sun's distance. 295 



traded are the daily hours of observation. It seems unlikely that 

 trustworthy results will be deduced from the observations of Venus. 



Figure 6. 

 ^A_____^^ ^ ParaMoM of Mars 



li: "OS 



The circumstances of Mars in opposition to the Sun (figure 6) are 

 much more favorable. Mars may then be compared with stars 

 through the whole night; he has two observable limbs, both admitting 

 of good observation; he remains much longer in proximity to the 

 Earth, and the nearer he is the more extended are the hours of ob- 

 servation. 



Here, however, a circumstance is to be considered which has not 

 previously called for attention. The orbit of Mars is much more 

 eccentric than those of Venus and the Earth. At some oppositions, 

 therefore, he will be so far from the Earth that little advantage will 

 be derived from attempting to observe his parallax. (It is understood 

 that such observations were made in the United States expedition of 

 a few years past, which, from the great distance of Mars, must have 

 been nearly useless.) At other oppositions he is almost as near as 

 Venus is about conjunction. The following table expresses roughly 

 the distance of Mars from the Earth, at some of the nearest and some 

 of the most distant oppositions. The unit of measure is the Earth's 

 mean distance from the Sun: 



1860 about July 21 0.38 



1862 about October 1 0.39 



1809 about February 13 O.GS 



1871 about March 22 0. 01 



1877 about September 3 0.37 



The years 1800, 1862, and 1877 are, therefore, favorable for the 

 determination of parallax. But they require the following special 

 considerations: 



When, as in figure 1, the method of comparison of observations 

 made at a northern observatory and a southern observatory ^s em- 

 ployed, the most favorable position of the planet is that of verticality 

 to the point midway between the two observatories. The north lati- 

 tude of the northern observatories (Greenwich, Berlin, Pulkowa) is 

 greater than the south latitude of the southern observatories, (Cape 

 of Good Hope, St. Jago.) Hence, copferis paribus, a north declination 

 of Mars will be preferable to a south declination. In this respect the 

 opposition of 1862 is preferable to that of 1860. 



But there is another method of making observations for parallax 

 not applicable to Venus, but applicable to Mars, namely, by observing 

 the displacement of Mars in right ascension, when he is far east of 

 the meridian and far west of the meridian, as seen at a single observa- 

 tory. Thus, in figure 6, conceive the pole of the Earth to be turned 

 towards the e} r e, and conceive the Earth and Mars to be stationary 

 in space, the Earth, however, rotating round its axis. By the diurnal 

 rotation, an observatory is carried from the position A to A'; and, at 



