NOTES. 



ASTRONOMY. 



Hy A. C. I). Ckommelin, D.Sc, H.A. 



THE SUN'S DISTANCP: DETEKMINia) 1 K( )M 

 EROS. — Last year Mr. A. R. Hinks published the result of a 

 very e.xhaustive discussion of the Sun's distance from the 

 observations of Eros at the time of its near approach in 1000- 

 1901. His final value for the Sun's parallax is S"-806. 

 Taking the Earth's equatorial diameter as 7925-45 miles, the 

 Sun's distance is 92.831,000 miles, with a probable error of 

 some jO.OOO miles. This result is very close to that previously 

 deduced by Harkness from a Least-Square adjustment of all 

 available material, and also to Gill's result from the minor 

 planets Iris, Victoria and Sappho. It is expected that Eros 

 will in the course of years afford a much more accurate 

 determination than any yet made, by the very large 

 perturbations which the earth causes in its motion. Careful 

 observation of these will give the ratio of the Earth's mass to 

 that of the Sun. from which the distance of the latter readilv 

 follows. These perturbations can be expressed as a series of 

 regular waves, or Sine-curves, of various periods. The 

 largest wave has a period of 40-6 years, the amplitude of the 

 Cosine part of it being — 703", and the argument seven times 

 Eros' Mean Longitude — four times that of Earth. There is 

 another large term with amplitude 259" and multipliers of the 

 longitudes 37 and 21; period 82-6 years. A more accurate 

 knowledge of the period of Eros is required before this last 

 term and others of longer period can be accurately calculated. 

 Forty-six revolutions are so nearly etiual to eighty-one years 

 that it is still uncertain in which direction they differ. The 

 shift in the planet's place due to these terms is increased by 

 the eccentricity of the orbit, and by the fact that the planet 

 is at times so near the earth that the heliocentric shift is 

 increased sixfold. A total range of some 3° may thus arise, 

 and as the range due to parallax is only 2' at most, it will be 

 seen how much the method of perturbations will eventually 

 surpass the other. It will probably not attain its full accuracy 

 till two or three times the forty-year period ha\e elapsed. 

 But it is likely that before the Transit of Venus in 2004 the 

 Sun's distance will be so well known that observations of 

 the Transit will be useless for this purpose, and will simply 

 be employed for correcting the elements of Venus' orbit. I 

 have taken the above figures from a paper by Heinrich 

 Samter in Astr. Nach., No. 4498. 



NEW DETICKMI NATION OF THE MOON'S 

 DISTANCE.— During the last six years the small- bright 

 crater Mosting A has been regularly olsserved on the meridian 

 both at Greenwich and the Cape. It is much easier to bisect 

 the crater with a micrometer wire than to place it tangential 

 to the Moon's limb, which is often serrated by mountains. 

 About a hundred nights are available on which the crater was 

 observed at both stations, and comparison of these has given 

 a new determination of the Moon's distance. The chief 

 source of uncertainty in the result is the fact that the shape 

 of the earth is not yet known with great precision. The 

 distance was accordingly computed on two assumptions of 

 the shape, \'u., compression L293i and 1/300; these about 

 cover the present uncertainty. On the 'Tfirst assumption 

 Hansen's parallax (which is 57' 2"-23) needs a correction of 

 plus 0"-50, and on the second of plus 0"-12. On the former 

 assumption the Moon's distance comes out 238,817 miles. 

 Another way of finding the distance is based on the observed 

 period and the force of gravity at the Earth's surface. This 

 method also gives results that vary, though to a less extent. 



with the Earth's figure. The deduced corrections to Hansen's 

 parallax are 0"-45 and 0"-36 on the above assumptions. 

 Hence to make the two methods agree we must take the 

 compression as 1 , 294A. This applies to the mean meridian 

 of Greenwich and the Cape, for it is cjuite possible that 

 it is sensibly different for different meridians. Sir David 

 Gill has for many years been endeavouring to have 

 South Africa geodetically connected with Europe, and 

 only a few links are now needed to complete 

 this great work, which would give a measured meridian 

 extending from the North Cape to South Africa, and would 

 give the compression of this meridian very accurately. It 

 seems to nie that it would be advisable for equatorial observa- 

 tories to measure the moon's distance by the diurnal method 

 (comparing observations made east and west of the meridian 

 at the same station). The equatorial parallax of the moon 

 would thus be given, free from uncertainty arising from the 

 figure of the earth, and it would in addition be possible to test 

 whether the equator has any ellipticity by comparing the 

 results obtained at different equatorial stations. The measures 

 might be made photographically, for Professor Pickering and 

 Mr. H. N. Russell have lately shown that good photographs 

 of the moon for position can be obtained by giving the 

 surrounding stars a time exposure, the moon being screened 

 by a disc except for a fraction of a second, the time of which 

 may be automatically recorded. This plan is analogous to 

 that used for many >-ears at Greenwich for photographing 

 Neptune and its satellite, and double stars that differ much in 

 brightness. 



BOTANY. 



P>y Professor F. Cavers, D.Sc. F.L.S. 



THi; llloLOGV OF LICHENS.— A vast amount of con- 

 troversy has centred around the relationship between the two 

 components of the thallus, or plant body, of the Lichens. .A 

 recent short paper by Tobler {Ber. liciitscli. hot. Gt's., 1911) 

 serves as a reminder that the question is by no means settled, 

 and the following notes may be of interest to the student of 

 plant life who knows something of the general characters of 

 these curious organisms. 



The Lichens form a quite exceptional group of plants, with 

 various peculiar features. A Lichen is a couipoimd plant, 

 consisting of a Fungus individual and numerous .^Iga 

 individuals. The Fungus, composed of branching and inter- 

 lacing threads, has grown around the Algae, enclosing them in 

 a sort of nest, in a manner which has often been compared to 

 the enmeshing of a fly by a spider. The result is that the 

 Lichen can grow in places which would be unsuitable for the 

 independent existence of either the Fungus or the .Algae of 

 which it is composed. Algae grow in water or in moist 

 places, and very few can live without a regular and abundant 

 supply of moisture, while (apart from the leathery bracket-like 

 forms) Fungi are very sensitive to cold and drought. Vet 

 the Lichens thrive in the bleakest positions and in the most 

 severe climates, as on bare mountain rocks where they may 

 get no water for weeks on end, or may be soaked with rain 

 and mist for equally long periods, and where they are exposed 

 to great extremes of heat and cold. 



In a typical Lichen the Fungus provides the organs of 

 fixation ; protects the Alga cells from drought and other 

 injurious infiuences; absorbs water with dissolved salts, 

 and air witli carbon dioxide; and it alone produces the 

 spore fruits. The Alga, on the other hand, manufactures 

 organic food, with the help of light, from carbon dioxide .and 



276 



