PREFACE.

THE OBJECT which I have proposed to myself in the preparation of this work has been to teach the beginner the stars in a manner which there can be no misunderstanding. I had the same object in view in preparing my Constellation-Seasons; but experience has shown me that to attain that object it is necessary to consult the beginner himself. I found on doing this that my Constellation-Seasons were not so well suited to the purpose I had in view as I had expected. Meridians and parallels, equator, ecliptic, and tropics, which had seemed to be absolutely necessary to the completeness of the maps, tended only to confuse the beginner. So also did the introduction of fourth-magnitude stars and the less important constellations. Other features of those maps, also, while increasing their utility (I think) to the more advanced student, rendered their meaning less obvious than is desirable for the beginner. In these maps I have discarded everything which could by any possibility be confusing. In place of letters indicating the points of the compass, the words eastern horizon, north-eastern horizon, &c. are written in full, and natural features round each map indicate the fact that the circumference of the map really corresponds to the horizon of the observer. The word 'overhead' is put on the centre of each map in place of zenith.' The hours to which each map corresponds for different days are written in according to the simplest mode of expressing them. And, lastly, the days proper for the use of each map will be found to run without interruption from the beginning of the year in Map I., to the end of the year in Map XII.; the interval between successive dates never exceeding four days.

I believe I am not claiming too much for these maps in saying that they are the first series ever published, which the beginner could not possibly misinterpret, even if he paid no attention to the accompanying letterpress.

SOUTH LAMBETH: 1869.

R. A. P.

HALF-HOUR ON THE USE OF THE MAPS.

IT IS VERY EASY to gain a knowledge of the stars, if the learner sets to work in the proper manner. But he commouly meets with a difficulty at the outset of his task. He provides himself with a set of the ordinary star-maps, and then finds himself at a loss how to make use of them. Such maps tell him nothing of the position of the constellations on the sky. If he happen to recognise a constellation, then indeed his maps, if properly constructed, will tell him the names of the stars forming the constellation, and also he may be able to recognise a few of the neighbouring constellations. But when he has done this, he may meet with a new difficulty, even as respects this very constellation. For if he look for it again some months later, he will neither find it in its former place nor will it present the same aspect,—if indeed it happen to be above the horizon at all.

It is clear, then, that what the learner wants is a set of maps specially constructed to show him in what part of the sky the constellations are to be looked for. He ought on any night of the year to be able to turn at once to the proper map, and in that map he ought to see at once what to look for, towards what point of the compass each visible constellation lies, and how high it is above the horizon. And, if possible (as the present work shows is the case), one map ought to suffice to exhibit the aspect of the whole heavens, in order that the beginner may not be confused by turning from map to map, and trying to find out how each fits in with the others.

It is to fulfil these requirements that the present maps have been constructed. Each exhibits the aspect of the whole sky at a given day and hour. The circumference of the map represents the natural horizon, the middle of the map representing the part of the sky which lies immediately overhead. If the learner hold one of these maps over his head, so as to look vertically upwards at it, the different parts of the horizon marked in round the circumference being turned towards the proper compass points, he will see the same view of the heavens as he would if he were to lie on his back and look upwards at the sky, only that the map is a planisphere and the sky a hemisphere.

But although this illustration serves to indicate the nature of the maps, the actual mode of using them is more convenient.

Let it first be noted that properly speaking the maps have neither top, bottom, nor sides. Each map may be held with any part of the circumference downward, then the centre of the map is to be looked upon as the top for that part of the circumference. The portion of the map lying beneath the centre represents the portion of the sky lying between the point overhead and a certain portion of the horizon—the part in fact corresponding to the particular part of the circumference which is turned downwards. Thus if on any night we wish to learn what are the stars towards the north, we look for the map corresponding to that night. At the hour named the stars towards the north will be those shown between the centre of the map and the top; and, of course, we hold the map upside down so as to bring the centre above the northern part of the circumference.

But this matter will be more clearly understood by comparing the account of any of the accompanying maps with the map itself.

Again, it must be noted that although the maps are necessarily arranged in a certain order, there is in reality no first or last in the series. The map numbered I. follows the map numbered XII. in exactly the same manner as the latter follows the map numbered XI. The maps form a circular series, in fact.

The only reason for numbering the maps as at present, is that the map numbered I. happens to exhibit the aspect of the sky at a convenient hour on the night of January 1. It will be found that the dates follow on with intervals of three or four days right round the year, the end of the year falling in the left-hand column of map I., while the beginning of the year is in the middle column of the same map.*

It will be seen at once that a map can always be found corresponding to a convenient hour on any night of the year-except only in Midsummer, when on a few of the dates, night has not begun at the hour named. It was impossible without spoiling the regularity of the dating, or adopting an inconveniently late hour for all the maps, to avoid this difficulty. But as a matter of fact the difficulty disappears at once when the student is told that on any date named under a map, the aspect of the sky two hours later than that named is that represented in the following map. Thus at eight o'clock in the evening of June 21, the aspect of the stars is as shown in map VI., but the stars cannot be seen because it is still broad daylight: at ten o'clock, however, on the same night, the aspect of the sky is that shown in map VII., as indeed the first date under that map shows. Applying this rule to the few occasions on which the hour named is not available for observation (five or six in all out of ninety-six dates) the observer can manage as well for those occasions as for any others.

* It may be mentioned in passing, that the dates have not been thrown in so as to fall regularly round the correspond with the variations due to the earth's variable motion round the sun.