v The Earth a Planet 69 



rises vanish from sight like tapers when an electric arc is 

 turned on. The altitude of the Sun at noon, observed at 

 any place, varies throughout the year, increasing day by 

 day until a certain maximum is reached, and then decreasing 

 gradually to a minimum. The period from highest Sun to 

 highest Sun, as observed in regions outside the tropics, 

 is about 365 days. The angular diameter of the Sun 

 when measured daily is found to gradually increase 

 from a minimum of 31' 32" to a maximum of 32' 36", and 

 then to diminish again to its former value, and this change 

 also takes place in about 365 days. Unless with the aid 

 of a very powerful telescope we cannot see the constellations 

 in daylight so as to be able to tell amongst what group of 

 stars the Sun appears at noon ; but we know that these 

 stars are just opposite those which cross the meridian at 

 midnight. In the course of 365 days all the constellations 

 of the star-dome successively cross the meridian at midnight, 

 and from this fact we know that the Sun, like the Moon, 

 moves amongst the stars from west to east, although in a 

 year instead of a month. 



1 06. Problem of the Earth and Sun. The most 

 natural explanation of the Sun's annual path amongst the 

 stars is that the Sun, like the Moon, revolves round the 

 Earth, but in a year instead of in a month. Another 

 hypothesis, that the Earth revolves round the Sun, would 

 also explain the facts. In Fig. 15 both hypotheses are illus- 

 trated. S represents the sun, E the earth, the arrow ESN 

 shows where the Sun appears amongst the stars at noon, 

 and the arrow EM shows what stars cross the meridian at 

 midnight. The dark circle is the hypothetical orbit of the 

 Earth round the Sun, the lighter circle the hypothetical 

 orbit of the Sun round the Earth. The arena is so vast 

 that the gyrating pair of globes are practically at the same 

 distance from the amphitheatre of stars. Whether we 

 assume that the arrow ESN, passing through the Sun, 

 turns round the centre E, or that the arrow ESN, passing 

 through the Earth, turns round the centre S, the arrow 

 would point successively to the same parts of the star-dome, 

 and observation of the stars would not decide which is the 



