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Popular Astronomy: A Series of Lectures Delivered at Ipswich. George Biddell Airy
Читать онлайн.Название Popular Astronomy: A Series of Lectures Delivered at Ipswich
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isbn 4064066442538
Автор произведения George Biddell Airy
Жанр Математика
Издательство Bookwire
As I said before, by the transit instrument we have found what is the proportion of a revolution through which the celestial globe must be turned in respect to a certain fixed star, in order that we may fix the position of the globe when the body passes the meridian; and by the observations with the Mural Circle, we have fixed the distance of the object from the Pole, when that object passes the meridian. These are the two co-ordinates which completely define the planet's place. If we had a globe we could mark down the place of the object at once. Or, instead of this, the result of the two kinds of observation may be registered in figures.
I have referred the times of transit of planets and small stars to one principal star, supposing it taken as a point of departure. This method was adopted by my predecessor, Dr. Maskelyne, and by several of the best astronomers. Dr. Maskelyne adopted the bright star of Aquila, as his fundamental star; others, however, have used several bright stars, whose relative positions have been well ascertained; and this is now the more usual course.
The methods of which I have spoken, give us the means of recording, with the greatest accuracy, the position of any object as viewed from any point of the earth, and we come to the same conclusions, as to the relative positions of the stars, wherever we may be placed. Thus, at the Cape of Good Hope, where there is an Observatory in the highest order, the relative positions of the stars are seen to be precisely the same as when they are viewed from the European Observatories. If you observe how many hours, minutes, and seconds, one star is before the other when it passes the meridian, and how many degrees, minutes, and seconds, one star is higher than the other when it passes the meridian—whether it it is observed here or at the Cape of Good Hope, it amounts to precisely the same thing there is not the slightest difference. From this we must draw one of two conclusions: either that the stars are, as it were, stuck in a shell of a very great size; or else, that the distance between the North of Europe and the Cape of Good Hope is unmeasureably small, compared with the distances of the stars; or that the distance of the stars is unmeasurebly great as compared with the distance from the North of Europe to the Cape of Good Hope.
I shall now conclude this lecture. In the next lecture I shall treat of the figure and dimensions and rotation of the earth; of the movements of the Sun amongst the stars; and of the motions of the planets.
1 ↑ The circumstances under which these Lectures were originally delivered are explained in the preface.
2 ↑ The reader is particularly desired to remark that the word perpendicular does not mean perpendicular to the horizon, or vertical, unless it is so expressed. When the expression perpendicular to the surface of the glass is used, it means what a workman would probably call square to the surface of the glass. The vertical direction at any place is that of a plumbline hanging there, or perpendicular to the surface of still water.
3 ↑ Thus, if we suppose the atmosphere to consist of a series of parallel beds or strata, Bc, cd, de, &c. each of which is of uniform density throughout, the ray AB falling on the boundary BD of the uppermost stratum will be bent in the direction Bc, so as to be more nearly perpendicular to BD. Again, when it reaches cE, the boundary of the next stratum, it will in like manner be bent in the direction cd. The same thing will happen every time it comes to the boundary of a new stratum; and at last, when it reaches the earth's surface at C, its direction will be Cg. The star, instead of appearing to the observer at A, will consequently be seen at I, in the direction of Cg.
4 ↑ The reader will easily understand this, if he remarks that upon opening a pair of compasses so that one leg points exactly upwards and the other leg points to the horizon; the two legs are then exactly square to each other, and therefore one leg has been turned away from the other by one-fourth part of the movement which would bring it round to the other again, or by one-fourth part of 360 degrees.
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