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According to the above statement, it therefore appearsthat there may he one lunar eclipse, or there may be nonenear to the time of each passage of the line of Nodes throughthe Sun , according to the number of days by which such anoccurrence is separated from the date of a Full Moon ; and thatif a lunar eclipse occur it will be within about eleven days ofthe Nodal passage.
The conclusion would consequently seem to follow that theremay be two lunar eclipses, or none, in any given year. Butowing to certain causes of a somewhat complicated natureconnected with the lunar orbit, which involve a slow turning-round of the line of Nodes, it is found that the above-namedinterval of six months is reduced to 173 days. From thisit results that there may be as many as three lunar eclipsesin the course of a year, one at the very beginning of the year,one in the middle of it, and one just before its close.
Although we have done our best, by means of some diagramsdifferent from those generally drawn, to render this explana-tion as lucid as possible, it is difficult so to do, owing to theperspective necessarily involved. A suitable model (such asmay be seen at Gresham College ) may, however, be easilyconstructed, and will be found much more efficient and of greatassistance both to teachers and students.
We must only refer very briefly to the Moon ’s connection withsolar eclipses, especially as the beautiful phenomena then wit-nessed have been already described in Lecture II. (see Plate IV.),in connection with the Sun ’s coronal and other surroundings.The most important difference between a solar and a lunareclipse is that the latter is visible from a whole hemisphere ofthe Earth at once, i.e., from every part of the Earth uponwhich the Moon would otherwise be shining; while a solareclipse is only seen from a small portion of the Earth at anygiven time. The shadow of the Moon , by which a solareclipse is caused, is, in fact, never much more than long enoughto reach the Earth . Sometimes, when the Moon is at itsfarthest from the Earth , and the Sun at its nearest, it evenfalls short of doing so. In such cases the shadow comes to apoint before it meets the Earth , and the Moon appears to be