot” vapour, and is enabled to shine by reason of its own light, which the planets and stars cannot do; they shine only by the sun’s reflected light. So we may conclude the sun to be entirely gaseous, but, thanks to the recent researches in spectrum analysis (already explained), by which the light of the sun has been examined by means of the spectroscope, and split up into its component colours, Mr. Lockyer and other scientists have discovered that a number of elements (metals) exist in the sun in a fused, or rather vapourous state, in consequence of the intense heat. Hydrogen exists in the sun, with other gases unknown to us here, and many metals, discovered by their spectra, which are the same under similar circumstances.

The sun is supposed to be spherical in shape,—not like the earth, flattened at the poles,—and to be composed of materials similar to what the earth is composed of, and what it would be if it were as hot as the sun is. Thus we can argue by analogy from the spectra of earthly elements, that, as the sun and star light gives us similar spectra, the heavenly bodies are composed of the same elements as our globe. We can thus form our opinion of the sun’s constitution. Mr. Neisen says:—

“With the aid, therefore, of the additional information given us by the spectroscope, it is not very difficult to form a true idea of the probable condition of the surface of the sun, which is all that we can see. It is the upper-lying strata of a very dense atmosphere of very high temperature—an atmosphere agitated by storms, whirlwinds, and cyclones of all kinds, traversed by innumerable currents, and now and then broken by violent explosions. Above the brilliant surface which we see is a less dense and somewhat cooler upper stratum, which, though hot enough to shine quite brightly, is quite invisible in the presence of the brighter strata beneath it.”

Sun Spots, as they are generally called, are hollows in the sun’s vapoury substance, and are of enormous extent; and there are brilliant places near those spots, which are termed faculÆ. These spots have been observed to be changing continuously, and passing from east to west across the sun, and then to come again at the east to go over the same space again. Now this fact has proved that the sun turns round upon his axis, and although he does not move as we imagine, from east to west, round the earth, the orb does move—in fact, the sun has three motions: one on his axis; secondly, a motion about the centre of gravity of the solar system, and a progressive movement towards the planet Hercules.

If we examine the surface of the sun through a proper telescope, we shall find that the even surface we can plainly distinguish at sunset is marked, and the brightness is greater towards the centre of the orb. We can perceive various irregularities; we shall find spots, faculÆ (little torches), etc. These spots were discovered by Galileo and other astronomers, and were, as we have stated, found to be surface markings, and not a series of bodies passing between the earth and the sun. The rotation of the sun was measured, and it was found that the orb revolved in about 25-1/3 days, and in such a manner as to be slightly inclined to the plane of the ecliptic.

Herschel observed a spot at least 50,000 miles in diameter, which is more than six times the diameter of the earth. The sun spots are observed to be constantly changing, and are naturally observed differently as the revolution proceeds. The dark pole, or “nucleus” (umbra), as it is called, is surrounded by a less dark surface called the penumbra, but the umbra is not really dark; it is extremely bright when viewed alone, as has been proved by Professor Langley, while the heat is even greater in proportion. But the umbra of a sun spot must be below the level of the penumbra, for the shape changed as the sun revolved on its axis. The penumbra was wider on the side nearest the edge of the solar disc, and the umbra may be due to the uprushing or downpouring of gas or vapour like “whirlpools in the solar atmosphere.”

Near the sun spots the long streaks, or faculÆ, are often observed by the borders of the disc, and a transition of the luminous part of the photosphere26 into darkness has been observed, and bright bridges crossing the spots, and then gradually getting dark, were seen by M. Chacomac. The sun spots vary in direction, but the same general course is continued. Sometimes they describe curves, sometimes lines.

During solar eclipses the sun exhibits what are termed “red prominences,” which are the luminous vapours existing around the sun. When the orb is eclipsed, we can see the bright-coloured vapours shooting out from underneath the dark shadow, and this light is termed the “coronal atmosphere”; the vapours are called the sun’s chromosphere. In the coronal atmosphere are certain curious shapes of vapour thrown up, and frequently changing,—projecting, in fact, from the gaseous envelope. These red prominences were first observed in 1842, and in 1851 it was proved that they appertained to the sun, for the moon hid them as the eclipse began. Before the prominences were discovered, the red light surrounding the solar disc was known, and called the “sierra” (now chromosphere), or chromatosphere. “The luminosity of these prominences is intense,” says Secchi, “and they rise often to a height of 80,000 miles, and occasionally to more than twice that; then bending back, they fall again upon the sun like the jets of our fountains. Then they spread into figures resembling gigantic trees, more or less rich in branches.” We give some illustrations of the appearances of these prominences.

The zodiacal light is often observed. It is a glow, and frequently of a rosy tint. It may be seen in England in March or April before sunset, or in the autumn before sunrise; and it is doubtful whether it be a terrestrial or an extra-terrestrial light—a lens-shaped object surrounding the sun. Some philosophers maintain that the light is caused by multitudes of minute bodies travelling round the sun; but Herr Gronemann has lately fully discussed the observations, and the drift of his contention is under stated:—

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.

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.