SJJCT. vn.] PARALLAX. 59 



SECTION VII. 



Parallax Lunar Parallax found from Direct Observation Solar Parallax 

 deduced from the Transit of Venus Distance of the Sun from the Earth 

 Annual Parallax Distance of the Fixed Stars. 



THE parallax of a celestial body is the angle under which 

 the radius of the earth would be seen, if viewed from the 

 centre of that body ; it affords the means of ascertaining the 

 distances of the sun, moon, and planets (N. 128). "When 

 the moon is in the horizon at the instant of rising or setting, 

 suppose lines to be drawn from her centre to the spectator 

 and to the centre of the earth : these would form a right- 

 angled triangle with the terrestrial radius, which is of a known 

 length ; and, as the parallax or angle at the moon can be 

 measured, all the angles and one side are given ; whence the 

 distance of the moon from the centre of the earth may be 

 computed. The parallax of an object may be found, if two 

 observers under the same meridian, but at a very great dis- 

 tance from one another, observe its zenith distances on the 

 same day at the time of its passage over the meridian. By 

 such contemporaneous observations at the Cape of Good Hope 

 and at Berlin, the mean horizontal parallax of the moon was 

 found to be 3459", whence the mean distance of the moon is 

 about sixty times the mean terrestrial radius, or 237,360 miles 

 nearly. Since the parallax is equal to the radius of the earth 

 divided by the distance of the moon, it varies with the distance 

 of the moon from the earth under the same parallel of lati- 

 tude, and proves the ellipticity of the lunar orbit. When the 

 moon is at her mean distance, it varies with the terrestrial 

 radii, thus showing that the earth is not a sphere (N. 129). 

 Although the method described is sufficiently accurate for 



