Heale. — On Moon Occultations. 59 



moon covers any poiut on the earth at the time the spectator is there, the star 

 will not be visible to him, or it will be occulted, and it is obvious that the 

 moment when the moon's image on the drawing just touches the part where 

 the spectator is at the moment, on either side, will be the time at which the 

 disappearance or occultation and egress will occur. 



To construct such a diagram as this does not require any considerable 

 calculations, but it is tedious, and, in practice, a pretty expert computer and 

 draftsman would hardly compute it in less than an hour, which is a good deal 

 of labour to expend on a mere preliminary, which may have to be many times 

 repeated before one is come to which proves to be available for observation. 

 A rather large book by Mr. F. C. Penrose, containing an elaborate method 

 of shortening the labour of this graphical process, was published in England 

 three or four years ago. In it there are diagrams ready made, upon which the 

 elements of an occultation, as taken from the nautical almanac, may be laid 

 down, the reduction necessary being made by means of a slide rule. I hardly 

 think that this method will be much used. It is possible that I may be so 

 wedded to methods to which I am accustomed that I do not readily take to 

 other ones ; but, to me, it appears quite as troublesome as the ordinary plan 

 as given in Loomis and many other astronomical books, to which, after giving 

 every attention to Mr Penrose's method, I have found it most convenient to 

 adhere ; but, in practice, I have adopted some mechanical aids which, without 

 in any degree altering or modifying the plan, assist so largely in carrying it 

 out that I now find that by their use I can predict at least four in the time it 

 used to take me to project one, and without any sensible diminution in 

 accuracy, the result being, thab when an occultation occurs while the moon is 

 within an hour or two of the meridian, the pirediction is true witliin a limit 

 of about two minutes, the possible error increasing to about double that 

 quantity when the moon is four hours from the meridian. 



In laying down a diagram by any process it is necessary to lay down in 

 their true relative values the magnitude of the earth, or, at least, of the ellipse 

 into which the observer's latitude-pai-allel is projected ; of the different hour 

 spaces upon it ; and of the moon and her position and motions. The ready 

 way of doing this is either to take the moon's horizontal parallax in seconds, 

 and to adopt that on a suitable scale as the earth's radius, in which case the 

 values of the hour intervals and of the observer's distance from where the star 

 is vertical must all be reduced to that I'adius ; or an arbitrary radius, as 1,000, 

 may be used, and the value of the hour intervals in the observer's latitude 

 may then be laid off one for all, the same diagrams being used, for an indefinite 

 number of predictions ; but then all the other quantities must be reduced to 

 that radius, and in either case the ellipses into which the observer's latitude- 

 parallel is projected must be set out fur each occultation or eclipse predicted. 



