The cover picture depicts the AIS system. AIS is an automatic tracking system for boats. It is compulsory for big ships and all passenger ships and optional on other boats. To be more accurate, the picture depicts the satellite based AIS (S-AIS) - the original version worked by sending messages between ships and between ships and land-based systems.
The signal from the S-AIS equipment on each boat is sent via satellite, and makes its way to a database. For North America that database is PostgreSQL. See AIS Database on the website of NOAA - the National Oceanic and Atmospheric Administration of the U.S. Department of Commerce. The data can then be used to show not only where each ship is, but also the track it took to get there.
The free service - MarineTraffic shows you the live system, worldwide. It stores half a billion ship positions per day and is accessed by 6 million users every month. This is a big data system.
A relational database is the ideal tool to manage this data and it is an example of just one of the unlimited number of interesting systems that we can build with relational databases.
The cover also reflects my fondness for classic sailing boats.