144" M r>E Zach's Ajlronomkal Ohfervathm, 



Obfervatlons of Jupiter's fatellites at Marfellles. 



IT is known, that the indh-ect method to calculate the orbits 

 of comets in a conic fedion, by means of three obfervatlons 

 given, is rendered more eafy and expeditious if there is a pofli- 

 bility of drawing a graphical figure that reprefents nearly the 

 orbit under confideration, by means of which the calculation 

 is directed, and the required elements of the comet's path may 

 be rigoroufly determined. To draw the orbit of a comet that 

 moves in a parabola or ellipfis, the problem is reduced to find 

 the potition of the axis and the perihelial diftance ; this por- 

 tion of the axis will be determined as foon as the angle Is 

 known, that the axis forms with another Hne, whofe pofition 

 is given ; this line may be an ordinate to a given point ot the 

 curve, or a tangent, or a radius vector, &c» The latter is to 



7 '^^ 



