FINDING THE WAY AT SEA. 731 



rapidity to act as a time indicator for terrestrial voyagers. It is hardly 

 necessary to explain why rapidity of motion is important ; hut the 

 following illustration may be given for the purpose. The hour-hand 

 of a clock does in reality indicate the minute as well as the hour ; 

 yet, owing to the slowness of its motion, we regard the hour-hand as 

 an unsatisfactory time-indicator, and only consider it as showing what 

 hour is in progress. So with the more slowly-moving celestial bodies. 

 They would serve well enough, at least some among them would, to 

 show the day of the year, if we could only imagine that such informa- 

 tion were ever required from celestial bodies. But it would be hope- 

 less to attempt to ascertain the true time with any degree of accuracy 

 from their motions. Now, the moon really moves with considerable 

 rapidity among the stars. 1 She completes the circuit of the celestial 

 sphere in 27 days (a period less than the common lunation), so that 

 in one day she traverses about 13, or her own diameter (which is 

 rather more than half a degree), in about an hour. This, astronomi- 

 cally speaking, is very rapid motion ; and, as it can be detected in a 

 few seconds by telescopic comparison of the moon's place with that 

 of some fixed star, it serves to show the time within a few seconds, 

 which is precisely what is required by the seaman. Theoretically, all 

 he has to do is, to take the moon's apparent distance from a known 

 star, and also her height and the star's height above the horizon. 

 Thence he can calculate Avhat would be the moon's distance from the 

 star at the moment of observation, if the observer were at the earth's 

 centre. But the Nautical Almanac informs him of the precise instant 

 of Greenwich time corresponding to this calculated distance. So he 

 has, what he requires, the true Greenwich time. 



It will be manifest that all methods of finding the way at sea, 

 except the rough processes depending on the log and compass, re- 

 quire that the celestial bodies, or some of them, should be seen. 

 Hence it is that cloudy weather, for any considerable length of time, 

 occasions danger, and sometimes leads to shipwreck and loss of life. 

 Of course the captain of a ship proceeds with extreme caution when 

 the weather has long been cloudy, especially if, according to his reck- 

 oning, he is drawing near shore. Then the lead comes into play, that 

 by soundings, if possible, the approach to shore may be indicated. 



schel's more matured views, his well-known " Outlines of Astronomy : " " For what 

 purpose are we to suppose such magnificent bodies scattered through the abyss of 

 space ? Surely not to illuminate our nights, which an additional moon of the thousandth 

 part of the size of our own world would do much better ; nor to sparkle as a pageant 

 void of meaning and reality, and bewilder us among vain conjectures. Useful, it is 

 true, they are to man as points of exact and permanent reference, but he must have 

 studied astronomy to little purpose, who can suppose man to be the only object of his 

 Creator's care ; or who does not see, in the vast and wonderful apparatus around us, 

 provision for other races of animated beings." 



1 It was this doubtless which led to the distinction recognized in the book of Job, 

 where the moon is described as " walking in brightness." 



