Zone time

A zone time ( English zone time , standard time ) is the time system assigned to a time zone . One or more (in countries of large west-east expansion) zone times serve as legal time for the states .

Zone times and politically defined time zones

World map with time zones, geometrically defined as spherical triangles , between two degrees of longitude drawn in black. In the map projection used , the time zones are in the form of strips of equal width. The brightly drawn vertical lines in the respective center of the zone are the reference meridians.

The politically defined time zones sometimes differ greatly from the geometrically defined time zones (recognizable only in the polar regions and in the oceans ). The map projection used is the same as above.

The use of a common time of day is advantageous for social life within a larger area on the earth's surface. In a simple geometric approach, the earth's surface is divided into time zones. According to the division of the day into 24 hours, 24 time zones are provided, each with a reference meridian whose longitude is a whole multiple of 15 ( ^{360 °} ⁄ ₂₄ = 15 °). The mean solar time of the reference meridian is the corresponding zone time. The 24 zone times thus differ from the mean solar time in Greenwich ( Greenwich Mean Time ) by a whole number of hours. In modern, u. a. According to the legal definition, the zone times are based on the Coordinated Universal Time (time zone UTC ± 0 ), from which Greenwich Mean Time can temporarily deviate by up to 0.9 seconds (see leap second ).

The zones ( spherical triangles ) of equal width in the geometrical approach and delimited by meridians do not, however, take into account any political, geographical, economic, social or other aspects and are therefore not practicable as actual time zones. By choosing one or more zone times as the legal time in states and other territories, time zones result that do not have degrees of longitude, but frayed state borders and internal borders as borders. They can differ greatly from the geometrically defined time zones and can also be interrupted. So is z. B. the time zone of Central European Time (CET) bulged to the west due to its choice as legal time in Spain and France . Some time zones are interrupted, e.g. B. in Asia , since in all of China instead of five zone times, only the Beijing zone time is legal time. The underlying ordering system, which is oriented towards the position of the sun , can only be seen clearly in international waters , where the boundaries of the real time zones largely follow the boundaries of the geometrically defined time zones.

A few countries do not have one of the 24 legal time that differs from UTC ± 0 by a whole number of hours, but by an additional half an hour or a quarter of an hour. Examples are Iran , India , Nepal , the Australian state of South Australia and the Australian federal territory of the Northern Territory .

Many states also have a temporarily different legal time with daylight saving time.

Conversion between zone times

The time shift between two zone times is the difference between the shifts given in their names and UTC (Coordinated Universal Time).

Example: Between Japan ( UTC + 9 ) and Finland ( UTC + 2 ) the time difference is +7 hours: (+9) - (+2) = (+7).

In countries with a large area such as the USA , Canada or Russia, there are also time differences within the country.

Example: Between Moscow ( UTC + 3 ) and Kamchatka ( UTC + 12 ) the time difference is −9 hours.

Conversion between zone time and solar time

Wall sundial with additional analemma-shaped hour markers, which enable reading of the evenly running time at full hours ( mean local time MONT)

The true solar time is directly derived from the apparent course of the sun (the true sun ). It is 12 o'clock every day when the sun is exactly in the south (in the southern hemisphere in the north).

Since the earth's orbit around the sun is not exactly a circle, but an ellipse and because the earth's axis of rotation is not perpendicular to its orbit around the sun and its direction in space does not change, there are seasonal fluctuations in the apparent celestial Movement of the sun and thus the true solar time (or true local time WOZ). The z. B. true solar time indicated by a sundial precedes the evenly passing time with a maximum of 16 minutes in November and a maximum of 14 minutes behind in February.

One defines a mean solar time (or mean local time MONT) derived from an artificial or mean sun . But like the true solar time, this depends on the longitude of the place where it is used. The seasonal shift between true and mean solar time is called the equation of time . The zone time is the mean solar time on the reference meridian of the time zone.

This results in several time shifts within a time zone, depending on the longitude of the location and the year date, e.g. B. for Hamburg on November 15th:

Time difference between true solar time and mean solar time:

November 15: Equation of time = +15 min (the true one is 15 minutes ahead of mean solar time)

Time difference between zone time CET (λ = 15 °) and mean solar time:

λ = 10 ° in Hamburg: (15 ° - λ) 4 min / ° = +20 min (CET is 20 minutes ahead of mean solar time),

Time difference between CET and true solar time:

Time difference between zone time CET (λ = 15 °) and mean solar time

minus time difference between true solar time and mean solar time

20 minutes - 15 minutes = +5 min (CET is 5 minutes ahead of true solar time).

literature

Hermann Mucke : Astronomical basics of the sundials . In: Hermann Mucke (Ed.): Sundials . 19th Sternfreunde Seminar, 1991. Planetarium of the City of Vienna and Austrian Astronomical Association , Vienna 1991, 1.3.4. Time and place of observation; Zone times , p. 8 (29-48) .
Robert Weber (Institute for Theoretical Geodesy of the TU Vienna) Editor = Hermann Mucke: Zeitsysteme . In: Modern Astronomical Phenomenology . 20th Sternfreunde Seminar, 1992/93, and 21st Seminar 1994. Planetarium of the City of Vienna and Austrian Astronomical Association, Vienna 1992, p. 33-54 .
PK Seidelmann, B. Guinot, LE Dogget: Time . Chapter 2. In: PK Seidelmann, US Naval Observatory (Ed.): Explanatory Supplement to the Astronomical Almanac . University Science Books, Mill Valley, CA 1992, pp. 39 (English).

Remarks

↑ The time zones UTC + 12 and UTC − 12 have the same reference meridian with the longitude ± 180 ° (historical definition of the date line ) and thus the same time deviating from UTC (or UTC ± 0) by 12 hours. Only the calendar date is different in both halves of the UTC ± 12 time zone. In the western half it is one day younger (in the calendar ahead) than in the eastern half. The division of the 12th time zone into two halves would be obsolete if the date line were 7½ ° east (only UTC + 12) or west (only UTC − 12). If this is exceeded, both the date and the time would be changed.
↑ Dennis D. McCarthy, P. Kenneth Seidelmann, TIME - From Earth Rotation to Atomic Physics . Wiley-VCH, Weinheim 2009, page 20, section 2.13 Time Zones :
"A worldwide system of standard time zones, based on increments of 15 degrees in longitude, provides the basis for local civil times that are related loosely to solar time. [. ..] For political and geographical reasons the time zones are not necessarily uniform longitude strips, 15 degrees wide, running from the north to the south pole. Rather the zone boundaries are set by individual countries and usually follow country, state, or province boundaries . "
↑ There are also officially used zone times such as UTC + 13 and UTC + 14 , which are more or less defined beyond the date line.

Web links

www.welt-zeit-uhr.de - List of selected places with current time
www.weltzeit.de - World map with time zones and current time in selected locations
www.dieweltuhrzeit.de - world map, current time in clickable locations
www.date-und-uhrzeit.de - world map, current time, date and coordinates of places typed in
www.zeitverschleiß.org - Calculate and compare time zones and time differences between all countries in the world

[1] The time zones UTC + 12 and UTC − 12 have the same reference meridian with the longitude ± 180 ° (historical definition of the date line ) and thus the same time deviating from UTC (or UTC ± 0) by 12 hours. Only the calendar date is different in both halves of the UTC ± 12 time zone. In the western half it is one day younger (in the calendar ahead) than in the eastern half. The division of the 12th time zone into two halves would be obsolete if the date line were 7½ ° east (only UTC + 12) or west (only UTC − 12). If this is exceeded, both the date and the time would be changed.

[2] Dennis D. McCarthy, P. Kenneth Seidelmann, TIME - From Earth Rotation to Atomic Physics . Wiley-VCH, Weinheim 2009, page 20, section 2.13 Time Zones :
"A worldwide system of standard time zones, based on increments of 15 degrees in longitude, provides the basis for local civil times that are related loosely to solar time. [. ..] For political and geographical reasons the time zones are not necessarily uniform longitude strips, 15 degrees wide, running from the north to the south pole. Rather the zone boundaries are set by individual countries and usually follow country, state, or province boundaries . "

[3] There are also officially used zone times such as UTC + 13 and UTC + 14 , which are more or less defined beyond the date line.