1843.] A perpetual Moon Table. 109 



It is not distinctly stated by Barlow, whether the era given by him 

 begins at noon or midnight, but as the French Astronomers about that 

 time attempted to introduce civil reckoning, and as the supposition of 

 this appears to agree pretty well with some old Tables, like Ferguson's, 

 which I have lately got hold of, I adopt it, subject to any correction 

 which better authority may hereafter shew to be requisite. 



Generally the marks on the card are put on the left of the numbers 

 to which they belong. On the outer card Leap years have two marks, 

 that beside which the number is written answering for the months 

 from March onwards, and the other answering for January and 

 February. 



The use of this Table is very much like that of the Table for week 

 days. The full century mark on the middle card is to be set to that 

 of the # $ on the outer. The given month-mark on the inner card is 

 then to be brought into line with the mark of the current year on the 

 outer : when opposite the days of the month will be seen those of the 

 Moon's age. Opposite ® will be the day of New, and opposite Q the 

 day of full Moon ; and opposite ]) and ([ the days of the first and 

 last quarters. 



The times thus found are of course the mean civil times at Green- 

 wich ; but there is no difficulty in reading the Table in astronomical 

 time, as any person will perceive. 



In order that the Table may serve for dates before the Christian era, 

 it is to be observed that the year 1 B. C. may be considered as the 

 year O of the Christian era, or the hundredth of the century — 1. 

 Hence this rule. Add 1 to the given century B. C. and reckon it — ; 

 then to the complement to 100 of the odd year add 1 and reckon 

 that as the current year of the century. In this way the year 721 

 B. C. is the 80th of the century — 8 ; and may conveniently be written 

 8 80, the mark — applying to the full century only, like the negative 

 index of a logarithm. 



Robert Shortrede. 

 November, 1841. 



The present Table in conjunction with the one for week days will 

 give Easter for ever with less trouble than by any other method at 

 present known. All that nee^be done is, to set the Table to March 

 in any given year, and if Full Moon falls after the 21st, find by the 



