52 PARALLAX. SECT. VII. 



SECTION VII. 



Parallax Lunar Parallax found from Direct Observation Solar Paral- 

 lax 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. 130). 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 distance 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 greatest terrestrial radius, or 237,608 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 latitude, 

 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. 131). 



Although the method described is sufficiently accurate for 

 finding the parallax of an object as near as the moon, it will not 

 answer for the sun, which is so remote that the smallest error in 

 observation would lead to a false result. But that difficulty is 

 obviated by the transits of Venus. When that planet is in her 

 nodes (N. 132), or within 1J of them, that is, in, or nearly in, 

 the plane of the ecliptic, she is occasionally seen to pass over the 



* Or more correctly 3422"'325 and 238,793 miles, as deduced from 

 Mr. Adams' more accurate calculations. 



