SUNRISE AND SUNSET. 



of TRUE SUNRISE or SUNSET, and for a long time this was the 

 time of sunrise and sunset given in the almanacks. 



The moment at which the centre of the sun's disc, seen as it 

 is through the atmosphere, is at s, fig. 1, is called the moment of 

 APPARENT SUNRISE or SUNSET, and is the time now given in the 

 almanacks. 



56. As we have already stated, the apparent altitude of objects 

 on or very near the horizon is greater than their true altitude by 

 more than half a degree. But the apparent diameter of the sun 

 being itself about half a degree, it follows that the sun is elevated 

 by the optical effect of the air to an altitude greater than its 

 real altitude by more than its own apparent diameter. 



If then we take a point s', at a height above s, fig. 1, equal to 

 that by which the atmosphere augments the apparent altitude, 

 this height, s' s, will be greater than the apparent diameter of 

 the sun, and when the real centre of the sun's disc is at s, it will 

 appear to be at s', and the disc of the sun, instead of being seen 

 at s, the horizon dividing it into two equal parts, will, in fact, 

 be seen at s', not only quite clear of the horizon, but with its 

 lowest part more than a quarter of a degree above the horizon. 



Let us take another case which is still more curious. Let the 

 true position of the sun's disc, that is, the position it would have 

 if there were no atmosphere, be that shown at s, in fig. 2, being 

 that which it has the moment before it begins to rise, or the 

 moment after it has completely set. In this position the disc 

 just touches the horizon, and the depression of the centre of the 

 disc below the horizon is a quarter of a degree. Now what is the 

 effect of the atmosphere ? to make the centre of the disc 

 appear to be more than half a degree higher, and consequently 

 more than a quarter of a degree above the horizon. The disc, 

 therefore, which is really altogether below the horizon, is in this 

 case seen in fact altogether above it as shown at s', fig. 2. 



57. The terms SUNRISE and SUNSET are commonly used, as 

 indeed most other terms are, in a loose and vague sense. The 

 sun may be said to be in the act of rising from the moment at 

 which the highest point of its disc begins to be seen until its lowest 

 point just touches the horizon ; that is, from the moment it has 

 the position s, fig. 3, until it has attained the position s'. 

 In the same manner it may be said to be in the act of setting 

 from the moment it has the position s', until it has sunk to the 

 position s. 



But in order to give a definite signification to the terms SUN- 

 RISE and SUNSET, it has been agreed to apply them to the 

 moment at which the centre of the sun's disc is on the horizon, 

 as it is shown at s, fig. 1. Thus the conventional moment of 



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