The Foreword is by astronaut Neil Armstrong. As a child, he and his neighbors kept track of time largely by the bells that rang from the church clock tower. Decades later, he visited England’s Greenwich Observatory, which features the first chronometers—clocks that solved the problem of knowing one’s location at sea. He notes that the prime meridian, the north-south line that divides the Eastern and Western Hemispheres, runs through Greenwich. This makes Greenwich Mean Time the standard for determining when each day on Earth officially begins.

As a child, the author was fascinated by the horizontal lines of latitude and the vertical lines of longitude that overlie the Earth on maps. In 150 CE, Ptolemy placed the Equator at zero degrees latitude so that it girdles the planet around its waist; he put the prime meridian of longitude at the Canary Islands. This north-south line has since moved several times and now runs through Greenwich in London: “The placement of the prime meridian is a purely political decision” (4).

The Earth spins through 360 degrees, and each hour it moves 15 degrees. At sea, when the sun reaches its highest point in the sky, sailors set their ship’s clock to local noon time and then consult a second clock with the time back at the home port.