

NAVIGATION NAUTICAL ASTRONOMY. 



oWrration worn the centre of the earth. As to the 

 stars, it u found that observations on them, though 

 wade at the turfaoe, would require no moditication if 

 made at the centre the radius of the earth being a mere 

 point in comparison to tlu-ir immense distance. We 

 >hall now define the principal circles of the celestial 

 sphere. 



The axis of the heaven* in the diameter of the 

 celestial sphere, about which the apparent diurnal revo- 

 lution of the celestial sphere takes place, and which, as 

 we have seen, U due to the real rotation of the earth. 

 The axis of the heavens is, therefore, only the axis of the 

 earth prolonged ; and tho extremities of this axis of 

 course the imaginary extremities are the poles of the 

 heavens. 



K'l<nnoct\al. Tho celestial great circle, to the plane of 



which the axis of the heavens is perpendicular, in called 



'i.ii---ti'i'. <>r the celestial equator. It is traced out 



merely by extending the plane of the terrestrial equator 



to the heavens. 



Meridians. The celestial meridians are, in like manner, 

 marked out by extending the planes of the terrestrial 

 meridians ; or they are semicircles terminating in the 

 poles of the heavens, and perpendicular to the equi- 

 noctial. 



X'Htth, Nadir. The zenith is that point in il.e heavens 

 which is directly over the head of the spectator ; or, 

 if a straight line be drawn from the centre of the earth 

 to any spot on its surface, and then prolonged to the 

 heavens, the point on the celestial sphere which it would 

 mark out in the zenith of that spot. The name line con- 

 tinued in the contrary direction would mark a point in 

 the celestial sphere called the nadir. These two points, 

 therefore, in reference to any place on the earth, are at 

 the extremities of that diameter of the celestial sphere 

 which U perpendicular to tho plane of the horizon of 

 that place : that is, they are the voles of the horizon. 



Vertical Circlet. Vertical circles of any place are 

 those which pass through the zenith and nadir of that 

 place ; they are all perpendicular to the horizon of that 

 place. They are hence, also, called circles of altitude. 



The altitude of a celestial body is its distance above 

 the horizon, measured on the vertical circle passing 

 through the body. The complement of the altitude in 

 the zenith distance. In the case of the sun and moon, 

 the tnte altitude is measured from the rational horizon, 

 and is a little greater thau the altitude measured from 

 the sensible horizon. In the case of the stars, as ob- 

 served above, the difference in altitude is insensible, 

 whichever horizon be referred to. 



The most important of all the vertical circles of any 

 place is the mtri'li'iu. When a celestial object is on the 

 meridian, its altitude is the greatest which that object 

 can possibly have ; it is called the meridian-altitude of 

 the object 



The vertical circle which cuts the meridian at right 

 angles, and which, therefore, passes through the east and 

 west points of the horizon, is distinguished next to 

 the meridian. It is called the prime vertical. When a 

 celestial object arrives at the prime vertical, it is either 

 due east or due west. 



Azimuth The azimuth of a celestial object is the arc 

 if the horizon, comprehended between the meridian of 

 the observer and the vertical circle passing through the 

 object. The arc of the horizon here spoken of is, of 

 online, the measure of the angle at the zenith, between 

 the meridian and the vertical, through the object. Ver- 

 tical circles are, sometimes, called azimuth circles. 



Amplitude,- Amplitude is also an arc of tho horizon. 

 It U the arc comprised between the east point of the 

 horizon and the point where tho body rises, or between 

 the west point and where it sets ; the former arc is 

 ailed the ruing amplitude of the body, and the latter 

 iU setting amplitude. Azimuth is measured either from 

 the north or south points of the horizon ; amplitude 

 ither from the east or west. When we speak of the 

 azimuth of a body, we refer merely to the azimuth of 

 UM vertical on which the body U, whatever its altitude 

 an that vertical niy bo ; wbeu we speak of its amplitude, 



we refer exclusively to its position with respect to the 

 east or west point <>f the horizon at rising or setting. 



Declination. The >!>< iiMthni of a celestial object is 

 its distance from tin- equim* -ti.il, measured on the 

 celestial meridian paasing through it, and is either north 

 or south. \Vlmt u latitude as respects a point on tho 

 earth, is declination in reference to a point in the 

 heavens. Celestial meridians are thus, sometimes, called 

 circles of declination ; and what are parallels of latitude 

 on the earth, become parallels of declination ou the 

 celestial sphere. 



The distance of an object from the elevated pole is the 

 polar distance of it. It is the complement of the decli- 

 nation when the elevated pole and the object are both on 

 the same side of the equinoctial ; but when they ape on 

 contrary sides the polar distance is the declination phi* 

 90. The elevation of the pole above the rational horizon 

 of any place is always equal to the latitude of that place, 

 for tin- latitude is equal to the distance of the zenith of 

 the place from the equinoctial ; the distance between 

 the zenith and the elevated polo is, therefore, the com- 

 plement of the latitude, and it is equally the complement 

 of the elevation of the pole above the rational horizon: 

 this elevation, therefore, is equal to the latitude of the 

 place. Consequently, the depression of the equator 

 below the horizon, or its elevation above the horizon, in 

 tho opposite quarter, is the complement of the latitude, 

 or, which is the same thing, the latitude is the measure 

 of the angle which the horizon makes with the equator. 



The celestial circles now defined, have especial re- 

 ference to the earth. The meridian and the equinoctial 

 are merely extensions to the heavens of corresponding 

 circles on the earth ; and the vertical circles, or perpen- 

 diculars to the horizon, are imagined for the purpose of 

 recording altitudes above the horizon, measured on the 

 earth. Jiut there are some circles peculiar to the celestial 

 sphere ; the principal of these are the ecliptic, or tho 

 circle of celestial longitude and the perpendiculars to it 

 the circles of celestial latitude. 



The Ecliptic. The ecliptic is the great circle described 

 on the celestial sphere by the sun in its apparent annual 

 motion about the earth : in reality, it is the path of tho 

 earth about the sun in the contrary direction ; but, as 

 already remarked, we are in this subject only concerned 

 with tho appearances. The ecliptic crosses the equinoc- 

 tial at an angle subject to continuous but very small 

 variation, determinable by observation. It is always 

 given, with the utmost attainable accuracy, in tho 

 Nautifal Almanac. The obliquity at present is about 

 23 274'. 



The two points whore the ecliptic crosses the equinoc- 

 tial are called the equinoctial points. The sun, in its 

 apparent annual course, passes through these points about 

 the 21st of March and the 23rd of September ; the 

 former being the time of the vernal equinox, and tho 

 latter of the autumnal equinox : these names being given 

 because the night is then equal to the day at all places 

 where the sun rises and sets. This is obvious, because 

 any point in the equinoctial, by the diurnal rotation of 

 the earth or the apparent rotation of the heavens is 

 just as long below the horizon of any place as it is 

 above it. 



Celestial Longitude. The circle on which the longi- 

 tude of any heavenly body is measured is the eclipt ><, 

 not the equinoctial ; and as terrestrial longitude is 

 measured from a fixed point of tho equator, the point 

 (with us) where the meridian of Greenwich crosses it, so 

 celestial longitude is measured from a fixed point in the 

 ecliptic namely, the vernal equinoctial point, which is 

 called the first point of the constellation Aries. 



As respects terrestrial longitude, tho fixed point from 

 which the reckoning commences is only fixed for parti- 

 cular nations, each kingdom choosing its own : this is 

 some inconvenience. But, as respects celestial longitude, 

 there is perfect uniformity of reckoning among astro- 

 nomers ; and this reckoning unlike that for terrestrial 

 longitude is carried on in one direction round the celes. 

 ; "hero ; so that a body may have any longitude 

 short of 360". 



