458 ARCHITECHNICS. 



3. Astronomical Navigation, or Nautical Astronomy, relates 

 to the finding of a ship's position, by means of celestial observations ; 

 whether of the sun, moon, planets, or fixed stars. It consists of two 

 principal operations ; finding the latitude, and finding the longitude : 

 as by these data the ship's place is defined on the earth's surface. 

 The instruments chiefly used in taking angles for these purposes, are 

 the reflecting quadrant, and sextant, and the repeating circle ; all 

 of which have a moveable arm, with an attached, or revolving mir- 

 ror, which is turned till the image of one of the objects, seen by a 

 second reflection from a fixed mirror, appears close by the side of 

 the other object seen directly ; when the vernier, on the arm, gives 

 the angle between them, read off on the graduated limb. 



The latitude, is most commonly found by observing the meridian 

 altitude of the sun's lower limb ; adding thereto the sun's apparent 

 semidiameter, for the altitude of his centre ; subtracting from this the 

 dip of the horizon, which, at sea, appears lower than its true place ; 

 and subtracting also the refraction, produced by the atmosphere. 

 Knowing, thus, the sun's altitude, and finding his declination by 

 means of the tables, we find, by addition or substraction, the altitude 

 of the equator, which is the complement of the latitude sought, or 

 the difference between it and 90. In the same manner we may 

 find the latitude by means of the moon, or a star ; but in the case of 

 the moon, we must allow for the parallax in altitude, to obtain the 

 direction in which the moon would appear if seen from the earth's 

 centre. 



The longitude, is found by means of the chronometer ; which 

 may either be used as a simple timepiece, for noting the local time 

 of any celestial phenomenon ; or, if well regulated, it may be re- 

 garded as showing the time at Greenwich, by comparing which with 

 the local time, the difference of longitude is found ; allowing 15 to 

 an hour. In the former of these two uses, if an eclipse of one of 

 Jupiter's satellites happens at 4 o'clock, P. M., by the ship's local 

 time, when, as found by the Nautical Almanac, it happens precisely 

 at noon, at Greenwich, then the ship must be 60 east of Green- 

 wich, or in 60 of E. longitude. This difference of time is more 

 commonly found, by observing the angle between the moon and 

 a star, and noting exactly the local time ; then finding, by the Lunar 

 tables, the exact time at Greenwich when the moon and star make 

 the same angle ; correcting it in both cases for the parallax. In 

 this way, the chronometer itself may be verified at frequent inter- 

 vals, by means of Lunar Observations: and thus, wherever the 

 ship may be, the actual time at Greenwich may always be known ; 

 and thence the longitude may be found, whenever the local time can 

 be found, by celestial observations. 



