To revise the main facts in this section, go to the section in the Grade 10 site.
Remember the following from that section :
- For every 15˚ of longitude, the sun time changes by 1 hour.
- Port A (to the east of Port B) will be ahead of Port B in time.
- Port C (to the west of Port D) will be behind Port D in time.
- A ship crossing the International Date Line from east to west (e.g. from the Pacific coast of Canada to China) will advance her clocks 24 hours of time.
- A ship crossing the International Date Line from west to east (e.g. from New Zealand to Panama Canal) will subtract (miss) 24 hours of time.
Calculating the Expected Time of Arrival (ETA) of a ship
Example A: What is the ETA of the ship in Port Y at Port Y’s time?
| Given facts : | |
| Port of Departure | Port X (15˚W) |
| Time of Departure | 10:00 on 3 June |
| Port of Destination | Port Y (135˚E) |
| Distance to be steamed | 5040 Nautical Miles |
| Average Speed | 15 knots |
But this time of arrival at Port Y is 10:00 on 17 June at Port X’s time.
We need to convert this to ETA the local time at Port Y.
Example B: What is the ETA of the ship in Port Q at Port Q’s time?
| Given facts : | |
| Port of Departure | Port P (45˚E) |
| Time of Departure | 07:00 on 15 August |
| Port of Destination | Port Q (120˚W) |
| Distance to be steamed | 5184 Nautical Miles |
| Average Speed | 18 knots |
But this time of arrival at Port Q is 07:00 on 27 August at Port P’s time.
We need to convert this ETA to the local time at Port Q.
Example C:
This is an interesting example because it includes actual ports, actual distance and a ship crossing of the International Date Line while steaming between those ports.
| Given facts : | |
| Port of Departure | Valparaiso, Chile (Takes its time from 60oW) |
| Time of Departure | 09:30 on 25 May |
| Port of Destination | Sydney, Australia (Takes its time from 150oE) |
| Distance to be steamed | 6281 Nautical Miles |
| Average Speed | 20 knots |
Let’s go to www.sea–distances.org and follow the steps below.
Click Voyage Calculator.
Click Add Port.
Go to the box, select Chile and then select Valparaiso.
Click Add Port.
Go to the box, select Australia and then select Sydney.
Enter also the speed (20 knots).
Now change the ETD from Valparaiso to 0930 on 25 May.
A calculation will be done automatically.
Now note how many days it will take to steam from Valparaiso to Sydney.
Note also the ETA at Sydney.
BUT the calculation has not taken into account the fact that the ship has crossed the International Date Line from east to west. This means that we need to advance the time by 24 hours, i.e. add 24 hours to the given ETA at Sydney.