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32 true pole of the heavens. Since this is the case the formula for finding the lati- tude from a meridian altitude below the pole will be polar distance plus altitude equals latitude. The general formula for a meridian al- titude is latitude equals zenith distance plus or minus declination. Zenith dis- tance is the. distance, measured in de- grees, from the point precisely over the observer's head to the observed. body. Let us suppose that we and the sun are both north of the equator. If now we can aScertain exactly how far we are _ north of the sun, and how far the sun is north of the equator, we will, by adding the two measurements together, know our latitude. 5 . Zenith distance is the difference be- tween the altitude of a celestial body and -go°. You know that it is 90° from the _ horizon to the zenith. Hence, having the altitude you have only to subtract it from «901 to: find how far you.are from 'the body in. question. The following exam- ple should explain this: Lae + In Lat. 44° N, what will be the alti- tude of Dubhe on the meridian above the pole; having determined the altitude find 'the latitude by the inverse process. Declination of true pole...../........ 90° eatittide: a... Aye ee es Se 44° N Colaiinde: 2a 46° N. Weer or DiDNee wie. ey ee hoes 62° N: More than 90° subtract at 108 PPP OM ee rk ce ee ee a wihle edie , 180 Altitude of Dubhe in latitude 44°..... 72° Deities ier Sak ee wa os 90° pO aOt DD UDHE. 6k Gaia oc en cas 72°. N, Pe eee a re 18° S Dec. of "Dubhew,. . 7 sic a wee 62° N Mgt eee 44° N, Note.--Call the altitude north if the body bears north of you; then name ze- nith distance just opposite; when zenith distance and declination are of the same name add them, but when opposite names 'subtract them, the smaller from the greater. The latitude takes the name of the greater. ' Astronomical Day.--The astronomical day begins at noon on the civil day of 'the same date. It also comprises twenty- TAE MarRINE REVIEW ary 9, 2h, astronomical time. Hence, we have the following rules: To convert Civil Time into Astrono- mical Time.--lf the civil time is marked A. M.\ take one from the day and add twelve to the hours,. and the result will _ be the corresponding astronomical time; if the civil time is marked P. M., take J when west, add it to the local time, or when east, subtract it from the local time. The result will be the correspond- ing Greenwich time. The data given in the Nautical Alma- nac for Greenwich noon is based upon astronomical time. Bear this in mind. Jf at any time there should be uncer- Celestial Meridian, 0 6G UOL{ CULT © D\% en 3 Oo @ SY | Pare} ' four hours, but they are.reckoned from © to 24, and run from the noon of one day to that of the next following. Astro- nomical time as well as civil time may be either apparent or mean, according as it is reckoned from apparent noon or from mean noon. The civil day begins twelve hours be- fore the astronomical day; therefore the 'first half of the civil day corresponds to the last half of the preceding astronomi- - eal day, and the last half of the civil day coincides. with the first half. of the astronomical day of the same date. ane january 0, 2 o'clock, a; m., civ. time, is . January. -- 8, "astronomical time; -and January 9, 2 o'clock P. M.,, civil time, is also Janu- | Fe E Pre 14h, : Fouator THE CELESTIAL CONCAVE, away the designation P.._M., and the as- tronomical time will result. To convert Astronomical Time into Civil Time.--lf the astronomical time is less than twelve hours, simply write P. M. after it. If greater 'than - twelve hours, subtract twelve hours from ' it, mark the result A. M., and add one to the days. For example, October 3, 23 hours astronomical time, is October 4, Ino clock, A, M.,: civil, time. To find Greenwich Time.--Express the longitude from Greenwich in time, and tainty concerning the Greenwich date, it may be determined as follows: Express the ship's time, that is, civil (mean) time, and proceed according to one of these two rules: If in west longitude, add the longitude in time to the ship's astronomical time; their sum, if less than 24 hours, will be the Greenwich time of the same date as at'ship; but if their sum is greater than 24 hours, reject 24 hours, and put the Greenwich date one forward. But if in east longitude, the longitude in time is less than the ship's astronomical