344 



KNOWLEDGE. 



September, 1913. 



a thrush's nest and eggs. A blackbird's nest in a 

 similar basket was begun on a Tuesday and was 

 seen completed with two eggs in it on the following 

 Sunday morning, while it may be added in this 

 connection that within two days of a new log box 

 being put up a great tit had begun to bring in 

 material, and before the week was out the nest was 

 finished and contained eggs. 



Some seasons ago, two examples of the Lyne 

 nesting bush, which we believe is intended for 

 aviaries, were sent to the Sanctuary by the inventor, 



and one of these put up in a hazel was occupied by 

 a thrush in the first year, but not afterwards. We 

 have pretty well exhausted the various places which 

 the birds with which we are dealing have chosen in 

 the Sanctuary, but we may mention that in The 

 Selbome Magazine for August a photograph is 

 reproduced showing a ladder hanging under the 

 eaves of a cottage with thirteen blackbirds' nests 

 built between the rungs. These were constructed in 

 the same season, and several of the birds were 

 sitting at the same time. 



NOTES. 



ASTRONOMY. 



By A. C. D. Crommelin, B.A., D.Sc, F.R.A.S. 



PROFESSOR NEWCOMB'S LAST RESEARCHES ON 

 THE MOON'S MOTION.— A pathetic interest attaches to 

 the volume of the American Ephemeris Papers that has lately 

 appeared ; for Professor Newcomb, who had given a large 

 portion of his life to researches on the Moon's motion, and 

 was the first to find the reason of the rapidly growing error of 

 Hansen's Tables, continued to dictate the matter of the 

 present volume till within a few days of his death. Some 

 thirty years have now passed since his earlier researches, in 

 which he showed that one of Hansen's large Venus terms 

 ought to be left out altogether, having no place in theory ; that 

 the mean motion and secular acceleration he used were also 

 wrong, and that it was necessary to introduce a large 

 empirical term with a period of about three centuries. These 

 four alterations were introduced into the Nautical Almanac 

 under the name of Newcomb's Corrections, and are used 

 there to this day. There were some who denounced their 

 introduction, which they called an introduction of empiricism 

 into exact astronomy ; this shewed a decided misconception of 

 the situation ; only one of Newcomb's four corrections was 

 empirical, the others were in accordance with theory ; besides, 

 Hansen's Tables were themselves just as empirical, for they 

 had a large Venus term which ought not to be there at all, its 

 coefficient being really quite insensible. In the last thirty 

 years many attempts have been made to find a theoretical 

 explanation of the great empirical term. It is now established 

 that it cannot be due to the action of any of the known 

 planets ; at least four causes have been suggested : it might be 

 due to a ring of planets inside Mercury's orbit, with period 

 nearly the same as a lunation ; or to want of symmetry in the 

 sun, whose rotation is nearly the same in length as a lunation 

 (this suggestion was made by Brown, but it does not appear 

 tenable in view of the great differences of rotation-time of 

 different zones of the sun) ; or that gravity was obstructed in 

 passing through matter, so that a lunar eclipse causes a 

 diminution in the sun's action on the moon (this is due to Dr. 

 de Sitter) ; or that there is some connection with the earth's 

 magnetism, the period of revolution of the magnetic pole being 

 something like three centuries. 



The object of Professor Newcomb's last research was to 

 try to find the nature of the great oscillation with more 

 accuracy ; for this purpose he has used a very long series of 

 observations of occultations of stars ; these are capable of 

 more precision than meridian observations, for the disappear- 

 ance or reappearance is instantaneous, and can be estimated 

 to a quarter of a second or less. The places of all the brighter 

 stars in the zodiacal region have now been found fairly 

 accurately, and the error of the moon's place can be deduced. 

 I would remark that the observation of these phenomena is 

 one in which amateurs may well take part ; a three-inch 

 telescope is amply large enough ; there must be some means 

 of getting correct time ; one way of doing this is to receive 

 the wireless signals from the Eiffel Tower. Writers in Nature 



have stated that they have succeeded in doing this, using an 

 ordinary wire mattress as collector ; a fairly good clock or 

 chronometer is also necessary, to measure the interval between 

 the occultation and the time signal. 



After calculating the results given by the long series of 

 occultations, which extend from 1620 to 1908, Newcomb 

 proceeds to discuss the mean motion and acceleration, and 

 the form of the empirical fluctuation. He finds that a single 

 sine curve will not represent the fluctuation ; there is evidence 

 of another with a period of about sixty-four years, which has, 

 in fact, been introduced into the new French Tables ; the 

 great term is best represented by the expression 



12"'95 sin { 1°-31 (t-1800) + 100°-6 } 



the period being two hundred and seventy-five years; his 

 result of thirty years ago was 



15"-49 sin { 1°-3187 (t-1800) + 93°-9 }. 



The coefficient has now been reduced by 2§", and the period 

 lengthened by four years. 



Newcomb gave a diagram of the fluctuations of the moon 

 from its calculated place since 1620 in the Monthly Notices 

 for January, 1909 ; it clearly shows the great wave and the 

 term of about sixty-four years, and there is an appearance of 

 a still shorter wave from the observations of the last century. 



The work also discusses ancient eclipses of sun and moon ; 

 Newcomb gives little weight to the former, considering that 

 the evidence that any eclipse was total at a given point is not 

 convincing ; indeed, this is admitted by all, and weight is only 

 assigned to corrections which will make most of the eclipses 

 of antiquity agree with the narratives. 



Several other astronomical constants are incidentally dis- 

 cussed, since this long series of occultations throws light on 

 them : among others the rate of change of the obliquity of the 

 ecliptic ; Newcomb favours Seeliger's suggestion that we 

 should look to the denser part of the Zodiacal Light for an 

 explanation of the discordance of this rate from theory, also 

 of discordances in the rate of motion of the perihelion of 

 Mercury and of the node of Venus (Newcomb had deduced 

 8"- 76 for the sun's parallax from the motion of Venus' node; 

 the accepted value now is 8" -806; the discordance may be 

 due to the action of the Zodiacal Light). The part of the 

 Zodiacal Light that seems to be gravitationally active is of 

 ellipsoidal form, and within the orbit of Mercury ; that is, it is 

 only a small part of the light that we see. but the density 

 doubtless increases rapidly as we approach the sun, so that 

 most of the mass lies inside Mercury's orbit. This would be 

 a more satisfactory explanation than the assumption that 

 gravity does not vary exactly as the inverse square. Another 

 suggestion for explaining the anomalies in the moon's motion 

 was that they might arise from changes in the rate of the 

 earth's rotation. Newcomb tested this question by the 

 observed transits of Mercury ; his earlier discussion seemed 

 to indicate changes that would explain half the moon's 

 fluctuations ; but the more complete discussion seems to 

 indicate no change in the rotation rate. The rather unsatis- 

 factory end of the entire work is that " Until the matter is 



