52 



SCENEKY OP THE HEAVENS. 



situated, the frequent interchange of day and night, with the gradual advance and 

 recession of both, is a benign and beautiful arrangement, the gentle, silent, yet emphatic 

 signal of nature, for man to go forth to his work and to his labour until the evening, 

 when the ".ploughman homeward plods his weary way," to give sleep to his eyes and 

 slumber to his eyelids. 



Owing to the refractive property of the atmosphere, the disk of the su-n when near the 

 horizon loses its circular form, and assumes an oval appearance. This is particularly 

 observable when sunrise or sunset is viewed from the summit of a mountain, or from an 

 eminence by the sea. The refringent power of the atmosphere being the greatest when 

 nearest to the horizon, it follows that the rays of light proceeding from the lower limb of 

 the sun are raised more than those which proceed from the upper point. This diminishes 

 the apparent vertical diameter, while the apparent horizontal diameter is scarcely at all 

 affected, as refraction acts only in a vertical direction. Measured by the micrometer, the 

 vertical height of the solar surface, in the circumstances named, is sometimes found to be 

 four, five, or even six minutes of a degree less than the horizontal width, and hence the 

 term given to the appearance, that of the horizontal sun. No such effect is perceived in 

 other situations of the solar body, because refraction operates more feebly away from the 

 horizon, and the difference between the refraction of the rays of light issuing from the 

 upper and lower extremities of the vertical diameter is too small to be observed. We are 

 accustomed to speak of the meridian glory of the sun ; and independent of the greater 

 purity of the atmosphere at noon, through the dissipation of mists and vapours, a greater 

 quantity of rays reach the eye from the sun when in the zenith than when near the 

 horizon. The air is an absorbent as well as a refractive and reflective medium : and 

 however transparent the medium may be, the quantity of light absorbed will increase or 

 diminish according to the extent of atmospheric space it has to traverse. This extent is 

 much smaller with reference to an object in the zenith than one near the horizon. By a 

 reference to the diagram, it will at once be seen that a ray of light passing from z the zenith 



will embrace a much less portion of the atmosphere included 

 between the two arcs than one from H the horizon ; conse- 

 quently a less* .quantity will be absorbed ; and hence a ce- 

 lestial object will appear the brighter as its distance from 

 the horizon increases. The comparatively dim and hazy 

 appearance of objects seen in the direction of the horizon, 

 is not only occasioned by the rays of light having to tra- 

 verse a larger space of the atmosphere, but of its lower 

 strata, where it is the most dense and most absorbent. It 



is estimated that the solar light is diminished thirteen hundred times in passing through 

 it, and we are thereby enabled to gaze upon the sun when setting without being dazzled 

 by his beams. It is commonly supposed that he presents a larger surface to the eye when 

 in the horizon than at any other altitude, but his apparent diameter is then somewhat 

 less. It is chiefly the diminished splendour in that situation, allowing a distinct view of 

 the solar outline to be taken, that gives rise to such an impression. 



The appearance of mock suns in the neighbourhood of the true orb a rare and 

 beautiful phenomenon called parhelia, that is, near to the sun, has been noticed by both 

 ancients and moderns. Aristotle remarks, that in general they are seen only when the 

 -sun is near the horizon, but mentions two that were observed on the Bosphorus through 

 the entire day. Pliny relates the repeated appearance of a triple sun, and mentions one 

 witnessed by himself in the reign of Claudius. The most remarkable exhibitions of this 

 kind occurred at Rome, when four suns were seen by Scheiner ; and at Dantzic, when 

 seven were noticed by Hevelius; but under various modifications, accompanied with 



