The Astronomy of Milton's 'Paradise Lost' by Thomas Nathaniel Orchard (read along books .TXT) 📖

Sun-spots usually appear in groups; large isolated spots are of rare occurrence, and are generally accompanied by several smaller ones of less perfect formation. The exact moment of the origin of a sun-spot cannot be ascertained, because it arises from an imperceptible point; it grows very rapidly, and often attains its full size in a day.

Prior to its appearance there is an unusual disturbance of the solar surface over the site of the spot: luminous ridges, called faculæ, and dark ‘pores’ become conspicuous, between which greyish patches appear, that seem to lie underneath a thin layer of the photosphere; this is rapidly dispelled and a fully formed spot comes into view. When a sun-spot has completed its period of existence, the photospheric matter overwhelms the penumbra, and rushes into the umbra, which it obliterates, causing the spot to disappear. The duration of sun-spots is subject to considerable variation; some last for weeks or months, and others for a few days or hours. A spot when once fully formed maintains its shape, which is usually rounded, until the period of its breaking up. Spots of long duration rotate with the Sun. Those which become visible at the edge of the Sun’s limb have been observed to travel across his disc in less than a fortnight, disappearing at the margin of the opposite limb; afterwards, if sufficiently long-lived, they have reappeared in twelve or thirteen days on the surface of the orb where first observed. It was by observation of the spots that the period of the axial rotation of the Sun became known.

Sun-spots vary very much in size—some are only a few hundred miles in width, whilst others have a diameter of 40,000 or 50,000 miles or upwards. In some instances the umbra alone has a breadth of 20,000 or 30,000 miles—three times the extent of the diameter of the Earth. Spots of this size are visible to the naked eye when the Sun is partially obscured by fog, or when his brilliancy is diminished by vapours near the horizon. A year seldom passes without the occurrence of several of such spots being recorded. The largest sun-spot ever observed had a diameter of about 150,000 miles. A group of spots, including their penumbræ, will occupy an area of many millions of square miles.

By long observation it has been ascertained that sun-spots increase and diminish in number with periodical regularity, and that a maximum sun-spot period occurs at the end of each eleven years. When spots are numerous on the Sun’s disc there is great disturbance of the solar surface, accompanied by fierce rushes of intensely heated gases. This solar activity is known to influence terrestrial magnetism by causing a marked oscillation of the magnetic needle, and giving rise to so-called ‘magnetic storms,’ accompanied by magnificent displays of auroræ, with variations in electrical earth-currents. It would therefore appear that sun-spots have a pronounced effect upon magnetic terrestrial phenomena, but how this is produced remains unknown.

Besides sun-spots, there are seen on the solar disc bright flocculent streaks or ridges of luminous matter called faculæ; they are found over the whole surface of the Sun, but are most numerous near the limb and in the immediate vicinity of the spots. They have been compared to immense waves—vast upheavals of photospheric matter, indicative of enormous pressure, and often extending in length for many thousands of miles.

Nearly all observers have arrived at the conclusion that sun-spots are depressions or cavities in the photosphere, but considerable difference of opinion exists as to how they are formed. The most commonly accepted theory is that they are caused by the pressure of descending masses of vapour having a reduced temperature, which absorb the light and prevent it reaching us. Our knowledge of the Sun is insufficient to admit of any accurate conclusion on this point; though we are able to perceive that the surface of the orb is in a state of violent agitation and perpetual change, yet his great distance and intense luminosity prevent our capability of perceiving the ultimate minuter details which go to form the texture of the solar surface. ‘Bearing in mind that a second of arc on the Sun represents 455 miles, it follows that an object 150 miles in diameter is about the minimum visible even as a mere mathematical point, and that anything that is sufficiently large to give the slightest impression of shape and extension of surface must have an area of at least a quarter of a million square miles; ordinarily speaking, we shall not gather much information about any object that covers less than a million.’[13] Since the British Islands have only an area of 120,700 square miles, it is evident that on the surface of the Sun there are many phenomena and physical changes occurring which escape our observation. Though the changes which occur in the spots and faculæ appear to be slow when observed through the telescope, yet in reality they are not so. Tremendous storms and cyclones of intensely heated gases, which may be compared to the flames arising from a great furnace, sweep over different areas of the Sun with a velocity of hundreds of miles an hour. Vast ridges and crests of incandescent vapour are upheaved by the action of internal heat, which exceeds in intensity the temperature at which the most refractory of terrestrial substances can be volatilised; and downrushes of the same photospheric matter take place after it has parted with some of its stores of thermal energy. Sun-spots of considerable magnitude have been observed to grow rapidly and then disappear in a very short period of time; occasionally a spot is seen to divide into two or more portions, the fragments flying asunder with a velocity of not less than 1,000 miles an hour. It is by these upheavals and convulsions of the solar atmosphere that the light and heat are maintained which illumine and vivify the worlds that gravitate round the Sun.

During total eclipses of the Sun, several phenomena become visible which have enabled astronomers to gain some further knowledge of the nature of the solar appendages. The most important of these is the Chromosphere, which consists of layers of incandescent gases that envelop the photosphere and completely surround the Sun. Its average depth is from 5,000 to 6,000 miles, and when seen during an eclipse is of a beautiful rose colour, resembling a sheet of flame. As seen in profile at the edge of the Sun’s disc, it presents an irregular serrated appearance, an effect created by the protuberance of luminous ridges and processes—masses of flame which arise from over its entire surface. The chromosphere consists chiefly of glowing hydrogen, and an element called helium, which has been recently discovered in a terrestrial substance called cleveite; there are also present the vapours of iron, calcium, cerium, titanium, barium, and magnesium. From the surface of this ocean of fire, jets and pointed spires of flaming hydrogen shoot up with amazing velocity, and attain an altitude of ten, twenty, fifty, and even one hundred thousand miles in a very short period of time. They are, however, of an evanescent nature, change rapidly in form and appearance, and often in the course of an hour or two die down so as not to be recognisable. These prominences, as they are called, have been divided into two classes. Some are in masses that float like clouds in the atmosphere, which they resemble in form and appearance; they are usually attached to the chromosphere by a single stem, or by slender columns; occasionally they are entirely free. These are called quiescent prominences; they consist of clouds of hydrogen, and are of more lasting duration than the other variety, called eruptive or metallic prominences. The latter are usually found in the vicinity of sun-spots, and, besides hydrogen, contain the vapours of various metals. They are of different forms, and present the appearance of filaments, spikes, and jets of liquid fire; others are pyramidal, convoluted, and parabolic.

These outbursts, bending over like the jets from a fountain, and descending in graceful curves of flame, ascend from the surface of the chromosphere with a velocity often exceeding 100 miles in a second, and frequently reach an enormous height, but are of transient duration. They are closely connected with sun-spots, and are evidence of the tremendous forces that are in action on the surface of the Sun.

The Corona is an aureole of