LATITUDE BY MERIDIAN ALTITUDE OF THE SUN. TO FIND THE GREENWICH TIME.-Begin by writing down the year, month, and day, oh om os. Turn the Long. into time, and place it below the oh om os. West Long., add; East Long., subtract. The Result is the Greenwich Apparent Time; mark it G.A.T. for brevity. TO FIND THE CORRECT DECLINATION.—This may be done by proportional logs., or by the hourly difference. Ist. By Proportional Logs.-From the Nautical Almanac at the given month, and page 1 of that month, take out the sun's declination for the Greenwich day and day following. If the declinations are of the same names, subtract; If the declinations are of different names, add. The sum or difference is the daily variation, and below it write the hours and minutes of the Greenwich time. From Table XXXIII., take out the Prop. logs of the D. Var. and G.A.T., and add these logs. together; the sum is the Prop. log. of the correction of the declination of the Greenwich day. Declination increasing, add (always); Declination decreasing, subtract (always). Note.--If the declinations are of different names (which occurs in March and September), the correction is always subtractive, but if more than the Dec. for Greenwich day, subtract Dec. from correction, and change the name of the result. 2nd. By the Hourly difference.-Take out the declination for the Greenwich day nd the hourly diff., which multiply by the hours and the tenths of hours in the Greenwich time, taking care to mark off from the right hand of the product as many decimal figures as there are decimal figures in the hourly diff. and Greenwich time. Divide by 60 if necessary; the result is the correction of the declination. Declination increasing, add (always); Declination decreasing, subtract (always). Note. The declination in all other problems is corrected by one of the above rules; therefore, in the instructions for working those problems, reference to these rules will be made instead of reprinting them for each. TO FIND THE TRUE ALTITUDE. Write down the observed altitude and correct it in the following order : INDEX ERROR.—If + add, if — subtract. DIP.-Table V., always subtract. REFRACTION.-Table IV., always subtract. SEMIDIAMETER.-Nautical Almanac, Page II., for Greenwich day, if Lower limb, add; if Upper limb, subtract. PARALLAX.-Table VI., always add. The result is the true altitude of the sun. Note. This method of obtaining the true altitude of the sun must be followed in the other problems where it is required. TO FIND THE ZENITH DISTANCE.-Subtract the true altitude from 90°, and mark the remainder of an opposite name to the bearing given in the question. TO FIND THE LATITUDE.- Below the Z.D. put the corrected declination. Z. Dist. and Dec. same name, add; Z. Dist. and Dec. diff. names, subtract. The sum or remainder is the Latitude, which is always of the same name as the greater of the Z. Dist. and Dec. EXAMPLE I. 1885.-September 22nd. In Longitude 169° 30′ W. The observed meridian altitude of the sun's lower limb 40° 1′ 20′′ bearing South. Index error 50" to add. Height of eye 20 feet. Required the Latitude. (Declination corrected by Proportional Logs.) The observed meridian Index error +2′; (Declination corrected by 1885. June 27th.-In Longitude 151° 45′ E. altitude of the sun's lower limb 61° 2′ 10′′ bearing South. height of eye 17 feet. Required the Latitude. Hourly Difference.) h. m. s. 1885, June 27th. O O o Long. 151° 45′ E. Long... Obs. alt... 61° 2′ 10′′ +2 O 4 G. A. T., June 26th 13 53 O 6,0)60,7 0 Dip... 61 4 10 -3 57 |