CONVERSATION XXIV.

Of the Fixed Stars, and Ecliptic.

TUTOR.

I dare say that you will

have no difficulty in finding the north polar star as soon as we go into the

e open air. James. I shall at once know where to look for that and the other stars which you pointed out last night, if they have not changed their places.

Tutor. They always keep the same position, with respect to each other, though their situation, with regard to the heavens, will be different at different seasons of the year, and in different hours of the night. Let us go into the garden.

Charles. The stars are all in the same place as we left them last evening. Now,

sir, if we conceive a straight line drawn through the two stars in the plough, which, in your figure (Fig. 1.) are marked d and g, and to extend a good way down, it will pass, or nearly pass through a very bright star, though not so bright as Arcturus or Capella, what is that called?

Tutor. It is a star of the second mag nitude, and if you refer to the celestial globe, in the same way as you were instructed last night, you will find it is called Regulus, or Cor Leonis, the Lion's heart. By this method you may quickly discover the names of all the principal stars, and afterwards with a little patience, you will easily distinguish the others, which are less conspicuous.

Charles. But they have not all names; how are they specified?

Tutor. If you look on the globe, you will observe, that they are distinguished by the different letters of the Greek alphabet; and in those constellations, in which there are stars of different apparent magnitudes, the largest is alpha, the next in size ẞ beta,

the third y gamma, the fourth ♪ delta, and

so on.

James. Is there any particular reason for this?

Tutor. The adoption of the characters. of the Greek alphabet, rather than any other, was perfectly arbitrary; it is, however, of great importance, that the same characters should be used in general by astronomers of all countries, for by this means the science is in possession of a sort of universal language?

Charles. Will you explain how this is? Tutor. Suppose an astronomer in North America, Asia, or any other part of the earth, observe a comet in that part of the heavens were the constellation of the plough is situated, and he wishes to describe it to his friend in Great Britain, in order that he may know, whether it was seen by the inhabitants of this island. For this purpose, he has only to mention the time when he discovered it; its position, as nearest to some one of the stars, calling it by the Greek letter by which it is designated; and

the course which it took from one star towards another. Thus he might say, that on such a time he saw a comet near in the Great Bear, and that its course was directed from to ß, or any other, as it happens.

Charles. Then, if his friend here had seen a comet at the same time, he would, by this means know, whether it was the same or a different comet?

Tutor. Certainly, and hence you perceive of what importance it is, that astrono mers in different countries should agree to mark the same stars and systems of stars by the same characters. But to return to that star, to which you just called my attention, the cor leonis, it is not only a remarkable star, but its position is also remarkable, it is situated in the ecliptic. James. What is that, sir?

Tutor. The ecliptic is an imaginary great circle in the heavens, which the sun appears to describe in the course of a year. If you look on the celestial globe, you will see it marked with a red line, perhaps an emblem

of the fierce heat communicated to us by

that body.

James. But the sun seems to have a circular motion in the heavens every day.

Tutor. It does; and this is called its apparent diurnal, or daily motion, which is very different from the path it appears to traverse in the course of a year. The former is observed by the most inattentive spectator, who cannot but know, that the sun is seen every morning in the East, at noon in the South, and in the evening in the West; but the knowledge of the latter must be the result of patient observation.

Charles. And what is the green line which crosses it?

Tutor. It is called the Equator; this is án imaginary circle belonging to the earth, which you must take for granted, a little longer, is of a globular form. If you can conceive the plane of the terrestrial equator to be produced to the sphere of the fixed stars, it would mark out a circle in the heavens, called the celestial equator or