This phenomenon never takes place so clearly as when the moon, after its shifting courses,See note 2, p. 10, § 4, below. brings back its monthly journey to the same starting-point after fixed intervals of time; that is to say, when the entire moon,I.e. the full moon; cf. § 7, below. in the abode of the same sign of the zodiac, is found in a perfectly straight line directly under the sun, and for a brief time stands still in the minute points which the science of geometry calls parts of parts.I.e. parts of degrees, or minutes; cf. Pliny, ii. 48, scripulis partium.

And although the revolutions and movements of both heavenly bodies, as the searchersThe natural philosophers. for intelligible causes had observed, after the course of the moon is completed,At the end of each lunar month. meet at one and the same point always at the same distance from each other,I.e. are in conjunction. yet the sun is not always eclipsed at such times, but only when the moon (by a kind of fiery plumb-line)According to Clark’s punctuation, based upon metrical clausulae (Introd., p. xxii); but igneo seems to be more naturally taken with orbi. is directly opposite the sun and interposed between its orb and our vision.

In short, the sun is hidden and his brightness suppressed, when he himself and the orb of the moon, the lowest of all the heavenly bodies, accompanying each other and each keeping its proper course, maintaining the relation of height between them and being in conjunction, as Ptolemy wisely and elegantly expresses itμαθνηατικὴ σύνταξις, vi. 6. have come to the points which in Greek we call ἀναβιβάζοντας and καταβιβάζοντας ἐκλειπτικοὶ σύνδεσμοιAscending and descending ecliptic nodes. The moon in its course shifts from one side to the other of the ecliptic, or sun’s course (see § 2, above). The nodes are the points where the moon passes the ecliptic; the node where she passes from the south to the north side is called ascending, that where she changes from north to south, descending. (that is, eclipse nodes). And if they merely graze the spaces adjacent to these nodes, the eclipse will be partial.