It is only by drawing that we can give an exact idea of the comparative sizes of the sun and the other planets. The reader will find in the accompanying illustration (Fig. 1) a figure which exactly represents the comparative dimensions of the sun, and the largest planets of our world. The earth, represented by a dot, gives an idea of what Mars, Mercury, and Venus, which are smaller than the earth, must be.
It takes three years to circumnavigate the earth. To circumnavigate the solar globe, under similar conditions, would take three hundred years. If human life be not more prolonged in the sun than on the earth, an existence would not suffice to enable a traveller to become acquainted with the surface of the globe he inhabits.
[Illustration: Fig. 1.--Comparative Dimensions of the Sun and the Planets.]
Weight is thirty times more intense on the surface of the sun than on the earth. We know that a body which falls upon the earth traverses, in the first second of its fall, a space of four metres, nine centimetres. In the sun a falling body traverses 144 metres in the first second of its fall. It follows from this, that a human body, if transported to the sun, would weigh about 2000 kilogrammes, the weight of an elephant. The body of a dog or of a horse would weigh twenty-eight times as much as upon our earth, so that these animals would remain fixed to the surface. The conditions of nature must therefore be entirely different in the sun from what they are in the group of planets to which the earth belongs.
The sun sheds rays from perpetual fire, a characteristic that appertains to him alone among all the stars of our world. Of himself he burns, and sheds abroad light and heat. The other stars are neither warm nor luminous, and if the sun did not exist, they would be plunged into eternal darkness and eternal cold. This privilege alone ought to make us comprehend the immense importance of the central star.
The light and heat which emanate from the sun are constant; they are never interrupted, and they never lose their force. Thus, a second characteristic--constancy of illumination--separates the sun from all the other celestial bodies of our world.
The intensity of the real heat of the sun has been measured by the physicists. This result was attained in an endeavour to determine by experience the quantity of heat which accumulates in a given time, upon a certain portion of the earth's surface, exposed to the sun's rays, and adding to that element the quantities of heat which would be absorbed by the atmospheric air, the ethereal spaces, and the soil.