Veiled Spots.—The late Father Perry described these objects at the R.A.S. meeting on May 9, 1884, and said they are to be seen all over the face of the Sun. They only exist for two or three minutes, and then disappear. In one instance he observed a train of these veiled spots stretching over “a tenth part of the Sun’s diameter, which was nearly as obvious to the eye as the penumbra of an ordinary spot; it split into two throughout its whole length, and disappeared in a minute. The veiled spots seem to be of two classes: the one appear like small greyish clouds, which disappear after a few minutes, as if they were formed and rapidly evaporated by the Sun’s heat, and the others seem to be connected with the umbra of ordinary spots; they appear about them, and are more permanent than the ordinary veiled spots, lasting sometimes two or three days, but never longer.” These markings appear to have been first detected by Trouvelot in 1875, and he gives some information as to this class of phenomena in the ‘Observatory,’ vol. viii. pp. 228 et seq.
Recurrent Disturbances[14].—It is supposed, and with good evidence affirming the idea, that certain regions of the Sun’s surface are subject to frequent outbursts of spots, which are possibly due to forces acting from below the Sun’s bright atmosphere. After the disappearance of large groups or isolated spots it is therefore advisable to watch the same region for some time afterwards, to find whether it remains perfectly quiescent, or whether it soon again becomes a seat of activity and change.
Recurrent Forms.—Certain spots observed at different times have exhibited appearances so nearly resembling each other that it has been considered the likeness may be due to something more than mere accident. Whenever such suggestive coincidences are recognized the observer should note them particularly, and secure drawings. It should be his aim to determine the exact intervals elapsing between the presentation of spots or groups of this character, and also whether they occupy the same latitude and longitude on the Sun’s disk.
Exceptional Position of Spots.—The ordinary spots are rarely seen more than 35° distant from the solar equator or within 8° of it. They usually appear in the zones from 8° to 20° N. and S. of the equator. A few exceptions may be mentioned[15]. Mechain saw a spot in July 1780 having a latitude of 40-1/3°; in April 1826 Capocci recorded one having 49° of S. latitude; Schwabe and Peters observed spots 50° from the equator. Lahire, in the last century, described a spot as visible in a latitude of 70°; but the accuracy of this observation has been questioned. Whenever a spot is seen near the equator, or very far removed from it, measures should be taken of its exact place; for it is desirable to learn something more of those disturbances which occasionally affect the more barren regions of the solar envelope.
The Solar Prominences.—Those amateurs who have included a spectroscope in their instrumental outfit will find the study of the chromosphere and prominences a most productive one. Huggins and Zöllner were the first to apply the “open-slit” method; and the study of the shape of the hydrogen prominences commenced in 1869. Tupman details (‘Monthly Notices R.A.S.,’ vol. xxxiii. p. 106) a series of observations which he secured in 1872 with a refractor of 3-inches aperture and a direct-vision spectroscope of five prisms. He mentions the cost of the entire apparatus as only £18, and says he entertains “no doubt that an equally effective instrument could be made for much less.” The prominences appear to be of different kinds, and are known as “cloud”- and “flame”-prominences. Both are liable to rapid changes. Trouvelot, in June 1874, noticed “a gigantic comma-shaped prominence, 82,000 miles high, which vanished from before his eyes by a withdrawal of light as sudden as the passage of a flash of lightning.” Since the study of these remarkable forms was rendered feasible by using a greater dispersion to open the slit of the spectroscope wide enough to see them, they have been made the subject of daily study and record. The results, so far as they have been investigated, show that the region of the Sun’s limb in which the prominences are most frequent reaches to some 40° on either side of the equator, which is somewhat greater than the area of sun-spot frequency. The chromosphere itself is probably of much the same character as the erupted prominences, and formed of little flames arranged thickly together like “blades of grass.”
In observing the Sun with a telescope the amateur will soon notice that the surface is far more brilliant in the central parts than round the margin of the disk. Vögel has estimated that immediately inside the edges the brightness does not amount to one seventh that of the centre. The difference is entirely due to the solar atmosphere, which is probably very shallow relatively to the great diameter of the Sun.
[CHAPTER VI.]
THE MOON.
Attractive aspect of the Moon.—Absence of air and water.—Only one Hemisphere visible.—Earthshine.—Telescopic observations of the lunar surface.—Eclipses.—Lunar changes.—Formations.—Plato and other objects described.—Table of Moon’s age and formations near terminator.—Occultations of stars.—Visibility of the new Moon.
“The western Sun withdraws: meanwhile the Moon,