Measurement of Time

Wg Cdr Rajagopal

Measurement of Time

Index

Hour Angle

  • Hour angle is the arc of the equinoctial which is equivalent of longitude
  • Hour angle is measured westwards from 0° to 360° between datum meridian and meridian of celestial body
  • Hour angle varies constantly due to rotation of earth
  • Hour angle is 000 degrees, when the celestial body transits a given meridian
  • When the body transits the anti-meridian, its hour angle is 180 degrees

Greenwich and Local Hour Angles

  • Hour angles are called either Greenwich or Local hour angles based on the selected datum meridian
  • Greenwich hour angle is the hour angle measured with Greenwich meridian as datum
  • If observer’s meridian is taken as datum, it is called Local hour angle

Mean Time

  • Mean Time is obtained by converting the meridians to time
  • Since, 360 degrees of rotation takes 24 hours, 15 degrees of rotation will take 1 hour
  • Similarly, 1 degree of rotation will take 4 minutes and 15 minutes of rotation will take 1 minute

Greenwich and Local Mean Time

  • Greenwich (GMT) and Local mean times (LMT) are Mean times on a selected meridian
  • Mean time is based on time elapsed from the meridian transit of mean sun
  • GMT is time elapsed from mean sun transit over Greenwich anti-meridian
  • LMT is time elapsed from mean sun transit over observer’s anti-meridian
  • The variation between GMT and LMT of a place is based only on its longitude
  • If the Longitude is east of Greenwich, GMT is less than LMT
  • If the Longitude is east of Greenwich, GMT is more than LMT
  • GMT corrected for slow down of half second each year results in UTC
  • UTC or Coordinated universal time, is calculated using a caesium clock
  • For all practical purposes, GMT and UTC can be considered equal

Change of Date

  • Local date changes when mean sun transits anti-meridian of any place
  • If mean sun at 45 East Longitude, it is 1200 LMT on 16th May at 45 East
  • At its anti-meridian of 135 West Longitude, LMT is 0000 LMT on the 16th May
  • We know that 0000 LMT on the 16th May is also 2400 LMT on the 15th May

Flight across International Date Line

  • Flight across the International Date Line would result in gaining or losing a day
  • Westward flight across the local anti-meridian, increases local date by one day
  • Flight from west to east would end up losing a day
  • Eastward flight across the local anti-meridian, decreases local date by one day
  • Flight from east to west would end up gaining a day

Zone Time

  • Zone times were used by ships for time keeping to align with periods of sunlight and darkness
  • Greenwich meridian is taken as datum reference for zone time
  • Earth divided into 25 zones of 15° longitudes with time difference of 1 hour
  • Zone time varies from UTC by increasing or decreasing periods of 1 hour

Zone Time Calculation

  • Knowledge of zone number would enable us to convert ZT to UTC and UTC to ZT
  • For westerly longitudes zone number is positive
  • For easterly longitudes zone number is negative
  • UTC = Zone Time + Zone Number
  • A zone is 7.5° about a central meridian, like Zulu time is 7.5 E to 7.5 W
  • Near International Date Line there are two semi-zones of M and Y

Standard Time

  • Standard times are official times established by law for each country or part of a country
  • Generally, it is the mid-longitude for a particular region
  • Standard times would be close but may not be equal to LMT or ZT
  • Standard Time corrections for most countries are listed in the Air Almanac
  • Some countries with large east west extent use Daylight saving time
  • Daylight saving time is normally 1 hour in advance of Standard Time
  • International date line is based on standard time hence not a straight line

Best of luck