103 



A perpetual Moon Table, By Capt. Robt. Shortrede, 1st Assist. 

 G. T. S. ; F. R. A. S. $c. With plates. 



I now send an account of a Table, which I have constructed for 

 finding the Moon's age for any date, past or future, somewhat similar 

 to that lately published for finding the week days. 



If the Moon's course were completed in an even number of days, 

 and with a uniform motion, its age and the time of any particular 

 phase would be found as readily and surely as the day of the week ; 

 but as neither of these conditions holds good, and as two lunations are 

 scarcely ever completed in equal times, if we wish to be correct, we 

 must either make a calculation at length, or avail ourselves of those 

 already made, or else use some other method by which correctness is 

 made to give way to convenience. 



The rule commonly given in books of Astronomy for finding the 

 Moon's age is by no means very simple, as it requires us first to know 

 the Golden Number and the Epact. The rule for finding the Golden 

 Number is tolerably simple, as also is that for the Epact at present, but 

 for this, after 1900, a new rule is necessary, which as given in Bar- 

 low's Dictionary, runs thus : ' Divide the centuries of the given year 

 by 4, multiply the remainder by 17; then to this product add 43 

 times the quotient, and also the number 86, and divide the whole 

 sum by 25, reserving the quotient: next multiply the Golden Number 

 by 1 1, and from the product subtract the reserved quotient, so shall the 

 remainder after rejecting all the 30's contained in it, be the Epact 

 sought.' This rule is such that few persons will be inclined to use it, 

 except in cases of urgent necessity, and even by means of it the Moon's 

 age may fall on the wrong day, as no account is taken of the great 

 Equations depending on excentricity, which may amount at a maximum 

 to 14 hours on either side of the mean time given by the Tables. 



The Table now given shews at once, without calculation, and with 

 scarcely any trouble, the mean times of New and Full Moon, &c, as also 

 the Moon's age to the nearest day, and by means of another similar 

 card for the two principal corrections, the true times of New and Full 

 Moon may be found within an hour or so of the results, which would 

 be found by a detailed calculation. 



