How Did We Know Where We Were While Sailing in the Middle of the Atlantic Ocean?

We were surrounded by big swells of blue water, the horizon stretching out in all directions unbroken by land or ship or anything but the next swell. For days and days. 

There was the sun to indicated east and west, but we needed a lot more than that to find out how far we had travelled and the location of the ship. 

Every morning there was a notice put up on a central board in the Tropical Bar that gave us the key information we all wanted to know. 

There was also a chart with more detailed information

At noon, just as we all moved slowly in a lunchward direction, there was a verbal updated over the loudspeaker, alerting is to our speed (usually 7-10 knots), the size of the swells (usually 2-3 metres), the depth of the ocean (usually 5,000 metres), the air temperature and the sea temperature (usually about the same), the distance we've come since Bridgetown, Barbados, and the distance we had to go to Lisbon, Portugal (usually 300 nautical miles a day). 

And we could always go on the bridge and ask. There were no hazards or weather concerns so the crew was pretty amenable to answering any questions we had. 

Plus there were many retired sailors on board who were a fount of all knowledge nautical. 

We had a lot of cloudless skies, which meant amazing starry skies at night and we could spot the movement of the constellations and planets so easily, there being absolutely no ambient light. We could even see the reflection of Venus on the sea. 

Every few days we were told to set our clocks ahead an hour, although it was as if time stood still if we didn't have a schedule of meals or activities to help adjust. All in aid of avoiding jetlag, or shiplag I suppose it would be.