Mapping the Earth
Second, to understand how sundials work, we must know how the Earth is mapped. Since ancient times, maps have been made by using simple "tic-tac-toe"-like grid lines. Grid lines allow us to easily locate points of interest. The entire Earth is mapped by "casting" a large, imaginary "net" of grid lines over the Earth. But because the Earth is a sphere, these map lines are actually curves, and curves are measured in degrees of a circle (also called angular distance). A circle or sphere is divided into 360 segments or degrees. The convention of dividing a circle into 360 degrees probably has its origins in ancient Babylonian calendars that used lunar cycles to mark the seasons, since there are 360 days in a lunar year (a lunar year is 12 cycles of New Moon to Full Moon phases of the Moon).
Grid lines that run east-west across the Earth, but divide north and south, are called latitude lines or parallels. The Equator is at 0° latitude and divides Earth into two hemispheres (north and south). Other important latitude lines are the Tropic of Cancer (23.5° N), Tropic of Capricorn (23.5° S), Arctic Circle (66.5° N), and Antarctic Circle (66.5° N). The United States-Canada border is at 40° North Latitude (the "40th Parallel").
Lines that run north and south across the Earth, from the North Pole to the South Pole, but divide east and west, are called longitude lines or meridians . The Prime Meridian* runs through Greenwich, England, and divides the Earth into eastern and western hemispheres. The International Date Line is on the opposite side of the Prime Meridian, at 180° E/W longitude (but it varies), and marks where the day changes. Each degree (°) of latitude and longitude is further divided into 60 minutes ('), and each minute of latitude and longitude is divided into 60 seconds ("). Both parallels and meridians are important in telling time, but meridians are especially important.
*Unlike the Equator, which naturally divides the Earth into northern and southern halves, there is no natural east-west divide. The location of the Prime Meridian at Greenwich, England, was decided in by international agreement in October of 1884 at the International Meridian Conference, convened in Washington, D.C. It was agreed to replace the several meridians then in use by a single, world meridian. Any principal meridian must extend from the North- to the South-Pole, but pass through one more point.