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30 which are now rising in the east, at any given hour of solar mean time, will be found setting in the west at the same hour six months hence;. while those which at any hour are now setting, will, at the same hour six months hence, be found rising, and six months before or atter it. Now, though roughly speaking, the right ascensions of the stars do not vary in themselves, it has just been shown that owing to the stars coming so much earlier each evening, a given right as- cension cannot be directly associated with any fixed hour of the day as indicated by -a solar mean time clock or chronometer ; and since sidereal chronometers or clocks are only to be found in observatories, and therefore, do not form a part of every vessel's navigating instruments, it becomes necessary in utilizing the right ascensions to make a trifling calculation, which, in its simplest form, is as follows: There are two things that we must first learn, and that is, how to find the sidereal time at Greenwich corresponding to any. given hour of mean time there, and secondly, how to find the sidereal time corresponding to any given hour at our own meridian. It is therefore obvi- ous that if we can find the first and this is very easily found, we can as easily get the latter by applying the longitude of our meridian to it by converting the lat- ter into time. Since the sidereal day is a little shorter than its counterpart in a solar day, it becomes necessary in turning' mean time into sidereal time to make some allow- ances. Table VIII, Bowditch, gives the allowances for changing sidereal to mean time, and table 1X, for changing mean to sidereal. Similar tables are to be found in the Nautical Almanac, back Bott Tables Al and II1- The Nautical Almanac gives us the sidereal time at Greenwich noon for every day in the year, and it is found by mere inspection. Hence the rule for convert- 'ing Greenwich mean time into Green- wich sidereal time is this: Add to Greenwich mean time the Greenwich sidereal time for the preceding noon, and the allowances given in Table III Greenwich mean time<...: 7a. 25m. 00S:. p.m. Sidereal time at Greenwich preceding': noon: > ):.2%%. 4. 17h Sinn. 52S. pean, From Table III Nautical Almanac, corr.' for. '7h. Ge a ele oils Soa sia dm, 13s. p, m: 24h. 58m. 05s. More than 25 hours (SUD) eae ee wre ek 24h. 00m. 00s. Equals Sidereal time at GHeen WIC 2 ie Gs. ss 58m. 05s. Ay for the number of hours, minutes and seconds in the Greenwich mean time. If the sun is more than 24 hours, sub- 'ttact 24 hours from it, because at the 'end of 24 hours sidereal time begins over again. TAE MarRINE REVIEW Example: Required Greenwich sidereal time when the Greenwich mean time as shown by the chronometer, was 7 hrs. 25. mins. p: m: on Dee. 15;. 1903. Now, what we want to know is how to find the sidereal time for the meridian we are on corresponding to any hour of mean time. The rule for finding sidereal - time at ship or right ascension of the meridian, which is the same thing, when longitude is known, is this:, Find the mean time at ship by applying your longi- tude turned into time to the time shown by chronometer, which always shows Greenwich time, provided it has been regulated to the time of that meridian. This performed add to mean time at ship the Greenwich sidereal time for the preceding noon and the allowance for Greenwich mean time by Table III of Nautical Almanac. If the result is' over 24 hours, subtract 24 hours from it. Example: Required the sidereal time at ship for Oct. 6, 1903, when the Green- wich mean time was Ithrs. 15min. p. m. Long. 85° W, equals 5 hrs. 40 mins., or right ascension of the meridian. Greenwich: mean" times. 45.26% 1ih. 15m. 00s. ong. west Gab.) hic. a iciss ass 5740 Mean (time: at ships. e050 c. 5 35 Sidereal time for preceding MOOD ee tes TA eee hee 12755 53 Allowance for 11h. 15m. from Mable, Wises cee ie oe ae 1, 51 Sidereal time at. SHIP. 2 as. vs eee 18h. 32m. 44s. Note.--We subtracted our longitude in time from Greenwich time, since the time at Geenwich is faster than any time west of it. When it is noon at Green- wich here on the lakes it is about 6 a. m. If the longitude had been east we would have added it to the Greenwich time, because the time in east longitude is in advance of the time at Greenwich. Now, supposing we desired to find the sidereal time on board of a lake vessel, where it is unnecessary to keep a time piece regulated to Greenwich time. Turn your standard time as shown by your watch into mean time by allowing the correction for longitude, that is, the dif- ference between your longitude and the longitude of the meridian your standard time is based upon. Having done this, turn your longitude into time and add it to your mean time and the answer will be the mean time at Greenwich for the same moment of your mean time at ship. Example: On Lake Superior (near Whitefish Pt.) in Long. 85° W, at 9 p. m. central standard time, desire to know the time at Greenwich corresponding thereto: Central Standard time..... 9h. 00m. 00s. p. m. Corr. "for" Long. (diff.:" be- tween 90° and 85°.== 5° and 5° equals 20m), sub- PACT titieere vob aselare tee culos 20m. Mean time at ship for the longitudes at. 366i. i aes 8h. 40m. p.m. Longitude 85° 'equals 5h. 40m. (add) ..-seeesees 5 40 Greenwich mean time......14h. 20m. p. m. More than 12 hours sub- ELACE eccis ole tierce hoc 1 2h. 20m. a. m. of the following day. Note.--Civil date (the date and time used in the business transactions of the world) commences at midnight and ends the following midnight, comprising two 12-hour intervals, one a. m., the other p. a Explanation of the foregoing example: We subtracted the correction of longi- tude of 20 minutes from the standard meridian, since a longitude east of the standard meridian has faster time than the time west of it; therefore, the time shown by watch being the mean time of the goth meridian, must be slow of the actual time of the meridian on which the ship is located. We added the longitude of ship converted into time to the mean time at ship to get Greenwich time, because the meridian of Greenwich is east of us, and therefore has earlier time, or 8:40 p. m. at Greenwich had taken place 5 hours and 40 minutes be- fore. Since the civil day begins at mid- night and ends at midnight, 8:40 p. m. lacks 3 hours 20 minutes of a finished day, hence 5 hrs. 40 mins. added to it must bring it into 2 hrs. 20 mins. of the next day Ga. mv.). Now, if we desired to get the sidereal time at ship in the above example we would proceed as already explained. All this data is given in the Nautical Almanac for Greenwich noon, therefore, if the time at Greenwich is a. m. the sid- ereal time at Greenwich for the preced- ing noon will be taken out for the day before, but if the time at Greenwich is p. m., the preceding noon will be of the same date, as your date. See Astronom- ical Time. The Right Ascensions and Declina- tions of all the stars available for the navigator are to be found in the back part of the Nautical Almanac, in the star tables. Those marked + are North, those -- are South declinations. The next thing to know is how to find out what star you can use at a particular hour. The rule is simple: We learned that the right ascension of the sun for any date or time is simply its distance in time from the point in the vernal equinox, and that the right ascension of any star is. its sidereal time of passing the meri- dian, hence, its distance in time from this point in the vernal equinox. It must be plain then, that the difference between the sun's right ascension and the star's right ascension must be equal to our own solar time. Therefore, we get this rule: To find the mean time of any star's passage over your meridian, subtract the sun's right ascension from that of the