History of Astronomy [CH. I. 



With these facts it is convenient to begin, taking them in 

 the order in which they most readily present themselves to 

 any ordinary observer. 



3. The sun is daily seen to rise in the eastern part of 

 the sky, to travel across the sky, to reach its highest position 

 in the south in the middle of the day, then to sink, and 

 finally to set in the western part of the sky. But its daily 

 path across the sky is not always the same : the points of 

 the horizon at which it rises and sets, its height in the sky 

 at midday, and the time from sunrise to sunset, all go 

 through a series of changes, which are accompanied by 

 changes in the weather, in vegetation, etc.; and we are 

 thus able to recognise the existence of the seasons, and 

 their recurrence after a certain interval of time which is 

 known as a year. 



4. But while the sun always appears as a bright circular 

 disc, the next most conspicuous of the heavenly bodies, the 

 moon, undergoes changes of form which readily strike the 

 observer, and are at once seen to take place in a regular order 

 and at about the same intervals of time. A little more care, 

 however, is necessary in order to observe the connection 

 between the form of the moon and her position in the sky 

 with respect to the sun. Thus when the moon is first 

 visible soon after sunset near the place where the sun has set, 

 her form is a thin crescent (cf. fig. u on p. 31), the hollow 

 side being turned away from the sun, and she sets soon 

 after the sun. Next night the moon is farther fr.om the 

 sun, the crescent is thicker, and she sets later ; and so on, 

 until after rather less than a week from the first appearance 

 of the crescent, she appears as a semicircular disc, with 

 the flat side turned away from the sun. The semicircle 

 enlarges, and after another week has grown into a complete 

 disc ; the moon is now nearly in the opposite direction to 

 the sun, and therefore rises about at sunset and sets about 

 at sunrise. She then begins to approach the sun on the 

 other side, rising before it and setting in the daytime ; 

 her size again diminishes, until after another week she is 

 again semicircular, the flat side being still turned away 

 from the sun, but being now turned towards the west 

 instead of towards the east. The semicircle then becomes 

 a gradually diminishing crescent, and the time of rising 



