LONGITUDE-JUPITER'S SATELLITES.] N A VIG ATION NAUTICAL ASTRONOMY. 



1111 



Obs. alt. of Pollux E. of merid. Obs. alt. moon's L.L. 



37 10' 10' 31 50 7 10" 



Index cor. - 1' 10* Index cor. + 1' 20" 



Obs. dist. farthest limb. 



103 20' 0" 

 Index cor. + 1' 30" 



The height of the eye was 18 feet, and the following par- 

 ticulars were supplied by the Nautical Almanac : namely, 



Right ascen. mean sun. Moon's semi. Hor. par. 



Jan. 9, 19h. 13m. 42 -27s. noon 14' 55 -8" 54' 47 '2* 



mid. 15' 0-2" 55' 3'G* 



The star's R.A. was 7h. 36m. 12s., and its dec. 28 22' 

 40" S. Also, the distance at Ch. was 103 8' 4", and at 

 9k, 101 37' 51". Required the longitude of the ship. 



Ans. 10 1'J' 15" E.* 



The examples now given will, wo think, sufficiently 

 illustrate the practical operations for determining the 

 longitude at sea by a lunar observation. In the Nautical 

 Almanac, the lunar distances are given for the planets 

 Mars, Venus, Jupiter, and Saturn, as well as for the 

 fixed stars near the moon's path. The calculations for 

 the true distance are, of course, the same for a planet as 

 f i >r a fixed star ; and, in deducing the time at ship from 

 the planet, we must proceed exactly as in deducing the 

 time from the moon ; that is to say, we must find the 

 planet's hour-angle, and thence, by means of its right 

 ascension, taken from the Nautical Almanac, we must 

 the right ascension of the meridian : the difference 

 between this and the right ascension of the mean sun 

 will as in last example be the sun's hour-angle with 

 meridian of the place ; that is to say, the time at 

 tliu ship. 



If either of the celastial objects observed be near the 

 horizon, where the refraction ia subject to considerable 

 irregularities, the mean refraction, which is that generally 

 employed, should be modified according to the state of 

 the atmosphere, as shown by the barometer and ther- 

 mometer. A table for properly correcting the mean 

 refraction ia to be found in every collection of nautical 

 tables. A neglect of this correction, when one of the 

 objects observed is not more than 8 or 9 degrees above 

 the horizon, may occasion an error of 1' in the lunar 

 distance ; and this error may be sufficient to introduce 

 one of upwards of 3V in in the longitude. 



LONGITUDE FROM OCCCLTATIOXS AND ECLIPSES OP 

 JUPITEE'S SATELLITES. What lias now been mentioned, 

 comprehends all the essentials for finding the latitude 



and longitude at sea. Other celestial phenomena besides 

 those here dwelt upon, may be occasionally made avail- 

 able for determining the longitude ; as, for instance; an 

 eclipse, or an occultation of a fixed star or planet by the 

 moon. Eclipses are of too infrequent occurrence to be 

 of much service to the navigator ; but the passage of the 

 moon over the stars and planets, in her path, is con- 

 tinually occurring ; and this occultation of the object by 

 the moon would furnish a very convenient means of 

 finding the Greenwich time, and thence the longitude, 

 if the motion of the ship did not, in general, preclude tho 

 possibility of keeping the telescope steadily directed to 

 the moon's edge. 



It is plain that at the instant of the occultation, that 

 is, at the instant of the disappearance of the star or 

 planet by the interposition of tho moon called the im- 

 mersion the apparent right ascension of the moon's 

 occulting limb must be the same as the right ascension 

 of the occulted star. By removing the elleet of parallax, 

 the moon's true right ascension at the instant of the 

 star's immersion may therefore be found, and the Green- 

 wich time corresponding to this right ascension may 

 thence be deduced. 



The eclipses of Jupiter's satellites answer a similar 

 purpose, since the entrance of a satellite into tho shadow 

 of the planet, is a phenomenon which takes place at tho 

 same absolute instant, wherever on tho surface of tho 

 earth the immersion be observed ; and so likewise does 

 the reappearance of the satellite, or its emersion. 



The Greenwich time, when these immersions and 

 emersions are predicted to happen, are given in the 

 Nautical Almanac ; so that if the ship time, when any 

 such phenomenon occurs, be known, the longitude may 

 be at once obtained. 



But here again, as in tho caso of occultations of tho 

 stars by tho moon, the frequent impracticability of keeping 

 a telescope sufficiently steady for the accurate observation 

 of the phenomena at sea, renders this short and other- 

 wise convenient method of finding the longitude of but 

 very limited application. In a calm sea, or in harbour, 

 a telescope of sufficient magnifying power may, of course', 

 be used without iucouvenieiice. Of tho four satellites, 

 tho Jir.it, or that which is at the least distance from the 

 planet, is the best adapted for the purpose of determin- 

 ing the longitude, on account of its mure rapid motion ; 

 it revolves round Jupiter, and is eclipsed by tho shadow 

 of the planet once in every forty-two hours : and the 

 instants of immersion and emersion are capable, in 

 genera], of being much more accurately noted, than the 

 instants of contact of the earth's shadow with the moon's 

 limb. 



CHAPTER V. 



NAUTICAL INSTRUMENTS. 



THE QUADRANT AND SEXTANT. These two instruments 

 are the same in principle both are equally employed to 

 measure angular distances ; but as the distance between 

 a celestial object and the horizon, fur the purpose of de- 

 termining the latitude at sea, ia a measurement more 

 frequently made than any other, the former of the two 

 above-mentioned instruments tho quadrant is con- 

 structed with exclusive reference to this purpose, and, 

 being less elaborate in its fittings and workmanship, is 

 by far the cheaper instrument of tho two. 



The arc of the sea quadrant is th>! eiylith part of an 

 entire circumference, or 45. This arc is therefore, 

 strictly speaking, not a quadrant, but an octant; but as 

 it is capable of measuring all altitudes from the horizon 



For more examples, see Mr. Jcan'd Nariijn'.'wn and Nautical At- 

 tnmomy, whence the above hare been taken. 



less extent than this must be estimated by the ere. 

 A skilful observer can initially uultc tuit estimate within two or three 



to the zonith as will presently bo explained a greater 

 extent of arc is unnec" 



Tho sextant is a more delicato instrument. Its arc 

 is the sixth part of an entire circumference, or GO", and 

 it is capable of measuring angular distances tip to 120. 

 The arc of the more common quadrants is divided and 

 that by rreans of an auxiliary scale attached to the index- 

 liuib into minutes only : those of a superior kind are 

 thus divided into half minutes ; but tho arc of a sextant 

 is frequently subdivided by aid of the Vernier scalo 

 just alluded to, and hereafter explained to every 

 10". f 



Artists generally extend the arc of a quadrant to a 

 few degrees beyond 45, and the arc of a sextant to a few 



seconds of the truth. Such an error, in tho measurement of a himr 

 distance, would not occasion an error of moo* than about a mile m 

 the resulting longitude. (Sue Astronomical Instruments, page 93'J, 

 et lc t .t 



