TOTAL ECLIPSE OF THE SUN.

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371. During a total eclipse of the sun, many singular appearances are usually observed. Soon after the eclipse has commenced, and as it gradually advances, jets of light are sometimes seen flashing over the lunar disk; and as the total obscuration approaches the bright portion of the sun changes color by degrees, either becoming fainter than before, or else assuming a reddish tinge.

When the sun is completely hidden, a beautiful ring or corona' of light appears around the dark body of the moon, like the crown of light or glory with which painters surround the heads of saints. In the eclipse of 1842, one observer describes it as a ring of peach-colored light, another as white, and a third as beaming with a yellowish hue. Its breadth likewise does not always appear to be the same; for in the eclipse just mentioned, while some observers estimated the width at one eighth of the moon's diameter, others saw radiations of the corona eight times as long as the moon's diameter. The breadth of the corona, noticed by Mr. Bond, during the eclipse of July 28, 1851, was about one half of the sun's diameter.

372. But the most brilliant phenomona remain to be described. When the sun is completely concealed, and the corona is displayed, rose colored flames appear to dart out from the edge of the moon, emanating from the bright ground of the corona, and so distinct that they are frequently visible without the aid of the telescope. They vary from two to four in number, and though mainly of a rose color, yet they are seen tinged with lilac, greenish blue, and purple. During the eclipse of July 28th, 1851, Prof. Bond of Cambridge, noticed these beautiful rose colored flames, two of which were connected by an arch of light, resembling a rainbow.

373. Fig. 64, represents this eclipse as seen by Mr. J. historical event occurred on the 19th of April, 481 B.C. A lunar eclipse which happened on the 21st of September, 331 B.C., fixes the date of the battle of Arbela, in which Alexander triumphed over Darius, king of Persia. The eclipse occurred 11 days before the victory.

1. Corona, a Latin word signifying a crown.

Describe the various appearances that are beheld during a total eclipse of the sun? What appearances were observed by Mr. Bond, during the eclipse of July 28th, 1851.

R. Hind, in Sweden. The eclipsed sun is here seen surrounded by a corona, the whiter portions of which near

TOTAL ECLIPSE OF THE SUN, AS SEEN BY MR. J. R. HIND, NEAR ENGELHOLM, IN SWEDEN, JULY 28, 1851.

the dark circle indicate the positions of the jets of flame and the arch of light.

374. DURATION OF A SOLAR ECLIPSE. No eclipse of the sun can last longer than six hours. The duration of a total eclipse never exceeds eight minutes, nor that of an annular twelve and a half minutes.

375. SOLAR AND LUNAR ECLIPSES-POINTS OF DIFFERENCE. When a lunar eclipse occurs, it can be seen from every part of that side of the earth, which is turned towards the moon. For this hemisphere is necessarily in the earth's shadow, and a spectator here situated beholds the moon eclipsed when she enters the shadow.

Describe Fig. 64. How long can any eclipse of the sun last? How long a total? How long an annular eclipse?

QUANTITY OF AN ECLIPSE.

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376. In the case of a solar eclipse, the shadow of the moon passes across the earth in less than four hours, (Art. 368 note 1,) and an eclipse can only occur in the path of the moon's shadow. Every part of the terrestrial hemisphere turned toward the sun will not therefore be eclipsed, but only those portions that are traversed by the lunar shadow.

The extent and path of the shadow must accordingly be determined before we can know in what regions of the earth the sun will be eclipsed.

377. These differences in respect to lunar and solar eclipses, arise from the different positions of the observer in the two cases. During a lunar eclipse he is on the body that forms the shadow, during a solar eclipse he is on the body that receives the shadow.

378. FREQUENCY OF ECLIPSES. Seven is the greatest number of eclipses that can occur in the course of a year, and two the least. If seven take place five may be solar and two lunar or three may be eclipses of the sun and four of the moon. Six eclipses in a year is an unusual number, four the average and two the least; in the last case the eclipses will be solar.

379. An eclipse of the moon sometimes happens the next full moon after an eclipse of the sun, and the reasons are as follows. The solar eclipse taking place at or near one of the moon's nodes, the shadow of the earth extends at this time across the moon's orbit, and is at or near the other node. Now the moon's orbitual motion is so rapid that after causing the solar eclipse, she may sweep round to the other node, before the earth's shadow has departed so far from it, as to be out of the moon's way. Under these circumstances she enters the shadow and a lunar eclipse occurs.

380. QUANTITY OF AN ECLIPSE. The quantity of an eclipse, is the extent of the obscuration of the eclipsed body, and is estimated in the following manner. In a

State in what respects solar and lunar eclipses differ? How do these differences arise? What is the greatest number of eclipses that can occur in a year? What the least? If seven take place what will be the number of solar eclipses, and what the number of lunar? What is an unusual number in a year? What the average? What the least number? If only two occur, are they solar or lunar? Explain why an eclipse of the moon may happen the next full moon after a solar eclipse? What is the quantity of an eclipse?

lunar eclipse, for example, the diameter of the moon is supposed to be divided into 12 equal parts, called digits, and the number of such parts that lie within the earth's shadow, at the time the moon's centre is nearest to the centre of the shadow, determines the quantity of the eclipse. When the moon is entirely immersed in the shadow, as in the case of a total eclipse, the quantity is found in like manner, by supposing a line to be drawn from the centre of the shadow to its outer edge through the centre of the moon, and then dividing the part included between the inner edge of the moon, and the outer edge of the shadow, by one twelfth part of the moon's diameter.

This subject is illustrated in Fig. 65, where M repre

sents the moon, N one of her nodes, NMP a portion of the moon's orbit, and NSCE the direction of the plane of the earth's orbit. The circle EOSK is a section of the earth's shadow, which completely envelopes the moon, causing a total eclipse: the line OC is a radius of the circle EOSK and passes through the centre of the moon. The quantity of the eclipse is obtained by dividing the line LO by one twelfth part of DL the moon's diameter.

If the eclipse instead of being total had been partial, and the moon's centre M, had been at the point Ò, then

What is meant by the term digit? How is the quantity of an eclipse estimated ? How is the quantity found in a total eclipse? Explain from Figure.

THE PERIOD OF THE ECLIPSES-THE SAROS. 189

one half of her diameter ML would have been in shadow, and the quantity of the eclipse would have been six digits.

381. THE PERIOD OF THE ECLIPSES-THE SAROS. It was discovered by astronomers centuries ago, that if the eclipses that happen during a period of about 18 years, are noted in their order, that the series is repeated during the next period in nearly the same manner as before.

The reason of this will be evident from the following considerations.

382. We have seen that eclipses depend upon the nearness of the moon to her node when new and full. But the node is in motion around the ecliptic, retrograding at the annual rate of about nineteen and a half degrees,' while the moon is also in motion around the earth. Now an inquiry may reasonably be made whether, supposing that an eclipse was to take place to-day exactly at one of the moon's nodes, in which case, the sun and the moon would be in the line of the nodes, there might not be such a relation between the motion of the moon and the motion of the node, that after a certain interval of time another eclipse would again occur at the same node; so that the moon and the sun during the next succeeding interval would go through the same series of positions in respect to each other as during the first, and reproduce the same set of eclipses, resulting from these positions.

383. Such a relation is found to exist very nearly. For if there was to-day a solar eclipse, the sun and moon as seen from the earth, being exactly at one of the moon's nodes, the moon would be there again in 229.53 days (a synodical month, Art. 275,) and the earth in its revolu tion about the sun, would bring the same node again to the sun in 346,62 days, a period which is termed the

1. The daily retrogadation is 3′ 10′′ (Art. 315,) which gives about 1910 for the annual rate.

2. This is the expression for the length of a lunar month in days and the decimals of a day. More nearly 29.5305887.

State what is said respecting the period of the eclipses? Explain in full the cause of this recurrence of a series of eclipses?