224 GEOGRAPHY AND ASTRONOMY. sea, a s o, differs very much from the extent of the real or ra- tional horizon, d s e. But as to the heavens where the fixt stars are, the sensible horizon b u g scarce differs at all from the rational horizon Is u r : For the eye placed in the centre of the earth e, or on the surface of it s, would find no evident difference in the horizon of the fixt stars, because they are at so immense a distance, that in comparison thereof half the diameter of the earth, that is s c or g r the distancebetween the surface and the centre is of no consideration. But let it be observedhere, that the planets are much nearer to the earth than the fixt stars are: And therefore half the dia- meter of the earth, that is, s e or g r is of some consideration in the horizon of the planets. It may not therefore be impioper to note in this place, that suppose a planet to be at g, if the eye of the spectator were op the surface of the earth at s, he would behold it as level with the horizon : But if his eye were at the centre of the earth at e, he would behold it raised several degrees or minutes above the horizon, even the quantity of the angle g e r, or (which is all one) s g c. Now, the difference between the place wherea planet appears to a spectator, placed on the centre of the earth, and to a specta- tor placed on the surface, is called the parallaxof that planet at that time z and therefore the difference between those two places g and r, or rather the quantity of the angle g c r, or s g c, is called its hórizontal parallax. And this is of great use to adjust the real distances, and consequently the real magnitudes of the several planets. But this doctrine of parcrli`axes belongs rather to the second or special part of astronomy. H. The meridian is a great brazen circle in which the globe moves ; it crosses the horizon at right angles, and divides the globes into the eastern and western hemispheres. It represents that line or circle in the heaven which passes just over our head, and cutting the horizon in the north and south points of it, comes just under our feet on the opposite side of the globe. This circle shews when the sun or stars are just at north or south and determines noon or midnight. When the sun is on the meridian and above the horizon to us in Great Britain, it is just in the south, and it is noon. When it is on the meridian and under the horizon, it is just in the north and it is midnight. Note, Whensoever we move on the earth, whether east, west, north, or south, we change our horizon both sensible and rational; for every motion or change of place gives us a hemis- phere of sky or heaven over our heart a littledifferent from what it was ; and wecan see less on one side of the globe of the earth and more on the other side. , Whensoever we move toward the east or west we charge
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