September 8, 1892] 



NATURE 



443 



Variable Star T Cassiopeiae. 

 From long-continued observations of the above star, irregu- 

 larities in the ascending light curve may be expected about 

 October or November next. I shall be happy to supply a dia- 

 gram of the field to any one interested in the question. 



CuTHBERT E. Peek. 

 Rousdon Observatory, Lyme, September 5. 



THE OPPOSITION OF MARS. 



'T'HE Times of Saturday contains a most important 

 *■ telegram, giving the results of Prof. Pickering's 

 observations in Peru during the present opposition of 

 Mars, which is one of the most favourable which has 

 occurred during the last half of the present century. The 

 work done at Arequipa in one respect contradicts, and 

 in others goes far beyond, the results recently announced 

 from the Lick Observatory. There can be no doubt that 

 a considerable advance has been made by this year's 

 results ; many prior observations which have been consi- 

 dered doubtful have been confirmed, and an additional 

 interest lent to the observation of the planet. 



The time, therefore, seems opportune for considering 

 several questions connected with Mars, and it will be 

 convenient to begin with the conditions of this year's 

 observations, especially since the least astronomical 

 among us has certainly noted with surprise the bright 

 red star which now nightly rises low down in the south- 

 east. Nor will he or she be less inclined to regard it 

 when it is recognized as the planet about which during 

 the last month so much has been written of human rather 

 than of astronomical interest. If everything that one 

 sees in print be true, the inhabitants of Mars are signalling 

 to us, and it only remains for us to choose our manner 

 of reply. Of course from signals the imagination of the 

 ready writer has passed at once to words, and having 

 got so far, each planet is about to become acquainted 

 with the history and present conditionings of the other 

 by means of a language understanded of our neighbours 

 as well as ourselves. 



But first as to the cause of its excessive brilliancy 

 during the last month or so, for this doubtless has had 

 something to do with the present general interest taken 

 in the planet. Mars was as bright in 1877, but on that 

 occasion nothing like the present amount of interest was 

 taken in its movements and possible structure. For this 

 there are two obvious causes — one the increasing interest 

 taken by people in science generally ; the other, popular 

 glosses on several recent discoveries made regarding 

 Mars itself. 



The popular idea that the changes which have been 

 recently observed on the planet are changes due to the 

 work of its inhabitants— an idea based upon a mistrans- 

 lation of a word — has, of course, generated the other one 

 — namely, that vast operations have been undertaken for 

 signalling purposes ; and from this idea the step to Mr. 

 Galton's or Mr. Haweis's method of signalling back is a 

 small one. Small though it be, however, the public 

 interest has thereby been greatly enhanced. 



One of the most serious suggestions in modern times 

 regarding signalling to bodies outside the earth we owe 

 to a German astronomer, who some while ago enriched 

 the world with the idea that the inhabitants of the Moon 

 might possibly be communicated with by establishing on 

 the vast plains of Siberia geometrical figures, such as 

 circles, &c., built up of fire-signals, to which signal, if 

 seen, the Lunarians would reply by reproducing them. 



Then the popular mind was content to bridge the chasm 

 of 240,000 miles which separates us from the moon. But 

 now Mars is the objective — Mars, which at its nearest 

 approach is 35,000,000 of miles removed ! 



. ..t Mars when in opposition may be very much further 

 away than that ; so far, indeed, that it is then observed 



NO. 1 193, VOL. 46] 



to be i-sth of its maximum brightness, and naturally with 

 very reduced angular diameter. The two preceding 

 oppositions at which its brightness has been at all com- 

 parable to its present one, took place in i860 and 1877, 

 so that we find the most favourable oppositions about 

 sixteen years apart. The reason of this will easily be 

 gathered from Fig. i, which shows with sufficient accuracy 

 the very elliptic orbit of Mars in relation to that of the 

 earth. The lines joining the two orbits are those con- 

 necting the two planets during some oppositions from 

 1830 onwards to 187 1. The outer planet, Mars, is re- 

 presented nearly at \h^ perihelion part of its orbit, that 

 is the point at which it is nearest the sun (and therefore 

 the earth, if we treat the earth's orbit as a circle), and 

 the reason that the 1830 and 1862 observing conditions 

 were so much better than those of 1 869 and 1 871 is at once 

 clear. The opposition of 1877 and the present are not 

 shown on the diagram, but they occurred at a time when 

 Mars was not far from its perihelion. 



The diagram also allows us to see that at the peri- 

 helion point of Mars' orbit the planet is very nearly at the 



I'iG. I.— The orbits of the Earth and Mars. 



time of [the southern solstice, the N. pole being inclined 

 away from the sun. Also that this must occur about four 

 months before the southern solstice of the earth, the 

 direction of the a.\is of which is also shown. 



So that at an opposition which occurs in August, as 

 the present one does, we observe what happens in the 

 summer solstice of the northern, and winter solstice of 

 the southern, hemisphere of the planet. In fact, generally 

 we have : — 



Time of opposition. 

 August ... 

 November 

 February .. 

 May 



N. hemisphere. 

 .. Winter , 

 .. Spring 

 ... Summer 

 ... Autumn 



S. hemisphere. 



Summer 



Autumn 



. Winter 



. Spring 



The perihelion point of a planet's orbit is astro- 

 nomically expressed by its heliocentric longitude, and 

 the apparent size of its disc (on which its apparent 



