Sir Frank Dyson 83 



that if ship-masters would observe these occupations 

 they could make certain of the exact time and there- 

 fore of their longitude. In practice this method failed to 

 work because the rolling deck of a small ship makes a 

 very poor observation platform, and also because the 

 disappearance of one of these moons does not happen 

 instantaneously, but takes some time. 



Longitude may be expressed as the difference between 

 the local time of the place where the observation takes 

 place and the local time of the place chosen as the stan- 

 dard meridian, or longitude nought, which is Greenwich. 

 That, you may say, is simple enough. Why not carry a 

 good watch ? Quite so, but please remember that there 

 were no time-keepers in those days except pendulum 

 clocks, and these, of course, could not be trusted on board 

 ship. There were no chronometers. 



About the middle of the seventeenth century a new 

 idea was mooted. The moon moves regularly and quickly 

 among the stars, and it was suggested that if a table were 

 drawn up of its distance from a number of fixed stars at 

 definite periods for a long time in advance, this would be 

 a good guide for the navigator. 



This plan came to the ears of Charles II, who was 

 extremely interested in scientific matters, and he at once 

 desired some of the leading scientific men of the time to 

 examine it and see if it were practicable. The Reverend 

 John Flamsteed was selected to inquire into it, and 

 presently reported that the scheme was a good one, but 

 that at present there was no table of the fixed stars 

 sufficiently reliable for the purpose. Whereupon the King 

 appointed Flamsteed his Astronomer Royal and ordered 

 the building of Greenwich Observatory. 



Now we must say a little about Flamsteed. He was a 

 Derbyshire boy, born in 1646, and was educated at the 

 free school in Derby. He was always weak and sickly, so 

 that even " one day's short reading caused him a desperate 



