CONVERSATION VIII. Of Day and Night. JAMES. You propose, now, sir, to apply the rotation of the earth about its axis to the succession of day and night. Tutor. I do: and for this purpose, suppose GRCB (in the diagram at page 72) to be the earth, revolving on its axis, according to the order of the letters, that is, from G to R, R to C, &c. If the sun be fixed in the heavens at Z, and a line H O be drawn through the centre of the earth T, it will represent that circle, which, when extended to the heavens, is called the rational horizon. Charles. In what does this differ from the sensible horizon? Tutor. The sensible horizon is that circle in the heavens which bounds the spectator's view, and which is greater or less, according as he stands higher or lower. For example; an eye placed at five feet above the surface of the earth or sea sees 24 miles every way: but if it be at 20 feet high, that is, 4 times the height, it will see 5 miles, or twice the distance. Charles. Then the sensible differs from the rational horizon in this, that the former is seen from the surface of the earth, and the latter is supposed to be viewed from its centre. Tutor. You are right; and the rising and setting of the sun and stars are always referred to the rational horizon James. Why so? they appear to rise and set as soon as they get above, or sink below, that boundary which separates the visible from the invisible part of the heavens. Tutor. They do not, however: and the reason is this, that the distance of the sun and fixed stars is so great in comparison of 4000 miles (the difference between the surface and centre of the earth), that it can scarcely be taken into account. Charles. But 4000 miles seem to me an immense space. Tutor. Considered separately, they are so, but when compared with 95 millions of miles, the distance of the sun from the earth, they almost vanish as nothing. James. But do the rising and setting of the moon, which is at the dis tance of 240 thousand miles only, respect also the rational horizon ? Tutor. Certainly; for 4000 compared with 240 thousand, bear only the proportion of 1 to 60. Now if two spaces were marked out on the earth in different directions, the one 60 and the other 61 yards, should you at once be able to distinguish the greater from the less? Charles. I think not. Just in the same manner does the distance of the centre from the surface of the earth vanish in comparison of its distance from the moon. There is a difference, however, connected with what astronomere called parallax; but this is not the time to explain that peculiarity. James. No: our present business is with the succession of day and night. Tutor. Well then; if the sun be VOL. II. 1 supposed at z, it will illuminate, by its rays, all that part of the earth that is above the horizon H O. To the inhabitants at G, its western boundary, it will appear just rising; to those situated at R, it will be noon; and to those in the eastern part of the horizon c, it will be setting. Charles. I see clearly why it should be noon to those who live at R, because the sun is just over their heads; but it is not so evident, why the sun must appear rising and setting to those who are at G and c. Tutor. You are satisfied that a spectator cannot, from any place, observe more than a semi-circle of the heavens at any one time; now what part of the heavens will the spectator at G observe? James. He will see the concave hemisphere z o N. |