62 



KNOWLEDGE 



[Makch 1, 1900. 



the sun, — the nature of the Light still remains more 

 or less of a mystery. We do not know yet whether it 

 lies in the plane of the ecliptic, or of the sun's equator, 

 or between the two, or whether even its plane may not 

 shift from time to time. It seems to vary in brightness, 

 both according to the seasjn of the year, and from one 

 year to anothei-, but the determinations of its bright^ 

 ness are usually far too vague and rough for any 

 definite period to have been yet fixed for its changes. 



So that we have in the Zodiacal Light the great 

 anomaly of a vast astronomical object requiring no 

 observatory and no telescope for its observation ; and 

 not only requiring none but permitting none ; and yet 

 to-day, when astronomy has lasted 5,000 years, we are 

 still in ignorance of many of the most fundamental 

 facts respecting it. 



This is due without doubt to the difficulties which 

 a.ttend its observation. Not that those difficulties are 

 in the .least insuperable, but they are very real. We 

 will suppose that someone has noticed the Light for the 

 first time, and desires to make a record of what he 

 sees. It at once strikes liim that a mere eye-sketch of 

 it is of very little good indeed; he must place it with 

 respect to the stars. In all probability most of those 

 which would be naturally used to define the outline of 

 the Light are unfamiliar to him. He has therefore to 

 have recourse to the star atlas. He painfully identifies 

 the stars one by one, but each recourse to the atlas, 

 which must necessarily be examined in the Light, 

 dazzles his eyes for his open-air work. He finds, there- 

 fore, that the process of recording what he has seen 

 is a very slow and tedioiis one, and, dicsatisfied with 

 what he has done, speedily gives up the work. So 

 that it happens that the names of the men who have 

 done really useful work in this field may be counted 

 almost on the fingers of one hand. 



Yet this difficulty can be siu-mounted without much 

 trouble. First of all and beyond everything, he who 

 would become "an astronomer wi^hrut a telescope" 

 must learn his stars. They form the very alphabet of 

 the language which he has to learn, and a little trouble 

 spent here will soon repay itself. Next, the difficulty 

 of recording his obsei-vations in the dark may be got 

 over in several ways. It is possible to learn to write 

 in the dark with sufficient clearness, and such little 

 dodges as having sets of cards prepared, ruled with lines 

 made by drawing a penknife across the back of the 

 card and cutting it partly but not entirely through, 

 will be found helpful. Or the note book may be 

 placed so that the rays from a ruby photographic lamp 

 may fall upon it. If the eyes are carefully screened 

 from the direct light of tlie lamp, it will be found that 

 the page may be lighted up quite sufficiently for the 

 pui-pose of writing without the sensitiveness of the 

 eye to the faint Zodiacal glow being much affected. If 

 a chart is needed for comparison with the sky this 

 might be done by tracing the map of the region re- 

 quired from some star atlas on to a piece of tlun card- 

 board and pricking little holes for tJie stars. A lamp 

 can be used behind the card, to show these, or a piece 

 of card painted with lumincus paint might be placed 

 underneath. Many similar dodges for getting over this 

 initial difficulty will suggest themselves to those who 

 seriously take up the wo"k. 



But it will be objected, since the Zodiacal Light is 

 seen so much better in the tropics than here, what is 

 the use of trying to observe it in England? There is 

 great use. Take for example one question ; the 

 question of its variability in brightness from year to 



year. In a way this could be as definitely determined 

 from observations made in England as from those made 

 in any other single country. A careful record year by 

 year for a term of years of the number of days when 

 the atmospheric conditions were favourable, and when 

 the Zodiacal Light was well seen, seen faintly, or not 

 seen at all, would soon show as to whether there was 

 any periodicity in its variation, and, if so, whether it 

 varied with the suiispot tycle or aot ■ just as Hofrath 

 Schwabe's record of the da.ys when the sun was seen 

 to be free from spots in each year was quite as effective 

 in determining the sunspot variation and the length 

 of its period as exact measurements of the areas of all 

 the spots would have Doen. In a certain sense our 

 less favourable position would serve as a kind of photo- 

 meter of the brightness of the Light, and our very 

 hindrance might transform itself into a help. 



Then, a more important point, observations in one 

 latitude alone are not sufficient. We want to ascertain, 

 either what is the amount of parallax which the Light 

 shows or else that it has no perceptible parallax at 

 all. Then, the degree to which its apparent outline 

 is affected by atmospheric absoi-ption is even more im- 

 l ortant, as othei-wise we cannot tell whether an ap- 

 parent shift in its plane is real or not. For both 

 these enquiries it is necessaiy that observations should 

 be made in several distinct latitudes. It is for this 

 puiioose that, in 1898, the British Astronomical Asso- 

 ciation initiated a Zodiac il Light Section, in order to 

 enlist the co-operation of observers in many lands, 

 under the directorship of Captain P. B. Moles- 

 worth, R.E., Trincomali, Ceylon, who has prepared a 

 set of eight ecliptic charts to assist in the study of the 

 Light. 



The principal points for observation in Zodiacal 

 Light work are, first of all, to note the character of 

 the evening. The magnitude of the faintest stars 

 visible in the west should be recorded. The visibility 

 of the Milky Way, and the distinctness with which its 

 lifts and streamers can b"- made out, would be most 

 useful for comparison. The evidence must be clear 

 that there is no mist or dust veil to hinder observation, 

 and here, it may be added, that the dwellers in towns 

 are necessarily too severely handicapped to enter upon 

 this class of work. The smoky atmosphere and the 

 glare of street lights are fatal to so delicate a research. 



The Light itself should then claim attention. It will 

 be perhaps easiest, first of all, to map out its extreme 

 border, and this will often be best detected by looking 

 a little way from the Light.; ' partially averted vision " 

 having a distinct advantage for very faint objects. 

 Then a definite area of the Milky Way may be taken 

 as a standard, and, so far as possible, the outline of the 

 Iiiglit where its brightness equals that of the selected 

 area of the Milky Way .'hould be 'aid down. Search 

 should be made in the pai-t of the ecliptic imme- 

 diately opposite the sun to detect the Gegenschein 

 Ci- Counterglow, the faint diffused light which travels 

 through the heavens in opposition to the sun. It 

 should be seen whether the Zodiacal Light extends to, 

 and is merged in the Counterglow, or whether there is 

 a space of dark sky between them ; and here it will 

 he found useful to take foi reference some of the 

 darkest regions of the sky which may be available. 

 The position of the apex of the Light is very im- 

 portant, and at this season of the year it should be 

 especially noted whether the Light can be definitely 

 traced beyond the Pleiades. There can be no doubt 

 that that group does seenf to exercise a strong 



