ASTRONOMY. 



917 



Southern signs, being south of the equinoctial. 



Autumnal Signs. 

 Libra, the Balance, Sept 23. 

 Scorpio, the Scorpio, Oct. 



23. 

 Sagittarius, the Archer, 



Nov. 22. 



Winter Signs. 

 Capricornus, the Goat, Dec. 



21. 

 Aquarius,the Water-bearer, 



Jan. 20. 

 Pisces, the Fishes, Feb. 19. 



The Colures are two great circles passing through the 

 poles of the heavens, dividing the ecliptic into four equal 

 parts, and marking the seasons of the year. 



One passes through the equinoctial points, Aries and 

 Libra, and is therefore called the equinoctial colure. 

 When the sun is in either of these points, the days and 

 nights on every part of the globe are equal to each other. 

 The other passes through the solstitial points, Cancer 

 and Capricorn, which mark the sun's greatest declination, 

 north and south of the equator, and is thence called the 

 solstitial colure. When the sun is in or near these 

 points, his meridian altitude undergoes scarcely any sen- 

 sible variation for several days ; and hence the term sol- 

 stitial applied to them. 



The Horizon is a great circle, whose plane passing 

 through the centre of the earth, and extended to the 

 sphere of the fixed stars, divides the heavens into two 

 hemispheres, of which the upper is the visible, and the 

 lower the invisible hemisphere. This is the rational or 

 true horizon, which determines the rising and setting of 

 the sun, planets, and stars. It is represented by the 

 wooden horizon of the artificial globe. The sensible or 

 apparent horizon is the circle which bounds our view, 

 where the land, or water, and sky, seem to touch each 

 other ; more or less extensive according to the position 

 of an observer. 



The sensible horizon of a person changes as he moves, 

 and, in an open country, enlarges or contracts as his sta- 

 tion is high or low. Standing on a plain, the eye having 

 an elevation of 5 feet above the surface, the radius of 

 the sensible horizon will be less than 2J miles. At an 

 elevation of 6 feet it will be just 3 miles. 



Rule^ to find the distance when the height is known 

 Increase the height in feet one half, and extract the 

 square root for the distance in miles. 



Thus, in the preceding case the eye is supposed to 

 have an elevation of 6 feet above the surface of a plain, 

 and 6 with its half is 9, the square root of which is 3 ; 

 that gives the distance in miles which a person will 

 be able to see in a right line upon that surface. 



Again a tower, 32 yards above the level of the ocean, 

 may be seen along that level from a distance of 12 miles. 

 For 32 yards = 96 feet, increased one half = 144, the 

 square root of which is 12. 



The poles of the horizon are the zenith and the nadir. 

 The zenith is the point in the heavens which is directly 

 over our heads ; the nadir, that which is exactly under 

 our feet. The zenith to us is the nadir to our antipodes, 

 and the nadir to us is their zenith. Circles drawn 

 through the zenith and nadir of any place, cutting the 

 horizon at right angles, are called azimuth or vertical 

 circles ; and that which passes through the east and west 

 points of the horizon, is the prime vertical. 



Meridians are imaginary great circles passing through 

 the terrestrial and celestial poles, cutting the equator 

 and equinoctial at right angles. 



A meridian is supposed to pass through every place 

 on the earth, and every point in the heavens ; but only 

 24 are drawn on the globes through every 15 of the 

 equator and equinoctial, including altogether 360". 

 These meridians mark the space which, in consequence 

 of the earth's diurnal rotation, the heavenly bodies 

 appear to describe every hour through the 24 in the day. 

 They are sometimes called, therefore, hour or horary 

 circles. As 15 answer to an hour, 1 answers to four 

 minutes of time, i to two minutes, and J to one minute. 



Longitude on the earth is distance east or west from a 

 fixed meridian measured on the equator. The Fortunate 

 Islands, supposed to be the Canaries, supplied the 

 ancients with their first meridian. The western ex- 

 tremity of Africa, as then known, was taken by Abulfeda, 



Fig 2. 



the Arabian geographer. The meridian of Terceira was 

 used by the Spanish and Portuguese in the sixteenth 

 century ; and that of Ferro by all nations in the seven- 

 teenth and eighteenth centuries. We now adopt the 

 meridian of the Greenwich, and the French that of the 

 Paris, observatories. 



Longitude in the heavens, is distance east from the 

 great meridian which passes through the first point of 

 Aries, or the equinoctial colure, measured on the ecliptic. 

 Right ascension, is distance east from the same me- 

 ridian measured on the equinoctial. 



Terrestrial longitude being reckoned in two directions 

 from a fixed point, east and west, can only extend to 

 180- Celestial longitude, and right ascension, are only 

 reckoned in one direction, east from the prime meridian, 

 and may, therefore, extend to 360. 



Parallels of Latitude are small circles supposed to 

 be drawn on the surface of the earth, north and south of 

 the equator, and parallel to it, dividing the globe into 

 two unequal parts. Let us suppose A (Fig. 2) to be 

 placed under considera- 

 tion, and P, E, Q, E' its 

 meridian, E E' the line 

 intersecting the equator, 

 and P Q the line of the 

 poles, it is here the aro 

 A E, or, which is the 

 same thing, the angle 

 A, O, E, which represents 

 the latitude sought. P, 

 O, E being a right angle, 

 the latitude is the com- 

 plement of the angle A, 

 O,P; but the angle A, O, 

 P, is only another thing 

 for the zenith distance 

 Z, A, H, of the pole of 

 the celestial sphere. For 

 to an observer placed at 

 A, P Q is a parallel to 

 the earth's axis ; the latitude of the point A then is the 

 complement of the zenith distance from the pole at that 

 point, to the height P', A, H, of the pole above the hori- 

 zon, A H, being equal to the distance Z, A, P'. We can, 

 therefore, say that the latitude of a place is equal to the 

 height of the pole above the horizon of that place. 

 Parallels of declination are such circles produced in the 

 heavens, north and south of the equinoctial, and parallel 

 to it. 



Latitude on the earth is the distance of a place from 

 the equator, measured on a meridian, north or south. 



Declin-ition is the distance of the heavenly bodies 

 from the equinoctial, measured on a meridian, north or 

 south. 



Latitude in the heavens is distance from the ecliptic, at 

 a right angle, north or south. 



Terrestrial latitude and declination may extend to 90. 

 The sun has no declination when in the equinoctial. His 

 greatest declination is 23i north or south. He has no 

 latitude, being always in the ecliptic. The greatest de- 

 clination of a plauet is 30^, -and latitude 8 north or 

 south, with the exception of the asteroids. It is more 

 convenient to describe the position of the heavenly 

 bodies by their declination and right ascension, than by 

 their latitjde and longitude, the former corresponding to 

 terrestrial latitude and longitude. 



The Tropic of Cancer is a small circle 23 north of the 

 equator, and parallel to it ; and the tropic of Capricorn 

 is a similar circle, at the same distance, on the south. 

 The polar circles are also small circles, each 66^ from the 

 equator, and at the same distance from the poles as the 

 tropics are from the equator. 



The tropics on the celestial sphere, mark the limits of 

 the sun's farthest declination, north and south. 



The tropics on the terrestrial sphere, divide the torrid 

 from the two temperate zones, and the polar circles 

 divide the temperate from the two frigid zones. 



Zones. Twice in the year the sun is vertical to those 

 who dwell in the torrid zone. Consequently, at noon 



