Each of the four circular charts in the body of the book represents the aspect of the _evening_ sky at one of the equinoctial or solstitial epochs. To be more precise, these charts show the sky as it appears, at about the latitude of New York, at 10 P.M., on, respectively, March 20th (the Vernal Equinox), June 21st (the Summer Solstice), September 23d (the Autumnal Equinox), and December 22d (the Winter Solstice). But the reader must not think that it is necessary to confine himself to the exact latitude, date, or hour just mentioned. Undoubtedly it would be better for the beginner to do that approximately, but it is not essential. The effect of a change of latitude is, perhaps, the least important. If the observer is farther south than about 40° north latitude, the southern stars will appear higher in the sky than they are shown in the charts, and some of the stars close to the northern horizon will sink from view. If, on the other hand, he is farther north (as in Canada or Northern Europe), the northern stars will appear higher, and some of those near the southern horizon will be invisible. But if he confines his attention to the stars and constellations represented in the central parts of the charts (which he should, in any case, do for other reasons), the effect of the shift due to difference of latitude will not be found very serious. As to the effects of a departure from the hours and dates for which the charts are drawn, they, too, can readily be allowed for. Suppose that, without changing the date, the reader makes his observations an hour earlier than that given, say at 9 P.M., March 20th. Then he will find that some of the eastern stars, seen along the left-hand edge of the chart when it is held upright, have not yet come into view above the horizon, while other stars, not seen on the chart drawn for that date, are visible above the horizon in the west. To the stars thus carried out of, or brought into, view he should pay no attention; he will find them again on other charts when they are better placed for observation.