April 21, 192 1] 



NATURE 



235 



the moon, our luminary must receive about 2,000,000 

 in twenty-four hours. The great majority of these 

 would necessarily be invisible. One-half of the number 

 would fall on her averted face. Of the remainder 

 more would fall during sunlight than during the hours 

 of darkness. Of those that fell during hours of dark- 

 ness the greater number would be concealed by terres- 

 trial cloud. Of those that were not so concealed one- 

 half would fall on the illuminated part of the moon's 

 disc, and, perhaps, be rendered invisible by the lunar 

 brightness. It is easy to see that large abatements 

 must therefore be made from the number of falls if 

 we wish to estimate the probability of making a 

 successful observation. This consideration has a 

 bearing, by the way, on the reasonableness of expect- 

 ing to be able to witness the arrival of Prof. Goddard's 

 projected rocket if the aim were good and a hit 

 secured; but that is by the way. 



If, in consideration of all these adverse con- 

 tingencies, we reduce the estimate of impacts to i per 

 cent, of the above-quoted figure, we have 20,000 hits 

 on an average moonlight night. Why has not one 

 of them ever been observed ? Among the number of 

 meteorites must be a certain proportion weighing 

 one or two hundredweight or more. When masses of 

 that magnitude enter our atmosphere they grow in- 

 candescent and light up a whole countryside, it may 

 be for some seconds. That is the result of impact 

 upon our yielding atmosphere. If they reached the sur- 

 face of the earth, as presumably they do that of the 

 moon, with cosmic velocities ranging up to 40 miles a 

 second, would they not break up there with an out- 

 burst of light like that of a nova among the stars? 

 Furthermore, as these impacts must include not only 

 single masses of considerable size, but also meteoric 

 showers, the areas affected must presumably at times 

 be large enough to be quite observable through a good 

 telescope. It may be suggested that when the fall is 

 normal, or at any large angle to the moon's surface, 

 the projectile buries itself too deeply in the substance 

 of the moon to be visible. But among the arrivals 

 must be some that arrive at grazing, or something 

 like grazing, incidence on the moon, penetrating little, 

 or not at all, beneath its surface. Why are their 

 glowing paths never seen and the furrows which must 

 so have been ploughed, in the course of ages, upon the 

 moon's ancient surface never described to us? 



Probably there is an easy answer to these ques- 

 tions, but, even if easy, it would be interesting to 

 those of us who are not astronomers. 



J. W^ Gordon. 



1 1 King's Bench Walk, Temple, E.C.4, April 12. 



The question of meteors on the moon is not now 

 raised for the first time. In my article on astronomy 

 in "Science in Modern Life," vol. i., p. 35 (I give 

 this, not as being the first mention of the subject, but 

 because it is the most accessible source), I wrote :— 

 "There is one puzzling question raised by Prof. Shaler, 

 i.e. how is it that the fall of meteors on the moon, 

 which must be as dense as those falling on the earth, 

 has not covered all the markings with a veil and 

 obliterated the differences of tint? It has, however, 

 been calculated that even if the atmospheric density 

 at the surface be only 1/10,000 of that on earth (a 

 quantity which it may well exceed), then, since the 

 rate of decrease is so much slower than on the earth, 

 at a height of something over 40 miles the densities 

 of the atmospheres would be equal, and at still 

 greater heights that of the moon would be the denser. 

 Now most of the meteors that enter our air are com- 

 pletelv burnt up at greater heights than this, so that 

 the thin lunar atmosphere may actually be as effec- 

 tive for stopping meteors as our own." 



It is comparatively rarely that meteors reach the 



NO. 2686, VOL. 107] 



earth's surface, and when they do so the speed has 

 been so diminished by friction that there is no intense 

 flash. The above reasoning makes it quite possible 

 that the conditions on the moon are similar. If so, 

 an impact-flash bright enough to be seen from the earth 

 would be extremely rare, and then it would be seen 

 only if an observer with a powerful telescope hap- 

 pened to be looking at the right spot at the right 

 moment. There are also very few meteors the flash 

 of which in the atmosphere of the earth would be 

 bright enough to be seen from the moon. Some 

 furrows on the Mount Wilson lunar photographs 

 might, however, possibly be due to meteor falls. 



Andrew C. D. Cromnielin. 



Calendar Reform. 



The simplified calendar proposed by the Rev. E. 

 Fanfani and described in Nature of March 17, p. 88, 

 is apparently inspired by a sound principle, viz. to 

 make the minimum of change in existing conditions. 

 It is, however, very desirable, if the months are 

 otherwise to remain unchanged, to secure that the 

 existing inequality in the lengths of the half-years 

 and quarters should be corrected. 



The late Prof. Millosevitch, of Rome, with whom 

 I corresponded on the subject, expressed the view 

 that this was the greatest — indeed, in his opinion, the 

 only great — defect of the present calendar. This 

 object can be effected by taking a day from August 

 and adding it to February— a change which was sug- 

 gested in Nature of F"ebruary 23, 191 1, although its 

 value was not, I think, fully appreciated at the time. 

 This change can be made without altering the date 

 of the vernal equinox (as fixed bv the Papal Bull of 

 February 23, 1582) by adding the day taken from 

 August to the February of the following year. This 

 alteration has the important advantage of giving us 

 four quarters each containing three months and 

 (the 365th and 366th days being apart from the week) 

 exactly thirteen weeks. A common measure for the 

 relation of monthly and weekly values would thus 

 be available — a matter of much importance in 

 accounting. 



As regards the exact relation to be established 

 between month-day and week-day, if, as M. Fanfant 

 proposes, the leap day is to be left in its present 

 position, which is in several respects desirable,, 

 facilities should be provided for terminating a 

 quarterly period at the end of February. This is best 

 accomplished by beginning with a Sunday on 

 December i. That would be the permanent date of 

 Advent Sunday — the true beginning of the ecclesias- 

 tical year. The central day of the ecclesiastical year- 

 would then be May 31, which might be most appro- 

 priately selected for exclusion from the weekly series. 



Of the five (or for the next 279 years four) dates; 

 of Easter Sunday possible under such a calendar one 

 would be April 12. When Easter Sundav fell on that: 

 day Pentecost would fall on Mav 31. If Easter 

 Sundav were fixed for that day, May 31 would be the 

 annual permanent Pentecost, the founders' day of the 

 Christian Church. 



If Easter Day were allowed to oscillate over the 

 four possible Sundavs, it would he ascertained bv the 

 existing Easter tables without disturbance, and still 

 always fall during evening: moonlight. 



Ecclesiastically. I submit that these proposals are 

 equally simple with, and present superior advantages 

 to, those suggested by M. Fanfani. From the point of 

 view of legal administration, commerce, and account- 

 ing they are effective in removing the defects in the 

 working of our present calendar. 



The above changes could be introduced without any 

 disturbance or interruption in 1924-25. 



March 19. Alexr. Philip. 



