Popular Astronomy: A Series of Lectures Delivered at Ipswich. George Biddell Airy

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СКАЧАТЬ on a globe, to turn the globe one hour, or one twenty-fourth part of a revolution, from the position in which the principal star was under the meridian, and then we know that the planet which we have observed, will be somewhere under the meridian, in that new position of the globe. That is the result of the observations with the transit instrument.

The next thing is, by means of the observation of the Polar Star with the Mural Circle, and by determining how high any other object appears when it passes the meridian, to determine the angular distance of any object from the Pole. These two observations amount to this:—the first gives the angular distance of the Pole from the north horizon. It is, however, rather more convenient to refer the position of the Pole to the point which is exactly upwards usually called the Zenith. The change is very easily made; for as the angular distance from the Zenith to the horizon is ninety degrees,^[4] we have only to subtract the elevation of the Pole (or of any other object) from ninety degrees, in order to obtain its zenithal ​distance on the north side of the Zenith. Thus we find that between the Zenith and the Pole there are so many degrees, and minutes, and seconds, of angular distance. That is obtained from the observations with the Mural Circle, directed to the Polar Star. By using the same instrument in the same manner, but directed to a planet or other object, we find the angular distance from the Zenith to the planet on the south side of the Zenith. We have then, got these two things: we have got the angular distance of the Polar Star from the Zenith on one side, and the angular distance of the planet from the Zenith on the other side. By adding these together, we have the angular distance of the planet from the North Pole. This is the other co-ordinate necessary to define the planet's place.

      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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