INTENSITY OP SUN'S HEAT AND LIGHT. 9 



temperature and sunlight similar to those which the preponderance of its mass 

 gives to the steadiness and uniformity of the planetary revolutions. Were the 

 same amount of heat and light, radiated from a smaller body like the Moon, the 

 effects would be restricted to a smaller portion of the Earth's surface; and the zone 

 of differential radiation would be reversed to one of cold and darkness. But in 

 the present beneficent arrangement, light and heat preponderate, counteracting 

 extremes of heat and cold with a warmer temperature. And this effect is further 

 prolonged by atmospheric refraction and reflection of the rays, which, rendering 

 the transitions more mild and gradual, lessens the reign of night. 



To estimate this effect of the Refraction of Light, we have only to find two 

 points on the spherical surface of the earth, at such distance that the inclination 

 of the two tangent rays from the Sun falling on them, shall be just equal to the 

 horizontal refraction. The terrestrial radii drawn to these points will evidently be 

 inclined at the same angle as their tangents, which is 34' nearly, or 40 English 

 miles. Thus it appears that the effect of refraction in widening the irradiated 

 zone of the earth is more than twice as great as that arising from the apparent 

 semi-diameter, or the mere size of the sun. Uniting the two effects, the sun is 

 found to illuminate more than half the Earth's surface by a belt or zone that is 58 

 miles in width, encircling the seas and continents of the globe. 



The advantage of the vast size of the sun is most conspicuous upon the planet 

 Venus, our evening and morning star, where the belt of illumination is sixty-one 

 miles in width, as shown in the preceding table. The next in rank is Jupiter, 

 whose belt of greater illumination is thirty-five miles wide ; while those of Mercury, 

 the Earth, and Saturn, are nearly eighteen miles in breadth. In the last column 

 of the table, it will be observed that the asteroid Vesta, though situated beyond 

 Mars, yet has, in consequence of its smaller size, a greater proportion of illuminated 

 surface than the Earth. 



From formula (7), it is found that the zone of differential illumination upon the 

 Earth extends over 455,400 square miles; or, including the additional area due to 

 34' horizontal refraction, it comprehends an aggregate of 1,430,800 square miles of 

 surface. The position of this great zone is continually changing, and in turn it 

 overspreads every island, sea, and continent. At the vernal equinox, when the 

 Sun is vertical to the Equator, it will readily be perceived that the larger base of 

 this zone is a great circle passing through the Poles and having the Earth's axis 

 for its diameter. From this position it gradually diverges, till at the summer 

 solstice, one extremity of its diameter will be in the Arctic, and the other in the 

 Antarctic circle. Thence it gradually returns to its former position at the Poles at 

 the autumnal equinox, all the while revolving like a fringed circle around the 

 globe, and accompanied with the lustrous tints and shadows Avhich variegate the 

 dawn and close of day. 



