Time zones

Earth rotates on its polar axis once every 23.9345 hrs. As an oblate sphere measuring a circumferential 360°, Earth rotates through almost 15 angular degrees per hour.

Local noon occurs when, on the hypothetical celestial sphere, the Sun is at the highest point during its daily skyward arch from east to west. When the Sun is at its zenith on the celestial meridian, this is termed local noon. In the extreme, every line of longitude, or fraction thereof, has a different local noon. In practice, however, because of Earth's angular rotation rate, it is more convenient to create a system of 24 time zones—each spanning 15 angular degrees. The central line of longitude in these zones establishes the local noon for individual time zones.

Earth's lines of longitude (meridians) are great circles that meet at the north and south polar axis. They are referenced by an east or west displacement from the prime meridian. Accordingly, lines of longitude range from 0° E to 180° E and 0° W to 180° W. Degrees are further divided into arcminutes and arcseconds.

The prime meridian runs through Greenwich, England and the line of longitude displaced 180° E and 180° W from the prime meridian is termed the international dateline. The international dateline generally runs through sparsely islanded areas of the Pacific Ocean.

With regard to the solar meridian, the Sun's location (and reference to local noon) is described in terms of being ante meridian a.m.) or post meridian (p.m.).

Standard meridians occur every 15° of longitudinal displacement from the prime meridian (e.g., 15° W, 30° W, 45° W, etc.) The standard meridians also establish the local noon for the time zone and, therefore, each time zone is defined as being 7.5° longitudinal displacement both west and east of the standard meridian. Accordingly, dividing the standard meridian by 15 yields the time correction for that time zone. For example, the standard meridian of 90° W runs near both Chicago and New Orleans. These sites are in the Central Time Zone of the United States (CST; Central Standard Time). To obtain the proper correction from Greenwich Mean Time (GMT)—also termed Universal Time (UT or UTC)—a division of 90° by 15° means that CST is six hours behind GMT. Accordingly, when it is noon 12:00 HRS GMT in London, it is 0600 HRS (6 a.m.) CST in Chicago or New Orleans. Because of Earth's rotation, displacements west are further designated with a negative sign. Accordingly, CST = GMT − 6 hrs.

Additional North American meridians and time zones include standard meridian 60° W for Atlantic Standard Time (e.g., as for Puerto Rico); standard meridian 75° W for Eastern Standard Time (EST); standard meridian 105° W for Mountain Standard Time (MST); standard meridian 120° W for Pacific Standard Time (PST); standard meridian 135° W for Yukon Standard Time (YST); standard meridian 150° W for Hawaii-Alaska Standard Time (HAST) and standard meridian 165° W for Bering Standard time.

Movement east of the prime meridian results adding time to GMT. For example, Rome, Italy, at a latitude and longitude of 42° N, 12° E, is 3° W of the 15° E standard meridian. Because a time zone ranges 7.5° east and west of a standard meridian, the applicable standard meridian for Rome is the 15° E standard meridian. Accordingly, the time differential between Rome and London (GMT) is 15°/15° = 1–interpreted as +1 hour time difference. Therefore, when it is noon in London, it is 1300 HRS, or 1 p.m., in Rome.

In reality, there are many local deviations of the time zone boundaries based upon geopolitical considerations (e.g., state and national boundaries). Actual time corrections are also influenced by whether or not a particular locality adopts daylight saving time shifts (usually one hour) to save energy by shifting daylight hours to clock hours more conductive to typical human work patterns. The United States shifts to Daylight Saving Time between April and October each year. Accordingly, time zone designations are changed from, for example, CST to CDST (Central Daylight Saving Time).

See also Cartography; Celestial sphere: The apparent movements of the Sun, Moon, planets, and stars; Solar illumination: Seasonal and diurnal patterns