Noon is defined as when the sun is highest in the sky. It is not defined as when the sun is directly overhead, as many people believe. The sun, in fact, never passes directly overhead in South Carolina, or anywhere else in the United States, except Hawaii. In those regions between the Tropic of Cancer and the Tropic of Capricorn, the sun does pass directly overhead, but on only two days in the year.
Noon, then is an easy point in time to determine. So it serves as a fundamental reference for measuring time.
However, when it's noon here, in Florence, SC, it's not yet noon in Columbia, SC. Noon occurs even later in Atlanta, GA, and hours later in California.
In earlier times, each town or community set its time by the sun. That meant, of course, that each town had a slightly different time. If you visited another town, you would have to change your watch to match. In days when it took all day to travel a hundred miles, this was no big problem. But then came travel by railroads.
On a train you could travel a hundred miles in just a few hours. Now this difference from town to town did become a problem. The railroads were concerned with how to make out their timetables. If you put each departure and arrival time according to the specific town's time, it was very confusing to everybody.
So, in the 1800s, the railroads suggested that the United States should be divided into four time zones, each fifteen degrees wide. Then everyone in a particular time zone would set his or her clocks to the same time, regardless of the sun's actual position in the sky. People in the adjacent time zone would also set their clocks to the same time, but one hour different from their neighboring time zones.
The idea was quickly accepted and the United States was divided up into, from east to west, the Eastern, Central, Mountain, and Pacific Time Zones. Now, when it's noon in the Eastern zone, it's 11 am in the Central, 10 am in the Mountain, and 9 am in the Pacific zone.
The actual time zone boundaries are not neat, straight lines, though. They are shifted various ways for social and political reasons. In some cases they are adjusted so all of a state falls in the same zone. However, there are several states that are split. Tennessee, for instance, is partly in the Eastern zone, and partly in the Central zone.
This worked so well that the entire world soon established time zones. These actual time zone boundaries are also very irregular, with some countries choosing to be in a different zone from what you would expect.
The Prime Meridian is a north-south line that passes through the observatory at Greenwich, England. This line is the reference for measuring longitude. Time there used to be referred to as Greenwich Mean Time (GMT). Today we call it Universal Time (UT). When the time for an astronomical event, such as an eclipse, is published, it is given as Universal Time. To find out when the event occurs for you, you must convert to your time zone. To convert to Eastern Standard Time (EST), you simply subtract five hours from Universal Time (to convert to Eastern Daylight Time, you subtract four hours.)