Questions

In each circle, starting at the  top left segment ,numbers increase, as you move clockwise,by 2 for the upper left circle,3 for the upper right, 4 for the lower right and 5 for the lower left

The Length of the Day is determined in Astronomical Units.

The length of a day corresponds to the time the Earth takes to turn around on its axis, which is approximately 24 hours. But it's not exactly 24 hours. 

This phenomenon is due to tides raised by the Moon which slow Earth's rotation. Because of the way the second is defined, the mean length of a day is now about 86 400.002 seconds, and is increasing by about 1.7 milliseconds per century.

The sheet number on the map is found in bold print in both the upper right and lower left areas of the margin.