If the line of the moon's nodes coincides with the line of the syzigies, or if the moon has no latitude at the time of her opposition to the sun, or at full moon, the moon will pass through the centre of the earth's sha dow, and will therefore undergo a central eclipse. When the moon is 6° from her node at the time of full moon, she will still pass through the earth's shadow, and be totally eclipsed in it ; and when she is full at 12° from her node, she will just touch the earth's shadow without being immersed in it. In Plate XL. Fig. 2., where S is the sun, E the earth, and P the moon in opposition, the moon will evidently begin to he eclipsed when she enters the earth's shadow at y : The middle of the eclipse will happen when she is at P, and the end of it when she leaves the earth's shadow at Itl• When the moon is cen trally eclipsed, the total duration of the eclipse, when she is in apogee, is 3h 57' 6"; and, when she is in peri gee, 27' 26". The moon's diameter is divided, like the sun's, into equal parts, called digits ; and when she is said to be 15 digits eclipsed, it is meant that she is completely covered by the earth's shadow, which ex tends three digits beyond her limb.
Even when the moon is completely immersed in the shadow of the earth, she may sometimes be distinctly seen of a red copper colour. This colour manifestly arises from the rays of light which are refracted into the carth's shadow by its atmosphere. Those rays which pass nearest the earth, or through the densest part of its atmosphere, are deprived of their blue rays, on account of their superior refrangibility, (see ATMO SPHERE') while the red rays force their passage through the resisting medium, and fall upon the moon.
cnt states of the atmosphere will produce different de grees of light ; and hence we find, that the moon, when eclipsed, does not always exhibit the same colour and brilliancy. It has been maintained by several astrono
mers, and more recently by Dr Herschel, that the moon. when eclipsed, shines with a native phosphorescent light, and that the refractive power of the earth's atmo sphere could not reflect the rays of light so as to make them fall upon the moon. It' this were the case, the moon should always shine with light of the same hue, whereas her light is often. variable, both in its colour and intensity.
The sun, moon, and earth, return into nearly the same position with regard to each other, after a certain num ber of revolutions of the moon. The Chaldaic period, or period of Pliny, as it is sometimes called, amounts to 18 years 11 days 7 hours 42 minutes and 31 seconds, comprehending 223 lunations, or revolutions °Idle moon; and, at the end of this period, the moon wants only 28' 10" of being as nearly in conjunction with the same node as at the beginning of the period. Hence, by adding this period to the time when any eclipse happens, we shall obtain the time when another eclipse will take place. Another period, much more exact than the form er, contains 6890 mean lunations, or 557 Julian years 21 days 18 hours 4 minutes 47 seconds, and at the end of this period the moon has advanced only 1 minute 41 seconds beyond the same distance from the node, which she had at the beginning of the period. See Fer guson's Lectures, vol. ii. p. 82.; Ferguson's Astronomy, vol. i. chap. xviii., where the subject of eclipses is treat ed at great length, and with uncommon perspicuity. See also Practical ASTRONOMY, Book ii., where we have given the method of calculating and projecting solar and lunar eclipses from new tables ; and the article