September i6, 1909J 



NA TURE 



)37 



(opposition), 23'9"; October i, 23"3" ; November i, 

 17'S" ; December i, i2'7". Tliis means tliat on Sep- 

 tember 18, an observer using a power of x So would 

 see Mars on the same scale as a naked-eye observer 

 sees the moon; the conditions of " seeing " would be 

 worse. Talving" another illustration, a land area of 

 about the size of Ireland would, roughly speaking, 

 appear as a spot of i'3" diameter, or a little longer 

 than I /1500th of the apparent diameter of the full 

 jnoon. 



Whilst the distance of the planet is an important 

 factor in determining the value of the observing con- 

 ditions at an opposition, it is by no means the sole 

 factor ; the altitude of the planet above the horizon 

 makes or mars the conditions for the users of large 

 instruments searching for minute detail. Thus, 

 although the opposition of 1892 produced a more 

 favourable distance-condition than that of 1894 (sec 

 Fig. i), the observing conditions at the latter were 

 not inferior,' because of the higher culmination of the 

 planet. At the present opposition, the declination of 

 Mars is 4° S., and this means that for observers in 

 our latitude (51° 30') the meridian altitude will not 

 e.\ceed 35° ; but this is a great improvement on the 

 conditions in 1907, when the corresponding altitude 

 was only 10°, and when, even from Flagstaff, Prof. 

 Lowell found it desirable to send an expedition to the 

 Andes for the observation of the planet. During the 

 present opposition the meridian altitude at Flagstaff 

 will be more than 50°. 



."Vs at all favourable oppositions, taking place about 

 August, the south pole of Mars is now tilted earth- 

 wards, the earth, at the date of opposition, being 

 about 20° below the plane of the planet's equator. 

 Therefore the southern hemisphere will be observed, 

 and as the summer solstice of this hemisphere, as 

 shown in Fig. 2, occurs but a few days before opposi- 

 tion, the southern snowcap is in the process of disso- 

 lution, and changes due to the melting of the snow are 

 taking place. .Already such phenomena have been 

 recorded by MM. Desloges and Jonckheere, among 

 others. As the rotation-periods of the earth and Mars 

 are approximately equal, the same regions can be ob- 

 served on consecutive nights. On September 19 the 

 Syrtis Major region will Ije in view, and on September 

 27 the region of the Mare Cimmerium. 

 . Probably at no opposition since the time that Fon- 

 tana suspected martcings on the ruddy planet, in 1636, 

 has the status of areographers been so critical as at 

 the present juncture. Thanks to the persistent 

 labours and unswerving faith of a few observers, of 

 whom Prof. Lowell is the foremost, the question as 

 to the subjective reality of the canaU discovered by 

 Schiaptirelli in 1877 ma}' be considered as settled. 

 Whether one follows Prof. Lowell's lead in the matter 

 of "artificial, irrigating waterways " or not, there 

 can remain but little, if any, doubt that these" long, 

 straight channels do exist. In describing his observa- 

 tions, made at Trincomali, Ceylon, during the un- 

 favourable opposition of 1903 (see Fig. i), when the 

 apparent diameter of the planet was but I4'6", the 

 late Major Molesworth said^ : — "Personally, I am 

 quite convinced of the reality of the great majority of 

 the so-called canals,: I think I could convince the 

 rhost sceptical on this point if thev could only have 

 spent an hour or two at my telescope on some of the 

 perfect nights in March and April this year." Major 

 Molesworth used a I2f-inch Calver reflector, with a 

 power of 450. Numerous observers, and the Flag- 

 staff photographs, have also testified as to the gemina- 

 tion of these features. Not only do these canals exist, 

 but, in the opinion of many experienced observers, 



1 Monlhly Notices, vol. Ixv., No. 8, p. 839, 1905. 



NO. 2081, VOL. 81] 



they also suffer changes which show a dependence 

 on the seasonal changes of the planet. 



Having settled the existence of the "canals," it 

 became necessary to account for the changes, and, in 

 one essential, this question remained more or less 

 open until the opposition of 1907. With regard to 

 the polar caps, Herschel's observations enforced the 

 natural conclusion that their changes were due to the 

 accumulation and dissipation of " snow " as the Mar- 

 tian winters wa.xed and waned. This coincidence of 

 snowcap and season was not to be denied, and in the 

 Martian spring, at the opposition of 1892, Prof. W. H. 

 Pickering observed the disappearance of some 1,600,000 

 square miles of the southern snowcap, an area about 

 the size of India, in a period of thirty-three days. 

 But there still remained the one essential factor, that 

 was the proof that this " snow " was really frozen 

 water; that the Martian atmosphere contained water- 

 vapour sufficient to produce these effects. On this 

 point the different observers were at issue. 



Beer and Madler, during 1830-9, found that 

 occasionally certain permanent features of the planet's 

 landscape were blurred, as though bv passing cloud 

 and mist. During tlie favourable opposition of 1862, 

 Lockyer's observations led to the definite conclusion 

 that " the daily — nay, hourly — changes in the detail 

 and in the tones of the different parts of the planet " ' 

 were caused by the transit of clouds over the various 

 features. 



" Clouds and mists " and " polar snows " inevitably 

 suggest to the terrestrian the presence of water, hence 

 a raison d'etre for the canals, and the • spectroscopic 

 evidence adduced by Huggins and Vogel went to 

 confirm the suggestion. But with the spectroscopic 

 equipment of the Lick Observatory at their disposal, 

 Campbell and Keeler could find no evidence for water- 

 vapour in the planet's atmosphere, and the critics of 

 a " terrestrial " Mars suggested that the snowcaps 

 might be caused bv the solidification and deposition 

 of some other compound, such as carbon dioxide. 



However, the spectrograms obtained by Mr. Slipher 

 at the last opposition, 1907, afford, according to our 

 present view, incontrovertible evidence that the atmo- 

 sphere of Mars does contain a detectable quantity of 

 water-vapour (see Nature, vol. Ixxvii., p. 442, March 

 12, 1908). Prof. Very estimates that at the time the 

 spectrograms were taken, the Martian atmosphere con- 

 tained sufficient precipitable water to give an average 

 layer 14 mm. deep, or about one-third or one-fourth 

 that in the earth's atmosphere. Nor is water-vapour 

 the only familiar atmospheric constituent which has 

 been shown to be present by the Lowell Observatory 

 spectra. When Jilr. Slipher described - the 1907 

 spectra, he explained the ditliculty of detecting the free 

 oxygen constituent of the Martian atmosphere, viz., 

 the probable relativelv slight increase in intensity, of 

 the oxygen bands, produced by adding the absorption 

 of a thin (Martian) atmosphere to that of a 

 dense (terrestrial) atmosphere, but expressed the 

 opinion that " its detection need not be considered 

 impossible." 



A recent message from the Kiel Centralstelle, dated 

 September 10, informs us that Prof. Very's measures 

 of the Lowell' Observatory spectrograms — which show 

 the spectra of the moon and Mars photographed side 

 by side when the respective objects are at equal alti- 

 tudes — show that oxygen is present in the atmosphere 

 of Mars; the relative intensification of the oxygen- 

 band b, in the planet's spectrum, is stated to be eight 

 times the probable error of the measures. Therefore, 

 although the details are yet to come, it appears fairly 



1 Memoirs R. A. S., vol. xxxii., p. 179, 1863. 



- Aslrophysknt Journal, vol. xxviii., p. 404, igo8. 



