DISTANCE AND DIMENSIONS OF THE SUN. 4.07 



be produced in the planet's position by a transference of the observer 

 from Washington to Santiago, or vice versa. 



The first modern attempt to determine the sun's parallax was 

 made by this method in 1670, when the French Academy of Sciences 

 sent Richer to Cayenne to observe the opposition of Mars, while Cas- 

 sini (who proposed the expedition), Roemer, and Picard, observed it 

 from different stations in France. When the results came to be com- 

 pared, however, it was found that the planet's displacement was im- 

 perceptible by their existing means of observation : from this they 

 inferred that the planet's parallax could not exceed half a minute of 

 arc, and that the sun's could not be more than 10". 



In 1752 Lacaille at the Cape of Good Hope made similar observa- 

 tions, and their comparison with corresponding observations in Europe 

 showed that instruments had so far improved as to make the displace- 

 ment quite sensible. He fixed the sun's parallax at 10", correspond- 

 ing to a distance of about 82,000,000 miles. 



In more recent times the method has been frequently applied, and 

 with results on the whole satisfactory. In 1849-'52 Lieutenant Gilliss 

 was sent by the United States Government to Santiago, in Chili, to 

 observe both Mars and Venus in connection with northern observa- 

 tories. In 1862 a still more extended campaign was organized, in 

 which a great number of observatories in both hemispheres partici- 

 pated. Prof. Newcomb's careful reduction of the work puts the 

 resulting parallax at 8.855". The method can be used to the best 

 advantage, of course, when at the time of opposition the planet is near 

 its perihelion and the earth near its aphelion ; these favorable oppo- 

 sitions occur about once in fifteen years, and the one which is next 

 to occur, in September, 1877, is so exceptionally advantageous that 

 already somewhat extensive preparations are on foot to secure its 

 careful and general observation. 



to M and the earth to E,' observe the planet's elongation from the sun, i. e., the angle 

 M' E' 8. Now, since we know the periodic times of both the earth and planet, we shall 

 know both the angle M 8 M 1 moved over by the planet in one hundred days, and also E 8 

 E described in the same time by the earth, the difference is M' 8 E,' called by some 



Fig. 3. 



writers the synodic angle. We have, therefore, in the triangle M' 8 E,' the angle at W 

 measured, and the angle M' 8 E known as stated above ; this of course gives the third 

 angle at M,' and hence we know the shape of the triangle, and by the ordinary processes 

 of trigonometry can find the relative values of its three sides. 



