Your vessel will sail from a position in LAT 8° 51.0' N, LONG 81° 31.0' W to a position at LAT 33° 51.5' S, LONG 151° 13.0' E. The distance by great circle is 7,635 miles, and you estimate an average speed of 15.0 knots. What is your estimated zone time of arrival if you depart at 1510 ZT, on 23 July?
• Great-circle distance and time calculation using speed in knots (nautical miles per hour) • Difference between Greenwich Mean Time (GMT/UTC) and Zone Time (ZT) and how to apply zone descriptions (ZD) • How longitude and date change when crossing the 180° meridian (International Date Line) going west to east
• First, compute how many hours the voyage will take from the given great-circle distance and speed. Then convert that into days and hours. How many whole days plus extra hours is it? • Determine the departure longitude’s zone description and the arrival longitude’s zone description. Is ZD positive or negative in each case, and by how many hours? • Once you have the ETA in GMT/UTC, convert it to the arrival port’s zone time, then carefully adjust the date when you cross the International Date Line going from west longitude to east longitude.
• Confirm your use of the speed formula: Speed = Distance / Time, rearranged correctly to solve for Time with units in hours and days • Verify you are using the correct sign convention for ZD: longitudes west are +ZD, longitudes east are −ZD when converting between GMT and ZT • Double-check how many calendar days you add to the departure date when you add the passage time, and whether crossing the 180° meridian advances or retards your date for eastbound travel
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