THE THEORY TAKING THE SIGHT Tom Cunliffe（看见汤姆坎里夫的理论）.pdf

18 CELESTIAL NAVIGATION Example 2 altitude. Whatever you do, don’t start to rack the What time is local noon in DR longitude 73°E on Sun up again as it begins to fall. Wait until the 1st May? lower limb bites positively into the horizon without altering the sextant again, and you Mer Pas at Greenwich 11h 57 m know you have it. Noon is past and gone for 73E E = minus 4 h 52 m another day. Note the log; go below, read the Local noon 07 h 05 m GMT sextant, put it away, then work out your latitude. If you happen to be sailing around within a few THE THEORY degrees of the Date Line on the opposite side of the globe from the Greenwich Meridian, a query The illustration on page 17 demonstrates the may arise as to which day it is. If this is so, refer noon sight set-up when viewed from the celestial to Chapter 4, page 24. If not, just remember elevated pole. that you have calculated the Greenwich time The picture below shows it as seen from the of noon for your approximate longitude, and celestial equator. Note how the celestial horizon read on. makes a right angle with the line dropped from the observer’s zenith, through his geographic position to the centre of the Earth. Zenith TAKING THE SIGHT Distance (ZD) is the only new concept to grab