WCP1393

[1]

16, Earlsfort Terrace, Dublin.

Nov[ember].19.1903

Dear Sir

In glancing over your book after writing last night I found there were a few points which I had omitted to notice[.]

You apparently accept Schiaparelli's¹ estimate of the total number of stars down to the 6th magnitude as 4303 (p. 48) and also Newcomb's² estimate of 7647 (p[.] 150). I fancy the truth lies about half-way between. (I do not know what however how with this high estimate of the stars up to map. 6 Newcomb works out his [1 word illeg.] of 3.5 for subsequent magnitudes. I would think it too high even if he had started with Schiaparelli's³ estimate instead of his own).

[2]

Some of the results I believe are taken from Argelander's⁴ Durchmusterung in which, as he did not use any figure below mag 9, a good many stars returned at that figure were really nearer to map. 10. (I do not believe that any star Catalogue which professes to give the total number of stars up to a given magnitude can be relied on as regards the last half-magnitude in the collection. The compiler passes over all stars that are ranked lower in some other Catalogue that he has chiefly relied on and would find on closer investigation that several of these had been ranked too [1 word illeg.] low).

P.188 you refer to the determination of the sun's distance by observations on Mars or Eros as bearing out our conclusions as to the velocity of light. I [3] think you will find that no computation of the sun's distance based on observations of Eros have^s yet yet been published. In fact I have seen numerous complaints of delay in publication.

With regard to run-away stars, I am not much of a mathematician but it occurs to me that if these velocities were impressed on these stars by the star-system to which they belong (and I see no reason for holding the contrary) the mutual attraction of the various members of a star-system cannot result in the expulsion of one member from it, which would require a repulsion instead of an attraction. So ^Therefore the system must be sufficiently extensive to admit of these velocities. But what are they? This for the most part depends on unreliable parallaxes. I don't think we have yet got a velocity of 100 [1 word illeg.] miles per second in the line of sight and part of this may be due to the sun's motion.

Looking at Newcomb's Astronomy For Everybody which was sent to me instead of The Stars I find that he thinks the short rotation period of Venus most [4] probable while he regards Mercury's rotation period as altogether uncertain. I think with several of the planets we are not in a position to say with any degree of probability whether they are habitable or not.

I remain | Sincerely yours | W H S Monck [signature]