268 
rOPULAR SCIENCE REVIEW. 
time her angular distance from a near star be known beforehand. 
Then if an observer at sea measure the distance minute by 
minute till he makes an observation which shows the distance * 
equal to the given one, he knows the Greenwich time for the 
moment of that observation ; and this time compared with 
the local or ship’s time of the observation gives, by mere 
difference, his longitude reckoned, in time, from Greenwich. 
If the distance of moon from stars be given at fixed intervals of 
Greenwich time, say hourly,! then it matters not when the 
navigator takes his distance, for he can always find, by inter- 
polation, the instant of Greenwich time corresponding to the 
moment of his observation. 
Now, to make this method practical two things are necessary. 
First, the positions of the fixed stars must be exactly known ; 
and second, the moon's place hour by hour must be accurately 
predictable a long time in advance, so that the mariner may 
carry the table of predicted Greenwich distances out with him. 
The plan, as we have said, was proposed more than three cen- 
turies ago, and the need of applying it was severely felt just 
two centuries ago. But there were no catalogues of fixed-star 
plans accurate enough for the purpose ; and the knowledge of 
the moon’s motions was utterly insufficient also. The double 
want was brought to the King’s (Charles II.’s) notice, and he at 
once ordered an observer to be appointed and an observatory to 
be founded to meet it. The right man for the post turned up 
in the person of Flamsteed, and he was commanded forthwith 
to “ apply himself with the utmost care and diligence to the 
rectifying the tables of the motions of the heavens and the 
places of the fixed stars, so as to find out the so- much-desired 
longitude of places for the perfecting the art of navigation.”' 
The same object is set forth in the tablet which still stands over 
what was the entrance-door of the building ; and a wording 
almost identical with that just quoted has been maintained in 
the warrants of all succeeding Astronomers Royal to the present 
day. 
It is desirable that this definition and its implied limitation 
of the duties of the Observatory should be borne in mind. 
There is no provision in its charter for the numerous modern 
subjects of inquiry which have been classed under the head of 
astronomical physics. It is true such subjects have been 
occasionally followed up, but only to an extent that precluded 
* Certain corrections are necessary to reduce the angular distance as 
measured at the earth’s surface to what it would be if measured from the 
earth’s centre ; but th^se form part of the after calculation. 
t As a matter of fact, they are given for every third hour in our 
u Nautical Almanacs.” 
