Conversations on Natural Philosophy, in which the Elements of that Science are Familiarly Explained

by Mrs. Marcet; Thomas P. Jones

The refraction of light prevents our seeing the heavenly bodies in their real situation: the light they send to us being refracted in passing into the atmosphere, we see the sun and stars in the direction of the refracted ray; as described in [fig. 4, plate 19.], the dotted line represents the extent of the atmosphere, above a portion of the earth, E B E: a ray of light coming from the sun S, falls obliquely on it, at A, and is refracted to B; then, since we see the object in the direction of the refracted ray, a spectator at B, will see an image of the sun at C, instead of its real situation, at S.

Emily. But if the sun were immediately over our heads, its rays, falling perpendicularly on the atmosphere, would not be refracted, and we should then see the real sun, in its true situation.

Mrs. B. You must recollect that the sun, is vertical only to the inhabitants of the torrid zone; its rays, therefore, are always refracted, in this latitude. There is also another obstacle to our seeing the heavenly bodies in their real situations: light, though it moves with extreme velocity, is about eight minutes and a quarter, in its passage from the sun to the earth; therefore, when the rays reach us, the sun must have quitted the spot he occupied on their departure; yet we see him in the direction of those rays, and consequently in a situation which he had abandoned eight minutes and a quarter, before.

Emily. When you speak of the sun's motion, you mean, I suppose, his apparent motion, produced by the diurnal motion of the earth?

Mrs. B. Certainly; the effect being the same, whether it is our earth, or the heavenly bodies, which move: it is more easy to represent things as they appear to be, than as they really are.

Caroline. During the morning, then, when the sun is rising towards the meridian, we must (from the length of time the light is in reaching us) see an image of the sun below that spot which it really occupies.

Emily. But the refraction of the atmosphere, counteracting this effect, we may, perhaps, between the two, see the sun in its real situation.

Caroline. And in the afternoon, when the sun is sinking in the west, refraction, and the length of time which the light is in reaching the earth, will conspire to render the image of the sun, higher than it really is.

Mrs. B. The refraction of the sun's rays, by the atmosphere, prolongs our days, as it occasions our seeing an image of the sun, both before he rises, and after he sets; when below our horizon, he still shines upon the atmosphere, and his rays are thence refracted to the earth: so likewise we see an image of the sun, previously to his rising, the rays that fall upon the atmosphere being refracted to the earth.

Caroline. On the other hand, we must recollect that light is eight minutes and a quarter on its journey; so that, by the time it reaches the earth, the sun may, perhaps, have risen above the horizon.