SCT. VII. PARALLAX. 51 



been the case, without any great change in the form of 

 the terrestrial spheroid. The variation in the length of 

 the pendulum was first remarked by Richter in 1672, 

 while observing transits of the fixed stars across the 

 meridian at Cayenne, about five degrees north of the 

 equator. He found that his clock lost at the rate of 

 2 m 28 s daily, which induced him TO determine the 

 length of a pendulum beating seconds in that latitude ; 

 and repeating the experiments on his return to Europe, 

 he found the seconds' pendulum at Paris to be more 

 than the twelfth of an inch longer than that at Cayenne. 

 The form and size of the earth being determined, 

 a standard of measure is furnished with which the di- 

 mensions of the solar system may be compared. 



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 center 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 center to the spectator and to the center 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, 

 ah" the angles and one side are given ; whence the 

 distance of the moon from the center 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 obser- 

 vations 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 



