Tourism Geography I: The Earth's Movements, Latitude and Longitude, the IATA Areas and Sub-Areas, City, Airport and Airline Codes, Time Zones, GMT and the International Date Line, Daylight Saving Time, and the Calculation of Local, Flying, Ground and Elapsed Time

The earth rotates on its axis from west to east once in about 24 hours, which gives day and night and makes the sun appear to move from east to west; places to the east therefore see sunrise, and noon, earlier than places to the west. It revolves around the sun in about 365¼ days on an axis tilted at about 23½° to the perpendicular of its orbit, which gives the seasons: the sun is overhead at the Tropic of Cancer (23½° N) at the June solstice and at the Tropic of Capricorn (23½° S) at the December solstice, and over the Equator at the equinoxes of March and September. Seasons are reversed in the two hemispheres, so the Australian and South African summer falls in the northern winter, a fact that drives travel seasons.

Latitude is the angular distance of a place north or south of the Equator (0° to 90°); parallels of latitude run east and west. Longitude is the angular distance east or west of the Prime Meridian through Greenwich, adopted by the International Meridian Conference of 1884 (0° to 180° E or W); meridians run from pole to pole. Because the earth turns 360° in 24 hours, it turns 15° of longitude in one hour and 1° in 4 minutes. Local mean time therefore differs by 4 minutes for every degree of longitude, later to the east and earlier to the west. Indian Standard Time is the local time of the meridian of 82½° E, which passes near Mirzapur: 82.5 × 4 = 330 minutes, so IST is 5 hours 30 minutes ahead of Greenwich.

For fares and rules IATA divides the world into three Traffic Conference Areas, usually written TC1, TC2 and TC3, each divided into sub-areas. A journey is described by the areas it touches (within TC3, between TC2 and TC3, and so on), and the fare rules, mileage figures and taxes that apply depend on this. A journey is also labelled by a global indicator showing the route taken between areas: EH (within the Eastern Hemisphere), WH (within the Western Hemisphere), AT (via the Atlantic), PA (via the Pacific), AP (via the Atlantic and the Pacific) and TS (via the Trans-Siberian route), among others. The same two cities can have different fares under different global indicators.

India lies in TC3, in the South Asian Subcontinent sub-area with Pakistan, Bangladesh, Sri Lanka, Nepal, Bhutan and the Maldives. A flight from Delhi to London is therefore a TC3–TC2 journey, Delhi to New York a TC3–TC1 journey, and Delhi to Singapore a journey within TC3 between two of its sub-areas.

IATA assigns a three-letter code to every commercial airport and to cities served by more than one airport. A city code covers all the airports of a city, so a fare quoted to LON applies to any London airport: LHR (Heathrow), LGW (Gatwick), STN (Stansted), LTN (Luton) and LCY (London City). In the same way NYC covers JFK, LGA (LaGuardia) and EWR (Newark), PAR covers CDG (Charles de Gaulle) and ORY (Orly), TYO covers NRT (Narita) and HND (Haneda), and MOW covers Moscow's airports. Airlines carry a two-character designator, letters or a letter and a digit (AI, 6E), used in flight numbers and reservations. ICAO uses different codes for air traffic control: four letters for airports (VIDP for Delhi, VABB for Mumbai) and three letters for airlines.

If every place kept its own local mean time, clocks would differ every few kilometres, which the railways of the nineteenth century found unworkable. Standard time zones solve this: in principle the world is divided into 24 zones, each 15° wide and one hour apart, centred on meridians that are multiples of 15°; in practice countries draw the boundaries to suit themselves, and some use half-hour or quarter-hour offsets. Greenwich Mean Time (GMT), the mean solar time at Greenwich, is the historical reference; Coordinated Universal Time (UTC), kept by atomic clocks, is the modern standard and for travel purposes is the same. India keeps a single zone, GMT + 5:30; Nepal keeps GMT + 5:45; China, though it spans about five geographical zones, keeps a single zone of GMT + 8; the contiguous United States has four zones, Eastern (GMT − 5), Central (− 6), Mountain (− 7) and Pacific (− 8); and Russia has eleven.

The International Date Line runs roughly along the 180° meridian but zigzags to keep island groups and countries on one date, bending round the Aleutians, Fiji, Tonga and Kiribati. At 180° a place is 12 hours ahead of Greenwich if reckoned eastward from the west and 12 hours behind if reckoned westward from the east, so the date on either side differs by one day. A traveller crossing the line westward (from the Americas towards Asia) advances the calendar by one day; one crossing eastward (from Asia towards the Americas) sets it back by one day. That is why a flight from Tokyo to Los Angeles can land on the same date and at an earlier clock time than it left. Some Pacific states have moved the line for trade: Kiribati adopted GMT + 14 for its Line Islands in 1995, and Samoa skipped 30 December 2011 to move to the Asian side.

Daylight saving time (DST, "summer time") advances clocks by one hour in summer to make better use of the evening light. The European Union's summer time runs from the last Sunday of March to the last Sunday of October, when the United Kingdom keeps British Summer Time (GMT + 1) and Central Europe GMT + 2; most of the United States and Canada observe it from the second Sunday of March to the first Sunday of November. Countries near the Equator gain little from it and India does not observe it. The offsets used for air travel, with and without DST, are those printed in the IATA standard clock time tables and the Travel Information Manual, and the offset in force on the date of travel is the one that counts.

Airline schedules print every departure and arrival in the local time of the airport concerned, so the times cannot simply be subtracted. The method is always the same: convert each local time to GMT by subtracting the zone's offset (a place ahead of GMT is brought back, a place behind is brought forward), then subtract GMT departure from GMT arrival. Flying time is the time in the air on one sector; ground time (the syllabus's "grounding time") is the time spent at an intermediate airport between arrival and onward departure, both of which are at that airport and so in the same zone; and elapsed time, or total journey time, is flying time plus ground time from the first departure to the final arrival. To convert a local time directly, add the difference between the two offsets: at 14:00 in Delhi (GMT + 5:30) it is 08:30 GMT, and therefore 03:30 in New York in winter (GMT − 5) and 17:30 in Tokyo (GMT + 9).