Given are the courses and speeds of 4 vessels. The navigator of which vessel would be required to know the actual time of meridian transit in order to take an accurate observation at LAN?
• Local Apparent Noon (LAN) and what meridian transit means for the Sun • Effect of a vessel’s course relative to meridians (north-south) vs. parallels (east-west) on longitude during the forenoon • Why a change in longitude during the LAN period matters for the accuracy of a Sun sight
• For which course will the vessel’s longitude be changing the most near the time of LAN, and why? • Which direction of travel (mainly east-west or mainly north-south) makes the timing of meridian transit most critical for an accurate LAN observation? • How does higher speed on an east-west course affect the rate of change of longitude and therefore the need to know the exact time the Sun crosses your meridian?
• Compare which courses are predominantly east-west vs. north-south • Consider how speed combined with an east-west heading changes longitude per hour • Ask: on which vessel would an error of a few minutes in meridian transit time cause the greatest error in assumed longitude during the LAN sight?
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