Next, suppose that without changing the hour of observation he changes the date, and instead of observing on the 20th of March he observes on the 5th. Then he will notice precisely the same difference that was manifest when his observation was made an hour too early on March 20th--_i. e._, some of the eastern stars on the chart will not yet have risen, and other stars, not on that particular chart, will be visible in the west. Although at first all this may be a little confusing to the beginner, he will soon find that he can make due allowance for the changes of aspect. The whole matter becomes very simple when it is remembered that the heavens have a double revolution toward the west; one of these revolutions, due to the earth’s rotation on its axis, being effected in twenty-four hours, and the other, due to the earth’s revolution round the sun, requiring an entire year. One hour of the daily revolution (represented by an hour of right ascension) produces the same effect on the position of the stars as two weeks of the annual revolution; or two hours of the first correspond to one month of the second. If the observations are made at a later date or a later hour than those indicated on the chart, the changes will occur in the reverse order--_i. e._, western stars will have disappeared and eastern stars will have come up into view. I particularly wish to impress upon the beginner the needlessness of being troubled about these discrepancies. He can avoid all possibility of perplexity by fitting his observations to the exact times of the charts. As I have already said, a difference of a few degrees in his latitude on the earth may be disregarded. The charts, with a slight allowance for the shift of position of the extreme northern and southern stars, are available for any of the middle latitudes of the northern hemisphere. And if the effects of a change of hour or date prove in the least confusing, the beginner has only to await the given date and the given hour, and all will be clear. Then, as soon as he has become familiar with a few of the leading constellations, the others, which in themselves are not so easily recognizable, will fall into their proper places, after which there can be no possibility of confusion. In fact, much less effort is required to become familiar with the aspect of the starry sky than is demanded for a similar acquaintance with the fundamental data of botany, mineralogy, geology, or any other of the observational branches of natural science.