TIHE.] 



NAVIGATION NAUTICAL ASTRONOMY. 



1075 



The ecliptic is conceived to be divided into twelve equal 

 parts, called signs : a sign is therefore an arc of 30. 

 The twelve signs have the names and symbols follow- 

 in?: 



9. 



(The 



10. 



(The 



f Sagittarius 



Archer). 

 Vf Capricornus 



Goat). 



11. s: Aquarius (The Wa- 



ter-bearer). 



12. X Pisces (The Fishes). 



1 . <r Aries (The Ram). 



2. Taurus (The Bull). 



3. n Gemini (Tlie Twins). 



4. 05 Cancer (The Crab). 

 6. ft Leo (The Lion). 



6. nji Virgo (The Virgin). 



7. ^ Libra (The Balance). 



8. m. Scorpio (The Scor- 



pion), j 



The first six of these lines are to the north of the equi- 

 noctial, and the others to the south : they are also called 

 Signs of the Zodiac the name given to a belt of the 

 heavens 8 on e;ich side of the ecliptic. 



Celestial Latitude. As the ecliptic is the circle of 

 longitude, the perpendiculars to it, that is, the great 

 circles through the poles of the ecliptic, are the circles 

 of latitude. The distance of a celestial object from 

 the ecliptic, measured on one of these perpendiculars, is 

 its latitude ; it is north or south, according as the 

 object U on the north or south side of the ecliptic. 



Right Asceiusitm The right ascension of a celestial 

 object is the arc of the equinoctial intercepted between 

 the first point of Aries and the decimation circle, or 

 meridian passing the object. 



The learner will perceive that the first point of Aries 

 i- the starting-point from which both longitude and right 

 Mcension are measured ; and that, what on the terres- 

 trial globe would be longitude and latitude, on the celes- 

 tial globe are right ascension and declination the first 

 ji'.int of Aries being substituted for the meridian of 

 Greenwich. 



Great circles, all of which pass through the poles of 

 any of the more important great circles of the sphere, 

 are frequently called secondaries to the latter. This is a 

 very convenient term : thus, vertical circles are secondaries 

 to the horizon ; meridians, or declination circles, are 

 secondaries to the equinoctial ; and circles of celestial 

 latitude are secondaries to the ecliptic. 



ON TIME. The most important portion of time, in 

 matters connected with nautical astronomy, is the day 

 and its sulxlivisions. There are several kinds of day 

 referred to in astronomy ; but the period occupied by a 

 single rotation of the earth, comprises, in each case, 

 nearly the whole of the time so designated. If tiio 

 heavenly bodies were all fixed, and the earth had no pro- 

 gressive motion, but only its present diurnal rotation on 

 its axis, all days would be alike as to length, since the 

 diurnal rotation is always performed in the same time ; 

 the interval between the departure from, and the return 

 t<>, the meridian of any heavenly body would then be 

 invariably the same. But, as the earth is continually 

 shifting its place in its orbit, and that by an ainuunt 

 which is not uuiform, the interval between two succes- 

 sive passages of the sun over the meridian of any place 

 is variable. This interval is called an apparent solar 

 day. 



Apparent Time. When the sun is on the meridian of 

 any place, it is apparent noon at that place ; when it is in 

 any other position, the angle between the meridian of the 

 place and that on which the sun is, is called the /tour 

 angle from noon at that place and instant ; this angle, 

 converted into time, at the rate of 15 to an hour, is the 

 apparent time at the place. 



M'lut Time. As, on account of the inequality of the 

 earth's motion in its orbit, the solar day is continually 

 varying in length, a day that is the average, or mean, of 

 these variable days is fixed upon for civil reckoning ; and 

 it is the length of such a mean d iy that is marked out 

 by the twenty-four hours of a common clock or watch. 

 This length of time is called a mean solar day ; and any 



The whole starry heavens have, however, a slow apparent movement, 

 arising from a real motion of the earth di-tlnct from iu rotation on iu 

 ftii-. Thu motion causes the axis to describe! a minute circle round the 

 poles of toe ecliptic in about 26,000 rears; the effect U to cause the ap- 

 parent approve* of some lUis towards the pole, and the recession at 



time shown by a correct clock or watch is mean solar 

 time, or simply mean time. At certain periods of the 

 year the sun will thus arrive at the meridian before the 

 clock points to XII. ; and at other periods the clock will 

 be in advance of the sun : the interval between the 

 arrival of the index of the clock to XII., and of the sun 

 to the meridian, is called the equation, of time. It is ' 

 given for every day in the year, at page 1 of the Nautical 

 Almanac, for the meridian of Greenwich ; that is to say, 

 when it is apparent noon at Greenwich, on any day of 

 the year, the almanac shows the time to be added or 

 subtracted to obtain the corresponding mean time at that 

 meridian. 



Sidereal Time. A sidereal day is the time occupied by 

 one complete rotation of the earth on its axis. This 

 interval is ascertained by observing the time elapsed 

 between two successive passages of the same fixed star 

 over the meridian. Such is the immense distance of the 

 stars, that the earth's change of place, from day to day, 

 produces not the slightest effect upon their apparent 

 positions, which are preserved the same as if the earth 

 were at rest Whatever star be observed, and whatever 

 be the place of the earth in its orbit, it is uniformly 

 found that the interval of two successive passages of the 

 star over the meridian is invariable namely, 23k. oG'4-O'J" 

 of mean time.* 



Besides the three kinds of day here described, there is 

 also the lunar day, which is the interval between two 

 successive passages or transits of the moon over the 

 meridian ; the average length of it is 24h. 64m. But 

 navigators have nothing to do with lunar time ; what 

 they are most concerned with are apparent time and 

 mean time the time that would be shown by a properly 

 constructed sun-dial, and the time shown by a well- 

 regulated chronometer. The time determined by obser- 

 vations at sea, which is in general deduced from the sun's 

 hour-angle with the meridian of the place, is, of course, 

 apparent time. It is turned into mean time by help of 

 the table of the equation of time at page 1 of the 

 Nautical Almanac the phenomena, predicted in that 

 important publication, for the use of seamen in finding 

 the latitude and longitude at sea, being recorded in muau 

 time, just like the transactions of common life. 



But there is this difference between the civil and the 

 astronomical mode of reckoning : the civil day reckons 

 from twelve o'clock, at midnight, and the whole twenty- 

 four hours is divided into two sets of twelve, the count- 

 ing recommencing at twelve o'clock, noon ; but astro- 

 nomers commence their day at noon, and count on 

 through the twenty. four hours, from hours up to 24 

 hours, when another day begins. Consequently, the 

 common or civil reckoning is always twelve hours in ad- 

 vance of the astronomical reckoning, both reckonings 

 being in reference to mean time : so that, to deduce the 

 civil from thu astronomical time at any instant, we have 

 only to add twelve hours to the latter. For instance, 

 Jan. 1, loh. 35m., astronomical time, is Jan. 2, 3h. 35ui , 

 in the morning, civil time. 



It is indispensably necessary that the learner have 

 clear conceptions of apparent and mean time : the former j 

 is at once ascertained by observation of the true sun's i 

 hour-angle from the meridian ; the latter is not pointed 

 out by nature, but is arbitrarily chosen for practical 

 convenience : its measure is not ascertained immediately 

 from observation, but computed from the actual phe- 

 nomena. Astronomers conceive an imaginary sun, called 

 the mean sun, to move uniformly in the equinoctial, and 

 with a notion in right ascension exactly equal to the 

 real sun's mean or average motion in right ascension, so 

 that the interval between two consecutive transits of the 

 mean sun is a mean solar day the mean, that is, of all 

 the variable solar days of the year of the true sun. It 

 is the motion of this imaginary sun that is measured by 

 a chronometer ; it completes every revolution in exactly 

 twenty-four common hours ; the twenty-four hours oom- 



others. Thus, the polo-star, as it is called, has, for many centuries, 

 been getting nearer to tile pole ; it if now about 1 34' from it : the Blur 

 will continue iu approach till within about 30', and will then recede. 

 The physical cause ol the phenomena ha* been noticed in treating of the 

 PrcceMion and N illation, in the previous section oil Astronomy. 



