THE SUN. 



often the case that we are able to measure large distances with greater ac- 

 curacy than small ones ; this is frequently so in the surveys conducted on the 

 surface of our own globe. If, then, the greatness of the magnitudes does not 

 . constitute of itself any difficulty, to what are we to ascribe the doubt entertained 

 | by the popular mind in regard to such measurement ? It will, perhaps, be 

 replied that the object, whose distance we claim to have measured, is inacces- 

 sible to us ; that we cannot travel over the intermediate space, and therefore 

 < cannot be conceived to measure it. But again, let us ask whether this cir- 

 cumstance of being inaccessible constitutes any real difficulty in the measure- 

 ment of the distance of an object ? The military engineer, who directs his 

 projectiles against the buildings within a town which is besieged, can, as we 

 well know, level them so as to cause a shell to drop on any individual building 

 which may have been chosen. To do this, he must know the exact distance 

 of the building from the mortar. Yet the building is inaccessible to him ; the 

 walls of the town, the fortifications, and perhaps a river, intervene. Yet he 

 finds no difficulty in measuring the distance of this inaccessible building. To 

 accomplish this, he lays down a space upon the ground he occupies, called the 

 base line, from the extremities of which he takes the bearings or directions of 

 the building in question. From these bearings, and from the length of the 

 base line, he is enabled to calculate by the most simple principles of geometry 

 and arithmetic the distance of the building in question. Now imagine the 

 building in question to be the sun, and the base line to be the whole diameter 

 of the globe of the earth . in what respect would the problem be altered ? The 

 building within the town is inaccessible so is the sun ; the base line of the 

 engineer is exactly known so is the diameter of the earth ; the bearings of 

 the building from the ends of the base line are known so are the bearings of 

 the sun's centre from the extremes of the earth's diameter. The problems are, 

 in fact, identical ; they differ in nothing except the accidental and unimportant 

 circumstance of the magnitudes of the lines and angles that enter the question. 

 In short, the measurement of distances of objects in the heavens is effected 

 upon principles in all respects similar to those which govern the measurement 

 of distances upon the earth ; nor are they attended with a greater difficulty, or 

 more extensive sources of error. 



By such means, then, it has been ascertained that the distance of the 

 sun from the earth is about 100,000,000 of miles. The distance is more .ex- 

 actly 95,000,000 of miles ; but let me counsel those, who for the mere pur- 

 pose of general information, and without any strictly or scientific object, study 

 subjects of this nature, to be content to confine themselves generally to round 

 numbers they are more easily remembered, and answer all purposes as well ; 

 for this reason I shall, in the course of these discourses, generally adopt, in 

 the expression of distances, magnitudes, motions, and times, the nearest round 

 numbers. 



MAGNITUDE OF THE SUN. 



Having explained the distance of the sun, let us now see how its magnitude 

 can be ascertained. There is one general principle by which the magnitudes 

 of all the heavenly bodies can be ascertained when their distance is known. 

 This is, in fact, accomplished by the device of comparing them with some ob- 

 ject of known magnitude anJ which at any known distance will have the same 

 apparent size. As this is important, considered as a general principle applied 

 to all objects in the heavens, it may not be uninteresting to develop it some- 

 what fully in its application to the present object, the sun. 





