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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 commonly 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.
HALF-IOUR ON THE USE OF THE MAPS.
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 inanner 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 year, but correspond with the variations due to the earth's variable motion round the sun.
HALF-HOUR ON THE USE OF THE MAPS.
Next as to finding the north point, or any point of the compass which will enable the observer to determine the rest. If he is only familiar with the aspect of those seven bright stars of the Great Bear which have been called Charles' Wain, the Butcher's Cleaver, and by other names, he can always determine the north point by means of the two stars called the pointers, since these seven stars never set. In the explanation of each map I have shown where the Great Bear is to be looked for on each night, the observer being assumed to have such a general knowledge of the direction of the compass-points as will suffice for the purpose of finding so marked a collection of stars. Thus the pole-star is found, and for the purpose of such observations as are here considered, this star may be looked upon as marking the exact direction of the north.
Perhaps nothing further is required, but if the observer prefer it he can determine the north point conveniently at noon by setting up a vertical stick in the sunlight and noting the direction in which the shadow lies. But this must be done at true noon; that is, when the sun is due south, and this does not agree with clock-time. However, if the observer notices that from December 25th to April 15th he must make the observation in the afternoon, thence to June 14th in the forenoon, thence to August 31st in the afternoon, and thence to December 25th in the forenoon; the time after or before noon being taken from the column in Hannay's Almanac, headed “equation of time' (the fourth column after the calendar for each month), he can learn the north point on any day of the year with the utmost nicety. Once the observation has been made, he can note what objects (these should be distant) lie towards the different points of the compass, and from that time he can use the accompanying maps without any reference to the Great Bear and the pointers.
It is worth noticing that the stars called the Guardians of the Pole form no bad time-piece when used with the aid of such maps as the present. They revolve round the pole once in twenty-four hours (less about four minutes), in a direction contrary to that of a clock's hands. But stars near the equator, wbose motions are much more rapid, afford a yet better measure of time, if the direction of the south point is well determined.
Of course, the observer who really wishes to become an astronomer, will not rest satisfied by learning only the principal stars shown in these maps. By means of regular star-maps he will be able to explore the depths of all the constellations, having once learned their position and general appearance from the accompanying maps. It will be well for the student to remember that the planets Venus, Mars, Jupiter, and Saturn, will at times appear among the constellations here shown. But besides that Venus and Jupiter can always be recognised by their superior light, and Mars and Saturn by the steadiness with which they shine, Hannay's Almanac will always show when those stars are above the horizon, their hours of rising, southing, and setting, and so on. As they never appear save among the zodiacal constellations, also, it becomes very easy to recognise them.
The following table exhibits the names of all the stars of the first three magnitudes to which astronomers have given names; at least, all those whose names are in common use :
a Andromedæ, Alpheratz B - Mirach, Mizar y
Almach a Aquarii, Sadalmelik B — Sadalsund ö - Skat a Aquilæ, Altair B - , Alshain
- Tarazed a Arietis, Hamal B - Sheratan y , Mesartim a Auriga, Capella B , Menkalinan a Boötis, Arcturus B — Nekkar E- , Izar, Mizar, Mirach
Muphrid a' Canum Ven., Cor Caroli a Canis Majoris, Sirius - Mirzam
Adara a Canis Minoris, Procyon B — Gomeisa a’ Capricorni, Secunda Giedi ů , Deneb Algiedi a Cassiopeiæ, Schedar B — Chaph a Cephei, Alderamin B - Alphirk y - Errai: a Ceti, Menkar
Ceti, Baten Kaitos 0- , Mira a Columbæ, Phact a Coronæ Bor., Alphecca a Corvi, Alchiba ở - Algores a Crateris, Alkes a Cygni, Arided, Deneb Adige ß
Albireo a Draconis, Thuban B-- Alwaid y
Etanin B Eridani, Cursa y Zaurac a Geminorum, Castor B — , Pollur y , Alhena - Wasat
- Mebsuta a Herculis, Ras Algethi B - , Korneforos a Hydræ, Alphard, Cor Hydra a Leonis, Regulus, Cor Leonis B - Deneb Aleet, Denebola, Deneb y , Algeiba ö - Zosma a Leporis, Arneb a Libræ, Zuben el Genubi B - Zuben el Chamali y , Zuben Hakrabi a Lyræ, Vega B Sheliak y , Sulaphat
a Ophiuchi, Ras Alhague
-, Homan a Persei, Mirfak B - Algol a Piscis Aust., Fomalhaut & Sagittarii, Kaus Australis a Scorpionis, Antares, Cor Scorpionis a Serpentis, Unukalhai a Tauri, Aldeboran B- Nath
- Alcyone (Pleiad) a Ursæ Majoris, Dubhe IB - Merak