1 8 LIGHT SCIENCE FOR LEISURE HOURS 



In applying this method to the sun, a serious diffi- 

 culty comes in. He is so far off that, instead of a 

 triangle with a respectable vertical angle, there is a 

 triangle having a vertical angle of about the 240th 

 part of a degree (under the most favourable conditions 

 which can be conveniently obtained). To know how 

 small such an angle is, let the reader note the minute 

 hand of a clock or watch, and observe how little it 

 shifts around its centre in a single second of time ; yet 

 this angular shift is twenty-four times as great as that 

 we have mentioned. 



It must not be forgotten that, in all such cases, the 

 question is not whether the astronomer can recognise 

 such and such an effect, but whether he can measure 

 it. It is not the whole quantity about which astrono- 

 mers are troubled. Unquestionably the observer at 

 Greenwich can recognise the depression of the mid-day 

 sun, 1 due to the fact that Greenwich lies above (or 

 north of) the earth's centre. For this depression is an 

 element which he has to take into account in his obser- 

 vations. The corresponding depression, even in the 

 case of bodies far more distant than the sun, as the 

 planets Jupiter and Saturn, is announced systematically 

 in our national astronomical almanac. But the direct 

 measurement of the depression is altogether out of the 

 question. 



If the stars which really bestrew the heavens beyond 



1 Only observations of the mid-day sun would avail, because the only 

 instruments having the requisite delicacy of adjustment are meridional. 

 There is an instrument suitable for observing the moon when she is not 

 on the meridian ; but it is quite unfit for the purpose we are considering. 



