1.8: Time

Standard time 

Early agricultural societies found that local noon could be determined by observing the changing length of the shadow cast by a stick placed perpendicular to the ground. Local noon is the time at which the shadow is the shortest length cast. Romans used this principle to design their sundials and called their noon position of the Sun the "meridian" (meridiem - the Sun's highest point of the day). It was difficult to compare time as one traveled to different localities as each city adjusted its clocks to their own local noon. Because the Earth rotates toward the east, towns to the east experienced solar noon earlier while those to the west later.

As cross-country travel and communication became faster and more efficient, a standardized system of global time was required. Given the Earth rotates once throughout a 24 hour period, 24 standard times zones were agreed upon at the 1884 International Prime Meridian Conference. The local solar time at Greenwich, England was designated the prime meridian. Each time zone extends 7.5^o on either side of a central meridian. For years the global standard for reporting time was Greenwich mean time (GMT). GMT is now referred to as Universal Time Coordinated (UTC) or Coordinated Universal Time but the prime meridian is still the reference for standard time. It uses the 24-hour time (military) notation based on the local standard time at the prime meridian of 0^o longitude. Midnight corresponds to 00:00 UTC and noon to 12:00 UTC. Please note that as we move to the east of the Prime Meridian time is ahead of GMT and as we go west of the Prime Meridian time is behind the GMT. For examples the time at India is 5 hours 30 minutes ahead of the time at Greenwich while GMT is 4 hours ahead of New York time. Yendi in North Ghana lies on the Prime Meridian so Ghana has the same time as the time in Greenwich. 

Figure \(\PageIndex{1}\): Universal Time Coordinated zones of the world. (CC BY-SA 4.0 ,Goran tek-en via Wikimedia Commons).

Figure \(\PageIndex{1}\) is a world map divided into vertical bands representing different time zones. It shows how regions across the globe are assigned different standard times based on their position east or west of the prime meridian. 

International Date Line

Ferdinand Magellan and crew in 1519 set out on their westward journey from Spain to circumnavigate the Earth. Upon their return three years later, they discovered that their meticulously kept logs were off by one day. This was one of the first recorded experience with changing global time. This earlier experience would ultimately lead to the establishment of the international date line. The International Date Line is an imaginary line the separates one day from another. It roughly follows the 180^o meridian from the North Pole to the South Pole through the Pacific ocean, deviating around some territories to avoid splitting countries or island groups into two different calendar days.Crossing the line when traveling east one turns their calendar back a full day. Traveling west one moves their calendar forward one day. The Prime Meridian lies opposite of the International Date Line. 

Time zones 

Time zones are regions on the Earth that make use of a uniform standard time for legal, commercial and social purposes. They were created for standardized time keeping across different geographical locations which became crucial with the emergence of fast travel and global communication. The Earth rotates at the speed of  approximately 1^o meridian for 4 minutes. As a result of this diurnal movement, time would change by an hour for every 15 ^o meridians of longitude. Every place set its time by the local solar noon, however, if time had to be adjusted for every meridian one crossed to the east or west it would be very complicated. Figure \(\PageIndex{1}\) is showing the Universal Time Coordinated zones of the world. 

Ideally, the Earth would be divided into 24 time zones, each 15 degrees of longitude wide, with a one-hour difference between adjacent zones. However, in reality, time zone boundaries are often adjusted to follow political borders (countries, states, etc.) for practical convenience. This means that some time zones aren't exactly 15 degrees wide, and some countries, despite their size, use a single time zone. Countries like India use the local time at 82 ° 30 'E which is a central median. The local time at this meridian is considered as the standard time for all of India. China spans 5 geographical time zones yet it uses the local time at Beijing (UTC+08:00) as the standard time for the whole country. It is currently the largest sovereign country in the world that observes only one time zone.  

Some countries, due to their large geographic size spanning multiple time zones or have multiple overseas territories, have multiple time zones. For a large country like United States of America (U.S.A) using a single time zone would mean that sunrise and sunset times would be drastically different between its eastern and western extremities, making daily life difficult. Contiguous U.S.A thus has 4 time zones (Eastern, Central, Mountain and Pacific) and when we include Alaska and Hawaii, then U.S.A has 6 domestic time zones. If we include all the territories for U.S.A., then there are 11 time zones. Russia which is the largest country by land area has 11 time zones. Canada has 6 time zones. Australia has three main time zones with additional zones for its external territories. Brazil has 4 standard time zones. 

Daylight Saving Time

Many countries observe daylight saving time - the practice of setting clocks forward one hour in the spring and back one hour in the fall. First proposed by Benjamin Franklin, the notion of extending daylight one hour into the evening didn't catch hold until World War One as a means of energy savings. Some countries,territories, and states in the U.S. do not observe daylight saving time.