CHAPTER II.

ON THE SUN O AND MOON (

THE Connection of the Sun, in some way or other, with the wondrous processes by which the fruits of the earth grow, mature, and decay, is too obvious to have been overlooked in any age. Yet in the infancy of science the most intelligent observer, equally with the most superstitious worshipper of the "Greater Light" that rules the day, could have formed but a very inadequate estimate of the marvellous forces which the Sun is perpetually exerting, and of the extent to which the earth is now known to be indebted for its vast treasures to solar energy.

Currents in the air and ocean, winds and rain, springs and rivers, volcanic eruptions, various geological changes and almost every motion observed on the earth's surface, modern science has shown to be wholly or chiefly due to the action of the Sun. To the same source we owe those treasures of hidden energy stored up in our coal fields, and that power which vegetables possess of drawing support from inorganic matter in order that, in their turn, they may furnish sustenance for animals and men. Further, from the Sunbeam, to which nature owes her charming beauty, Spectroscopic Analysis has elicited the most interesting information with respect both to the actual condition and the constituent elements of the Sun.

That the Moon shines by reflecting the light of the Sun was taught among the Greeks by Anaximander, and was probably known or suspected at a still earlier period. The Moon, however, was long believed to have a light proper to herself, and to this was attributed the faint light, sometimes seen on that part of the lunar surface not directly illuminated by the Sun, till Mæstlin, the preceptor of Kepler, furnished the true explanation of the phenomenon. Ptolemy detected the inequality in the Moon's motion called the Evection; Galileo discovered the Diurnal Libration of the Moon, and showed that the Moon is not a perfectly round, smooth body, as had been asserted, but that her surface, like that of the earth, is diversified by mountains and valleys. The same philosopher first announced the dark, irregular Spots, sometimes appearing on the solar disc, and from their motion he concluded that the Sun rotates on a fixed axis.

Eclipses of the Sun and Moon are phenomena which have excited the superstitious terrors of all nations. The Hindoos are said to have a custom of beating gongs and uttering wild screams and shouts of vengeance during

a Solar Eclipse, in the belief that some horrid monster is devouring the Sun. When the Eclipse passes off these frightful noises are exchanged for shouts of joy, as the natives fancy they have driven the greedy monster to disgorge their deity. Large numbers of Eclipses were recorded by the Chinese and the Chaldeans, nor were the ancients without the means of foretelling Eclipses of the Moon. Thales is said to have predicted one remarkable Solar Eclipse which, occurring while the Medes and Lydians were engaged in battle, put an end to the conflict and led to the conclusion of peace. By calculating backwards astronomers have fixed the date (May 28, B.C. 585) of this Eclipse, and even determined the locality, till then unknown, in which the battle was fought.

LESSON 7.-THE SUN: SOLAR SPOTS.

1. Q. Why does the Sun appear no larger than the Moon?

A. Though the sun's diameter is 396 times greater, he appears no larger than the moon, because his distance is nearly as many (385) times greater than that of the moon. The sun is really one of the Stars of our universe-brighter and bigger, only because nearer, than any other star 2. Q. What is the length of the sun's Diameter ?

A. To appear as large as the sun, at the distance of 91 millions of miles, a body must have a diameter of 853,000 miles-a diameter exceeding the earth's in the proportion in which 3 yards exceed an inch. 30 such globes as the earth piled one on the other would reach to the moon, but 108 would lie along the sun's diameter.

3. Q. What is the Figure of the sun?

A. The uniformly circular appearance of the solar disc, in all positions, renders it extremely probable that the sun is a globe, but his globular figure is more fully proved by the motions of

certain dark Spots, called Maculæ, which are often observed on his surface.

4. Q. How do such motions prove the sun to be a globe?

A. Sun-spots move regularly in the same direction, from the left towards the right side of the sun, where they disappear, and afterwards, if they continue long enough, reappear on the left to pursue a like path across the solar disc-their identity being recognised by their keeping the same relative positions to other Spots. The actual motions and paths of the Spots could only be produced by the rotation of a globe.

5. Q. What are the direction and period of the sun's rotation?

A. Sun-spots, moving from the eastern towards the western limb or border of the solar disc, prove that the sun, like the earth, rotates from west to east the time of a rotation being about 253 days, and the inclination of the solar equator to the ecliptic about 7° 15′. If seen from the centre of the sun, the direction of a Spot's motion would be from west to east, or the opposite of what it appears to an observer on the earth.

6. Q. On what part of the sun are the Spots seen?

A. They never appear in the polar regions, and very rarely on the sun's equator, or beyond 35° on either side. A belt about 8° broad on each side of the equator is comparatively free from Spots and their occurrence mostly within a zone of 25° or 30° north and south of the equator, shows that the distribution of Spots is influenced by the sun's rotation, much as are the Trade Winds by the rotation of the earth (Less. 4).

7. Q. What are the usual appearances of Sun-Spots?

A. Bright curved streaks, called Faculæ, usually. precede or attend the formation of Spots, which are saucer-shaped hollows below the general surface of the photosphere or bright surface of the sun. A Spot consists of (1) a shaded border or Penumbra; (2) at a lower level, a dark Umbra, in which coloured matter has been observed; and (3), near the centre of the Umbra, when the Spot is fully developed, a perfectly black hole or Nucleus (Lesson 40).

8. Q. How do Sun-Spots vary in number and duration?

A. The Spots are very irregular and subject to continual change, but Schwabe's observations, extending over more than 40 years, prove their recurrence to be periodical. The sun may appear spotless for weeks or months together; then a few Spots of immense size, or groups of small Spots may occur as many as 200 small ones having been counted (1846) in a single groupand the same Spot may last for months, or disappear during a single transit across the sun's disc.

9. Q. What are the dimensions of Sun-Spots ?

A. The least Spot visible in a powerful telescope must have an area of 50,000 square miles, but Spots are sometimes large enough to be visible to the naked eye. Spots covering 700 or 800 millions of square miles are not uncommon; the area of one measured by Sir J. Herschel (1837) was 3,780 million square miles; the breadth of a Spot in 1858 was 107,520 miles, and another in the same year had a breadth of 143,808 miles.

10. Q. What is the general appearance of other portions of the solar disc?

A. Faculæ, which are probably elevations or ridges in the photosphere, are usually seen near Sun-Spots and the unspotted portions of the disc often appear mottled, as if dotted over with dark spots or "pores." These "pores" have been regarded as the interstices between certain ovalshaped luminous objects-possibly the source of solar light and heat-which lie across one another in all directions and which have been compared in shape to "rice-grains" and "willow-leaves."

11. Q. At what intervals are Sun-Spots most abundant?

A. About nine times in a century, or once in 11.11 years, solar Spots are most plentiful, and about as often, few or no Spots are seen; thus in 1837, 1848, 1859, and 1870 Spots were abundant, but few were seen in 1833, 1844, 1855 and 1866-7. The prevalence of Sun-Spots appears to be associated with magnetic storms and great displays of the Aurora Borealis.

12. Q. Why is the prevalence of Sun-Spots supposed to be associated with Magnetic Storms and Auroral displays?

A. Observations prove that magnetic disturbances occur at intervals of 11.11 years, and that they always accompany splendid displays of the Aurora which are more frequent when Sun-Spots are abundant and very rare in the absence of Spots. Magnetic storms are indicated by frequent sudden jerkings, to and fro, of the compass needle, which may continue for days, and extend over large portions of the earth.