ON THE HORIZONTAL MOON. 235 



If you think it worthy a place in your coiledion, it will 

 be confidered as an additional obligation conferred on, 

 SIR, 



Your obedient fervant, 



H. STEINHAUER. 



Obfervations on the feemingly enlarged apparent Diameters of the 

 Sun and Aloon, xvhen viewed in or near the Horizon. 



Every one who views the fun or moon, when they are in General faft 

 ,, , . I • I 1 I 1 1 II ftated th?c the 



trie horizon, thinks that they appear larger than when they are hcaveniv bodies 



feen in any more elevated part of the heavens. And aftro- feem larger at 

 nomers know, that the diRance of the fame fixed fiars is ap- °^^'""*^^ 

 parently greater when feen near the horizon, than when they 

 are more elevated. But it is likewife well known to afirono- 

 mers, that the apparent magnitudes of the fun and moon, as 

 well as the apparent diflancesof any given fixed f^ars, as inea- 

 fured by the micrometer, are the fame in that part of the hea- 

 vens which is near the horizon, as, in the fame circumftances, 

 they are found to be in any other part, except that tlie moon, 

 being really perceptibly farther from an obferver, placed on 

 the earth's furface, when flie appears in the horizon, than 

 ■when (he appears in the zenith, is found to have a fmaller 

 apparent magnitude, agreeing with the caufes to which it is 

 known to be owing. The firfl mentioned phenomena muft 

 therefore belong to the head of optical deceptions. Let us 

 enquire from what fourcethis deception arifcs. 



I am not the firft by wiiom the fource was fought for In the 

 apparent flatnefs of the Iky ; but I differ as far as I know, 

 from all others in my manner of connecting one appearance 

 with the other. 



In order to explain my idea of the matter, I fliall firfl at- Explanation dff 

 tempt to fhew that the flattened appearance of the vifible hea- appe-,ranc" of 

 vens is not an illulion, but a reality; or in other words, that the heavens. 

 an obferver placed on the earth is really at a greater d'fiance 

 from a point of the fky, fituate in the horizon, than from a 

 point fituate in the zenith. 



This will appear in the cleareft manner if we endeavour to T>e Iky i.- a real 

 give an anfwer to the two following queflions: What is he Jky ? "M"» '^ , 

 and Where is thcjky ? coacivity. 



By the f!<y, I mean that blue concave fuperficles, within 

 which every obferver on the furface of the earth finds himfelf 



placed 



