1 SECTION II. R23 2d, Though the latest and best astronomers have found that the sun is fixt in or near the centre of our world, and that the earth moves round its own axisonce in twenty-four hours with a circular motion, and round the sun once in a year witha progres- sive motion ; yet to make these things more easy and intelligible to those that are unskilful, we shall here suppose the sun to move round the earth, both with a daily and yearly motion, as it ap- pears to our senses; viz. daily going round the earth, and yet every day changing its place a little in the heavens, till In a year's time it returns to the same place again. SECT. II. Of the greater Circles. THE greater circles are such as divide the globe into two equal parts, and are these four; (viz.) the horizon, the meridian, the equator, and the ecliptic. I. The horizon is a broad flat circle, or the wooden frame in which the globe stands: Its upper edge divides theglobe into the upper and lower halves or hemispheres, and represents the line or circle which divides between theupper and the lower parts of the earth and heavens, andwhich is called the horizon. This circle determines the rising or setting of sun or stars, and distin- guishes day and night. When the sun is in the east part of the horizon, it is rising. When in the west part, it is setting. When it is above the hori- zon, it is day ; When below it is night. Yet till the sun be 18 degrees below the horizon it is usually called twilight ; because the sun -beams shooting upward are re- flected down to us by the atmosphere after sun-set or before sun- rise : And it is upon this account that in our horizon at Lon- don, there is no perfect night in the very middle of summer for two months together, because the sun is not 18 degrees below the horizon. The horizon is distinguished into the sensible and the ra- tional, See Fig. 1. The sensible horizon supposes the spectator placed on s the surface of the earth or water, and it reaches as far as the eye can see. But the rational or true horizon supposes the spectator pla- ced in the centre of the earth c, and thus divides the globes both of the heavens and the earth into halves. Suppose in Figure 1. the circle s d p e is the earth, u b h n r g the heavens, b s g the line making the sensible horizon, h r the rational horizon. The sensible horizon on the earth or sea includes a s o, and it reaches but a veryfew miles ; for if a man of sixfeet high stood on a large plain, or on the surface of the sea, at s, he could not see the sea itself, or the land, further than three miles round: Thus it appears that the sensible horizon on the earthor
RkJQdWJsaXNoZXIy OTcyMjk=