198 TJie Planet Mars in 1873, [April, 



is singularly favourable for observing the gibbous phase of 

 Mars after opposition. For it will be perceived that, even 

 when the line joining the sun and Mars is immediately in- 

 clined to the line joining the earth and Mars, — as E 7 M 7 , 

 E 8 M 8 , and E 9 M 9 , — the distance of the planet is not very 

 much greater than when Mars is in opposition at M 5 . Thus 

 the disc of the planet, it will be seen (from the illustrative 

 plate), diminishes much more slowly in size after opposition 

 than it had increased before opposition. The telescopist 

 should not lose this excellent opportunity for studying the 

 way in which the disc seems coloured near the terminator. 

 A careful comparison between the part of the disc near the 

 terminator and on the opposite side cannot but prove most 

 instructive. It will be observed that the terminator marks 

 the place where morning is breaking on Mars (before oppo- 

 sition, of course, the terminator marks the place of Martial 

 sunset) ; accordingly the occasion is favourable for deter- 

 mining whether, supposing there is whitish light near the 

 terminator, that light is sharply denned towards the middle 

 of the disc. Of course this can be done at any epoch of the 

 opposition-period ; but it can be best done near quadrature, 

 because either the morning or evening part of the planet is 

 then less foreshortened than at other times. 



Next notice another circumstance. Whereas the motion 

 of Mars on his orbit causes the solar elevation north of the 

 Martial equator to continually diminish throughout the 

 period dealt with in the figure,* the elevation of the earth 

 north of the Martial equator does not change in the same 

 way. It is easy to see why this is. We may regard Mars, 

 during the opposition-period, with reference to its bearing 

 from the earth when Mars is at M„ his bearing from the 

 earth is the same as his bearing from the sun when he is 

 near M 6 , and accordingly the elevation of the earth north of 

 the equator-plane of Mars is nearly the same on February 

 26th as the elevation of the sun north of the same- plane on 

 May I2th.t Then, as seen from the earth, Mars sways 



* Precisely in the same way, of course, as in the case of the earth, as 

 specially illustrated in my " Sun-Views of the Earth." 



t The table towards the end of the present note gives the actual relations 

 of the Martial globe, as well with reference to the circles and parallels of 

 declination as to the sun. For the convenience of the reader who may care 

 to test the results here tabulated, I give the formulae and elements from which 

 the table lias been calculated. 



The elements on which the determination of the axial position of Mars has 

 been based are those given in No. 858 of the " Astronomische Nachrichten,*' 

 in a paper by Dr. Oudemans upon the observations made by Bessel with the 

 Konigsberg heliometer. between the years 1830 and 1837. He gives (as quoted 

 in a note by Mr. Hind in 1867) — 



