10. Chap. 10. (13.)-On The Recurrence Of The Eclipses Of The Sun And The Moon.

CHAP. 10. (13.)—ON THE RECURRENCE OF THE ECLIPSES OF THE SUN AND THE MOON.

It is ascertained that the eclipses complete their whole revolution in the space of 223 months[1], that the eclipse of the sun takes place only at the conclusion or the commencement of a lunation, which is termed conjunction[2], while an eclipse of the moon takes place only when she is at the full, and is always a little farther advanced than the preceding eclipse[3]. Now there are eclipses of both these stars in every year, which take place below the earth, at stated days and hours; and when they are above it[4] they are not always visible, sometimes on account of the clouds, but more frequently, from the globe of the earth being opposed to the vault of the heavens[5]. It was discovered two hundred years ago, by the sagacity of Hipparchus, that the moon is sometimes eclipsed after an interval of five months, and the sun after an interval of seven[6]; also, that he becomes invisible, while above the horizon, twice in every thirty days, but that this is seen in different places at different times. But the most wonderful circumstance is, that while it is admitted that the moon is darkened by the shadow of the earth, this occurs at one time on its western, and at another time on its eastern side. And farther, that although, after the rising of the sun, that darkening shadow ought to be below the earth, yet it has once happened, that the moon has been eclipsed in the west, while both the luminaries have been above the horizon[7]. And as to their both being invisible in the space of fifteen days, this very thing happened while the Vespasians were emperors, the father being consul for the third time, and the son for the second[8].

1. According to the remarks of Marcus, it appears probable that this sol-lunar period, as it has been termed, was discovered by the Chaldeans; Ajasson, ii. 306, 307.

2. "coitus."

3. "Hoc enim periodo (223 mensium) plerumque redeunt eclipses, non multum differentes, denis tamen gradibus zodiaci antecedentes;" Kepler, as quoted by Alexandre, in Lemaire, ii. 238.

4. The terms "sub terra" and "superne" are interpreted, by most of the commentators, below and above the horizon respectively; see Marcus in Ajasson, ii. 307.

5. "globo terræ obstante convexitatibus mundi." The term convexus, as applied to the heavens, or visible firmament, simply signifies arched; not opposed to concave, like the English word convex.

6. This point is discussed by Ptolemy, Magn. Const. vi. 6; "De distantia eclipticorum mensium." See also the remarks of Hardouin in Lemaire, ii. 260, 261; and of Poinsinet, i. 67.

7. These are styled horizontal eclipses; they depend on the refractive power of the atmosphere, causing the sun to be visible above the horizon, although it is actually below it. Brotier states, that eclipses of this description occurred on the 17th July, 1590, on the 30th November, 1648, and on the 16th January, 1660; Lemaire, ii. 260.

8. This is supposed to have been in the year 72 of our æra, when it is said that the sun was eclipsed, in Italy, on the 8th, and the moon on the 22nd of February; see Hardouin and Alexandre, in Lemaire, ii. 261.