Читать книгу The Friendly Stars - Martha Evans Martin - Страница 4
ОглавлениеTHE RISING AND THE SETTING OF THE STARS
The most favorable time for identifying a star is when it is rising, or has just risen, especially if its rising takes place anywhere between twilight and midnight, the portion of the day most likely to be convenient for observers. It is then in the eastern part of the sky, where the afterglow of the sunset does not dim its lustre, and it is far enough down towards the horizon for one to look at it face to face in a comfortable attitude. Moreover, at rising a star is more nearly by itself, more separated from neighboring stars, and is, therefore, much more easily recognized than when one has to pick it out from among the host of other stars already scattered over the sky and in as plain view as it. For near the horizon the atmosphere obliterates the multitude of small stars which are plainly visible higher up in the sky.
It is true that, owing to the density of the atmosphere there, all stars twinkle more, and hence shine with less steadiness, when they are near the horizon than when they are higher up in the sky, and most stars are not to be seen at all until they have actually gotten some distance above the horizon. Then there is to be allowed for, the interference of buildings, trees, and other objects in the landscape; but, even in spite of these interferences, most any star comes well into view before it has risen so high that it cannot be easily separated from the others and looked at with comfort. One has the advantage given by the comparative isolation of a rising star whether it rises from below the horizon or from behind the trees and houses.
If one were observing a star through a telescope and for other purposes than that of identification, he should choose, as nearly as possible, a time when the star is in the middle of its course from east to west, or as nearly overhead as it ever reaches. There the atmosphere will be clearer and, in consequence, the star will shine with a steadier light and offer less opportunity for optical illusions. But this means either a neck-breaking attitude or some special preparations for a comfortable reclining posture, either of which is more or less discouraging to one who has not already become pretty familiar with the stars. When he has come to know them, there is no greater delight than to lie stretched on a cot in the open with the whole expanse of the heavens above him, and watch them as they rise and pass and go again. This is the stage of acquaintance that binds one forever in friendship with the stars. But a speaking acquaintance may be maintained that will give much pleasure with lighter demands upon one.
The rising of the stars is the time of recognition and greeting. But it is no less important to speed the parting guest; and the next most satisfactory time to watch a star is when it has reached the western sky and is about to leave us until it comes again to the eastern door on its never-ceasing round. Here, while the star remains the same in its individual attributes, all its setting is the reverse of what it was when the star first appeared above the eastern horizon. The parallelogram of twinkling stars that trailed behind Vega as she rose in the east now waves above her as she sinks below the line of vision in the west. The Northern Cross, which heaved itself into sight in the east almost wrong-side up, with Deneb weighting the head of it, has, twenty hours later, practically righted itself and is lost to view in the glowing west with Deneb dancing on the tip of its upright. So complete identification must include the recognition of a star both in the east and in the west.
In the north there is a circle of stars around the pole which are always visible to us. They complete their small round once in twenty-four hours; but their path lies entirely above the horizon, so they never rise or set. Within this circle are the Pole Star and the stars of the Great Dipper, which will be illustrated in the next chapter; but it includes none of the brightest stars.
Next comes a line of stars describing a larger circle around the pole, one that takes them below the horizon for a longer or shorter time according to their distance from the pivotal point, the pole. These stars rise in the northeast, make a long, slow sweep of the sky, and set in the northwest, finishing their circle below the horizon and rising not long after they set. Among these are some of the brightest stars.
Then follows a belt of stars that rise somewhere near the exact eastern point of the horizon, cross the heavens to the western point in about twelve hours, and remain below the horizon another twelve hours.
And, finally, there are the stars in the southern section of the sky, which rise south of east and set in the southwest, never rising very high in the heavens, the portion of their circle, or course, over which we can see them pass being smaller and smaller as that course lies farther and farther south of us. Among these southern stars are Sirius, the star that outshines all the others, and Fomalhaut, that hides from us for sixteen hours out of each twenty-four.
All the stars that rise south of the exact eastern point on the horizon are called southern stars; for the sky is divided into a northern and a southern hemisphere by an equator running half-way between the north and the south pole, just as the earth is divided. The northernmost stars of the southern hemisphere are visible to us just as the southernmost stars of the northern hemisphere are visible to people living in southern latitudes. These distinctions are of no great importance, but it is well to understand that, while we cannot see the southern stars as a body, yet some of them are visible to us and still are spoken of as southern stars. Three of the brightest stars that come within our view, Rigel, Antares, and Spica, are southern stars, in addition to Sirius and Fomalhaut, which have already been mentioned.
Unlike the sun and moon and the planets, each star rises always at practically the same point in the heavens. It comes back to the starting-point each day four minutes earlier than the day before, which makes it rise about an hour earlier each fortnight, and two hours earlier each month; so that in twelve months it comes twenty-four hours earlier. This brings it back to the same rising point at almost the same second once a year.
These changes from day to day in the time of the rising and the setting of the stars are regular and definite; and so it is an easy matter, after one has once learned to recognize a star, either by its appearance or its surroundings, to know where to look for it at any given date. If a star, for instance, is due to rise at six o'clock in the evening on the first of March, it will rise at four in the afternoon in April, and at eight in the evening in February; at two o'clock in the afternoon in May, and at ten in the evening in January; at noon in June, and at midnight in December; and so on throughout the year. Hence, almost any star can be seen at some time in the night for six months in the year. The other six months, if it is among the northernmost stars, it can be seen more or less each night; but if it is among those that are below the horizon many hours, it may be above it during the hours of sunshine and thus be rendered invisible to the naked eye, though even in the daytime one can usually find a bright star with a telescope.
The times of rising and setting given in this book are for about the latitude of New York City, which is nearly forty-one degrees north latitude. For any place within five degrees of this latitude on either side the difference in time of rising is so slight that for mere purposes of identification it will not matter. The time of the appearance of the stars to persons within this belt of latitude is the same all the way across the country from Maine to California. It includes the region around all the large cities of the Eastern and Middle States and most of the Western ones. Within it lie Portland, Boston, Philadelphia, Washington, Baltimore, Cincinnati, Indianapolis, Chicago, St. Paul, Detroit, Louisville, St. Louis, Denver, and San Francisco. On the southernmost edge of it is Memphis, and just a little outside of it on the north is Olympia, Washington. Above or below this belt one must take latitude more or less into consideration; which means only to remember that when an observer is placed farther south the stars to the north of him rise somewhat later and those to the south of him somewhat earlier, and that if he is placed north of this belt the stars to the south of him rise somewhat later and those to the north of him somewhat earlier. The rising-times of the stars as given here will answer approximately for any place in the United States.
When the rising-time of a star is referred to in these pages, it is usually given as nearly as possible in round numbers to avoid any complications. If the actual time, for instance, is 6:55, it is given as 7:00. Variations of this kind seldom extend over more than a few minutes and they more often give a time a little later instead of earlier than the exact minute. This makes the inexactness still more immaterial, since for naked eye observation one cannot hold the stars to their scheduled minute of rising, and anywhere within half an hour is a reasonable expectation in that regard. They are there, to be sure, but variations of climate between two places and variations of atmosphere from day to day will make a considerable difference in their visibility when near the horizon. After they are well up in the sky, of course, one need not make so many concessions.
It is to be remembered also that one is rarely ever situated where he can really see the horizon, but owing to various causes sees usually a line somewhat above the true horizon.
