33(> 



NATURE 



[Septemher :6. 19-^9 



THE APPROACHING OPPOSITION OF MARS. 



THE ))lanet Mars is now a conspicuous object 

 in the evening sky, its ruddy, brilliant disc 

 appearing just below the great square of Pegasus, 



1899 



(894 





Y iMAagl 



^ 1 



SEP'-^a 



-^1907 



°^A//\RS 



and astronomers the world over are once more seizing 



the opportunities presented by a favourable opposition 



for the further solution of the Martian enigma. The 



actual opposition will not take 



place until September 23d. 22h., or 



10 a.m. on September 24, civil 



date. 



As Prof. Lowell says in his 

 classical memoir on Mars, " Study 

 of Mars at one opposition is 

 material to its study at the 

 next. ... At any one opposition 

 We may scan Mars but for 

 a few months through only a frac- 

 tion of its circuit round the sun." 

 Therefore, no opportunities mav 

 be missed by the students of the 

 ruddy planet, whenever, and under 

 whatever conditions, an opposition 

 takes place. But only at one op- 

 position in every seven, or about 

 once every fifteen years, are the 

 conditions so favourable as at 

 present; Figs, i and 2 show this 

 diagramniatically. The orbits ot 

 the earth and Mars are drawn to 

 scale, but as eccentric circles, and 

 /'rom Fig. 1 it will be seen that 

 the opposition of this month will 

 be, as regards the distance separ- 

 ating the two planets, the most 

 favourable we have experienced 

 since 1892. Owing to the eccen- 

 tricity of the planet's orbit, the distance between 

 the earth and Mars, when at an opposition, may range 

 from 61,000,000 to 35,000,000 miles, the corresponding 

 range of the apparent diameter being 13" to 25". The 

 NO. 2081, VOL. 81] 



most favourable oppositions occur when Mars is near 

 its perihelion (n, Figs. 1 and 2), with the earth near 

 its aphelion (a), point. The ideal condition would 

 be for opposition to take place about the last week 

 in August, whilst the most 

 unfavourable conditions would 

 obtain if it occurred about the 

 thi:d week in February; thus the 

 opposition of 1877 was the most 

 favourable during last century, 

 except that of 1845, whilst that 

 of I go I, 'February 21, was about 

 as unfavourable as is possible. 



The advantage of proximity 

 was well illustrated in 1S77, wher> 

 Asaph Hall discovered the two 

 Laputan satellites, and Schia- 

 |jarelli first observed the much- 

 discussed canali. 



On September 23, when at op- 

 position. Mars will be about 

 36'4 million miles from the earth, 

 but the nearest approach of the 

 two bodies will take place on 

 September 18, when the distance 

 separating them (E^-M,, Fig. 2) 

 will be about 160,000 miles less. 

 After the opposition, as the planet 

 lags behind the earth, as shown in 

 Fig. 2, the distance will continue 

 lo increase, and the apparent 

 diameter of the planet will, of 

 course, decrease, as shown by the 

 circles drawn on the right of the 

 diagram. These circles show the 

 relative apparent diameter of 

 the planet on .-Xugust 13, when 

 Mars is at perihelion, on .September 18, when 

 at least distance from the earth, and on November i, 

 when a suh-,t.inti.Ll increase in the distance 



1909 



1877 



separating the two bodies will have taken place. 

 The following are the apparent diameters of the planet 

 at different epochs during the present opposition : — 

 September i, 228"; September 18, 24"; September 23 



