WCP1392

[1]¹

16, Earlsfort Terrace, Dublin.

Nov[ember].18.1903

Dear Sir

I am obliged for yours, I have not got a copy of Dr. Newcomb's² book on The Stars though I ordered it some days ago from my book-seller. But I think there is still some mistake about the proper motions. The number set down as having a proper motion of 10'' per century is much too small. It is about the number with a proper motion of half-a-second per year or 50'' per century. There are over 250 of these stars in Bossert's³ Catalogue of 1890 and others have since been discovered.

I do not think the great majority of the parallaxes at all reliable but there is another objection to using them big that the results so far attained are incomplete. [2] Assuming that the parallaxes are correct, the list is no doubt deficient and much more so at the lower end than at the upper. If a list of parallaxes thus made out seems to indicate a gap at a particular distance this affords very little evidence of a real gap gap. If we ascertained the parallaxes of all stars where the figure exceeded 0.01'' we might arrive at very different results. As far as I know neither the magnitudes of the stars nor their proper motions show any trace of a gap between the solar cluster and the Galactic Ring and I think the parallactic evidence is on two grounds insufficient. The gap may exist, but I do not think there is any evidence of it.

You mention the agreement of the period of [1 word illeg.] rotation of Venus Venus's rotation as proved to ^be equal to that of its revolution. This I think is one of the most disputed of astronomical questions. The fact is that ^the markings on Venus are so badly defined that there is great difficulty in fixing its pe [3] riod of rotation. The case I believe is still worse with Mercury. In fact the only three planets (other than the earth) with respect to ^{whose rotation} which we have any certainty are Mars, Jupiter and Saturn.

In one passage I think you give the mass of the moon as 1/50 of that of the earth and in another as 1/80. I do not know ^see that this affects the argument. The latter figure is I believe correct.

With regard to relative parallaxes I believe comparison stars are sometimes used whose [1 word illeg.] proper motion is unknown. In fact there are not enough faint stars with known proper motion near enough for the purpose. But then proper motions are assumed to be very small and if this were not the case the fact would probably come to light in the course of the observations and computations. Suppose the real parallax to be 0.10'' and that one of the comparison stars had a real parallax of 0.05'' while that of the others was unsensible it would be found that this star gave a different relative parallax from the others. When [4] the agreement among us all is pretty close however an average of the lot ought to come near the truth. But there are instances in which after several ^fairly accordant determinations of parallax a new one gave a totally different result. Of course an error of 0.01'' in measuring an angle may utterly derange the result when the real parallax is 0.001''.

I do not regard Sir R Ball's⁴ theory of meteorites as proved but I think that there are strong reasons for regarding them (I mean those which fall to the earth in the solid state) as members of the solar system and not visitors from space — in fact as very minute asteroids. They seem to be moving in elliptic orbits not very [1 word illeg.] or very oblique to the eclipse and with direct motion round the sun. Small relative velocity is in fact necessary to prevent them from being dissipated high up in the air. I do not believe that we shall ever catch a Leonid, a Perseid, or a Lyrid and analyse it in a laboratory. They are too swift for that.

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

[5]

P.S. Is not the density which you assign to Saturn too small? As regards Uranus and Neptune while there is no doubt that their densities are low then diameters are not sufficiently certain to enable exact figures to be given. The latest measurement that I saw increases the previous figure for Uranus's diameter and diminishes it for Neptune's. This would involve an opposite change in their assumed densities. It is very difficult to measure such small object with the necessary accuracy.