Measurement of Time

Wing Commander Rajagopal

Time

Time calculations in aviation include Local Time, UTC (Coordinated Universal Time), and Zone Time. The difference in time between locations is calculated using longitude differences (15° longitude = 1 hour). Accurate time calculation helps pilots with navigation, flight planning, and communication.

Hour Angle

  • The Hour Angle (HA) is the arc of the equinoctial (celestial equator) and is the celestial equivalent of longitude.
  • It is measured between a reference (datum) meridian and the celestial body’s meridian.
  • Hour angle is measured westward from the reference meridian.
  • Its value ranges from 0° to 360°.
  • The hour angle changes continuously due to the Earth’s rotation.

Position of a Celestial Body and Hour Angle

  • The hour angle is 0° when the celestial body transits the reference meridian.
  • When the body transits the anti-meridian, its hour angle is 180°.

Greenwich and Local Hour Angles

  • Hour angles are classified according to the reference meridian used.
  • Greenwich Hour Angle (GHA) uses the Greenwich Meridian as the datum.
  • Local Hour Angle (LHA) uses the observer’s meridian as the datum.

Arc to Time Conversion

  • Mean time is obtained by converting angular rotation into time.
  • 360° of rotation = 24 hours.
  • 15° of rotation = 1 hour.
  • 1° of rotation = 4 minutes.
  • 15′ (minutes of arc) of rotation = 1 minute of time.

Greenwich Mean Time (GMT)

  • Mean time is based on the elapsed time since the meridian transit of the mean Sun.
  • Greenwich Mean Time (GMT) and Local Mean Time (LMT) are mean times measured on different meridians.
  • GMT is the time elapsed since the mean Sun crossed the Greenwich anti-meridian.
  • LMT is the time elapsed since the mean Sun crossed the observer’s anti-meridian.

Coordinated Universal Time (UTC)

  • UTC is the modern international time standard that has replaced GMT for precise timekeeping.
  • UTC is maintained using highly accurate caesium atomic clocks.
  • The difference between UTC and a location’s LMT depends only on its longitude.
  • For east longitudes, UTC is less than LMT.
  • For west longitudes, UTC is greater than LMT.

Change of Date

  • If the mean Sun is over 45°E, the LMT is 1200 hrs on 16 May at 45°E.
  • At the corresponding anti-meridian (135°W), the LMT is 0000 hrs on 16 May.
  • The same instant can also be expressed as 2400 hrs on 15 May.
  • The local date changes when the mean Sun crosses the anti-meridian of a location.

Flight Across the International Date Line

  • Flying westward across the International Date Line advances the local date by one day.
  • A westbound flight effectively gains one calendar day.
  • Flying eastward across the International Date Line moves the local date back by one day.
  • An eastbound flight effectively loses one calendar day.

Zone Time

  • Zone time was originally introduced for maritime timekeeping.
  • It keeps daylight and darkness reasonably aligned with local time.
  • The Greenwich Meridian is used as the reference for zone time.
  • The Earth is divided into 24 standard time zones, each approximately 15° of longitude wide, with a one-hour difference between adjacent zones.
  • Zone time differs from UTC by whole-hour increments.

Zone Time Calculation

  • Each standard time zone extends approximately 7.5° on either side of its central meridian.
  • Near the International Date Line there are two half-zones designated M and Y.
  • UTC = Zone Time + Zone Number.
  • For westerly longitudes, the zone number is positive.
  • For easterly longitudes, the zone number is negative.

Standard Time

  • Standard time is the official time established by law or custom.
  • Each country or region may adopt its own standard time.
  • Standard time is generally based on the central meridian of the region.
  • It is usually close to both Local Mean Time (LMT) and Zone Time.

Calculation of Standard Time

  • Standard time corrections for most countries are listed in the Air Almanac.
  • Some countries with a large east-west extent observe Daylight Saving Time (DST).
  • Daylight Saving Time is typically 1 hour ahead of standard time.
  • The International Date Line follows standard time boundaries and is therefore not a straight line.