DAY AND NIGHT THE SEASONS 79 



that, at the moment when the sun once more casts its shortest 

 shadow, almost precisely twenty-four hours have elapsed since 

 the last observation. Here, then, is a measure of time ready 

 for use, and if we were shipwrecked sailors we should know 

 that it was dinner-time as soon as the shadow came back to 

 yesterday's mark. Similarly, by noticing that the shadow grows 

 gradually shorter up to noon, and then lengthens out in precisely 

 the same fashion as it shortened, it becomes clear to us that 

 we could make on the ground a series of marks which would 

 help us to divide our days equally, and make sure that we had 

 our meals at the same time each day. It seems a sound principle 

 to look at the movements of the earth in this way in their practical 

 bearings on daily life, before attempting any wider consideration 

 of them. 



If the class manifests interest in the subject, and shows some 

 skill in the manipulations involved, it might be well to make 

 similar observations in November, if the weather conditions 

 permit. Suppose that in the beginning of that month we are 

 fortunate enough to be able to make shadow observations at 

 noon, we shall find that, as compared with our June observation, 

 noon, as shown by the shortest shadow, occurs sixteen minutes 

 earlier than it did in June. 



This is because the equation of time is large in November, 

 a correction of sixteen minutes having to be applied to convert 

 apparent noon into mean noon. 



Without going further into these difficult points, it may be 

 worth while to suggest to intelligent pupils that while the sun 

 is our great timekeeper, he has the disadvantage of not keeping 

 perfect time, moving sometimes faster and sometimes slower. 

 We have therefore to invent a theoretical sun, which keeps 

 perfect time, and that is one of the reasons why in large towns 

 Greenwich time is telegraphed from headquarters, instead of 

 every town simply making observations of the sun for itself. 



The questions connected with time and longitude deserve 

 fuller consideration, for nearly all pupils find them difficult. 

 It is important to keep as close as possible to the practical side 

 of the matter, and illustrations from continental or Irish time- 

 tables should be employed freely. 



