LET. X. OF THE LATITUDE AND LONGITUDE. 17X

eclipses of the moon. The satellites of Jupiterare bodies of this kind, whose eclipses afford usmore frequent means of information than thoseof the moon : they pass so often into the shadowof that planet, that there is scarcely a night inwhich one or other of them is not eclipsed; andas they appear and disappear almost instantane-ously, the time of the phænomenon, by meansof a good telescope, may be much more accu-rately observed. The first and second satellitesare esteemed the best for this purpose; for, be-sides that their theories are more accuratelysettled, the5 r are quicker in their motions thaneither of the rest, and will, therefore, be morefrequently eclipsed.

Jupiter, as I have already informed you, hasfour satellites, or moons, which revolve roundhim in different orbits, in the fame manner asthe moon revolves round the earth. A simpleinspection of Fig. 13. Plate IV. will give you aperfect idea of the manner in which these satel-lites are eclipsed. S represents the fun, J Jupiter,88 his shadow, and 1, 2, 3,4, the satellites, ac^cording to the order in which they move. Wheneither of them enters into the shadow S S, itbecomes immediately invisible; and the momentwhen it disappears, is called the immersion ofthat satellite. After being for some time totallyhid from our sight, it again instantly appears, onthe contrary side of the shadow; and the moment

when