52 SCENERY OF 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 sun 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. "\Y~e 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 high in heaven 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 n 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. The apparent diameter of the grand orb varies slightly at different seasons 

 of the year. This is owing to an actual variation of distance, for the ellipticity of the 

 earth's orbit alternately increases and diminishes our proximity to the luminary. The 

 solar diameter appears the least about the summer solstice, because the sun is then in 

 apogee, or most remote from the earth. It is the greatest about the winter solstice, when 

 the sun is in perigee, or at the nearest point to us. It' seems extraordinary at first 

 sight, that at mid-winter, when the streams are ice-bound, the snow lies upon the fields, 

 and the traveller shivers in the blast, we should be nearer to the sun than when, at an 

 opposite season of the year, the greensward is burnt up, the cattle pant in the shade of 

 the trees, and men seek a covert from the solar heat. But the effect of the sun's rays is 

 increased or modified by two circumstances, more than sufficient to counterbalance that 

 of the varying distance ; the length of time during which they act continuously, and 



