Standard Time Zones

As we know, the earth rotates in an eastward direction, causing the sun, moon, and stars to appear to be moving westward through the sky. Solar noon is defined as that point in time when the sun's rays strike a point on the earth's surface most directly. The noon meridian is that meridian at which solar noon occurs at any given time. Solar time at any location is determined by the noon meridian.

Until the nineteenth century, individual localities kept their own solar time. Such a system worked well in preindustrial societies, but the development of modern industrial society forced people to confront the problems associated with variation in local solar time with even small distances eastward and westward. The noon meridian moves westward at about fifteen degrees of longitude per hour, or one degree every four minutes.

Nineteenth-century railroads providing east-west service had difficulty adjusting their schedules to local differences in standard time. For example, travelers on trains from Philadelphia to Washington were obliged to set their watches back about eight minutes because Washington is two degrees of longitude west of Philadelphia. When it was noon in Washington, for example, it was already 12:24 p.m. in Boston but only 1 1:43 a.m. in Savannah. Georgia.

In order to overcome this problem, standard time zones were established in the United States in 1883. By agreement, every locality within a standard time zone sets its clocks to the same time. Standard time within each zone is the same, even though solar time varies. By the late nineteenth century, standard time zones had been established throughout the world (Figure B 1-1).

Figure B 1-1. Standard Time Zones. In the 1880s, standard time zones centered on the prime meridian at Greenwich, England, were established by international convention. Places within each time zone maintain the same standard time, even

In the United States, fifty-six local time zones were reduced to the four standard time zones used today. An international conference held in 1883 established the meridian passing through Greenwich as the prime meridian and used this as the basis for developing standard international time zones. (Piqued that Greenwich rather than Paris was chosen as the location of the prime meridian. France refused to accept international standard time until 1911.)

In modern industrial society, most activities begin well after dawn and continue well after dark. In response, many localities have changed standard time in order to increase evening daylight. This goal is achieved by setting clocks ahead. For example, cities such as Cleveland and Detroit, which are now in the Eastern Standard Time Zone, were placed in the Central Standard Time Zone in 1883. In the early twentieth century, both voted to switch to the Eastern Standard Time Zone.

In order to take advantage of long hours of summer daylight, many countries have adopted daylight saving time. In most of the United States, daylight saving time is observed by setting clocks forward one hour between the first week of April and the last week of October.