Familiar talks on astronomy, with chapters on geography and navigaton . der to calculate the distance of themoon, we have first to find its horizontal paral-lax; knowing this, it is very easy to computethe distance, — and this applies to all the heav-enly bodies; as soon as we know their parallax,we readily find their distances, diameters, andvolumes. So I must first tell you what parallaxis. Parallax is the change in the direction ofan object as viewed from different places; or,it is the angle at the object made by two linesdrawn from it to the two places. The horizontalparallax of a heavenly
Familiar talks on astronomy, with chapters on geography and navigaton . der to calculate the distance of themoon, we have first to find its horizontal paral-lax; knowing this, it is very easy to computethe distance, — and this applies to all the heav-enly bodies; as soon as we know their parallax,we readily find their distances, diameters, andvolumes. So I must first tell you what parallaxis. Parallax is the change in the direction ofan object as viewed from different places; or,it is the angle at the object made by two linesdrawn from it to the two places. The horizontalparallax of a heavenly body is its change of di-rection as seen from the centre of the earth andthe surface of the earth, — the body being on 80 Familiar Talks on Astronomy, etc. the horizon, — or, it is the angle at the objectsubtended by the radius of the earth. In the diagram (Fig. 4) suppose the circleA B C to represent the earth, O its centre, andlet A be an observer on its surface. Let M bethe moon on his horizon A M. The angle A M Ois the moons horizontal parallax. A -—^Jif. Fig. 4. Now in order to find the moons horizontalparallax, two observers, one as far north as prac-ticable and the other as far south, and both being
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