{"id":14247,"date":"2025-06-03T16:22:27","date_gmt":"2025-06-03T10:52:27","guid":{"rendered":"https:\/\/ibexaviation.com\/pilot-training\/?p=14247"},"modified":"2025-06-14T07:01:18","modified_gmt":"2025-06-14T01:31:18","slug":"time-measurement","status":"publish","type":"post","link":"https:\/\/ibexaviation.com\/pilot-training\/time-measurement\/","title":{"rendered":"Measurement of Time"},"content":{"rendered":"\n<h2>Measurement of Time<\/h2>\n\n\n\n<h3>Hour Angle<\/h3>\n\n\n\n<ul><li>Hour angle is the arc of the equinoctial which is equivalent of longitude<\/li><li>Hour angle is measured westwards from 0\u00c2\u00b0 to 360\u00c2\u00b0 between datum meridian and meridian of celestial body<\/li><li>Hour angle varies constantly due to rotation of earth<\/li><li>Hour angle is 000 degrees, when the celestial body transits a given meridian<\/li><li>When the body transits the anti-meridian, its hour angle is 180 degrees<\/li><\/ul>\n\n\n\n<h3>Greenwich and Local Hour Angles<\/h3>\n\n\n\n<ul><li>Hour angles are called either Greenwich or Local hour angles based on the selected datum meridian<\/li><li>Greenwich hour angle is the hour angle measured with Greenwich meridian as datum<\/li><li>If observer&#8217;s meridian is taken as datum, it is called Local hour angle<\/li><\/ul>\n\n\n\n<h3>Mean Time<\/h3>\n\n\n\n<ul><li>Mean Time is obtained by converting the meridians to time<\/li><li>Since, 360 degrees of rotation takes 24 hours, 15 degrees of rotation will take 1 hour<\/li><li>Similarly, 1 degree of rotation will take 4 minutes and 15 minutes of rotation will take 1 minute<\/li><\/ul>\n\n\n\n<h3>Greenwich and Local Mean Time<\/h3>\n\n\n\n<ul><li>Greenwich (GMT) and Local mean times (LMT) are Mean times on a selected meridian<\/li><li>Mean time is based on time elapsed from the meridian transit of mean sun<\/li><li>GMT is time elapsed from mean sun transit over Greenwich anti-meridian<\/li><li>LMT is time elapsed from mean sun transit over observer\u00e2\u20ac\u2122s anti-meridian<\/li><li>The variation between GMT and LMT of a place is based only on its longitude<\/li><li>If the Longitude is east of Greenwich, GMT is less than LMT<\/li><li>If the Longitude is east of Greenwich, GMT is more than LMT<\/li><li>GMT corrected for slow down of half second each year results in UTC<\/li><li>UTC or Coordinated universal time, is calculated using a caesium clock<\/li><li>For all practical purposes, GMT and UTC can be considered equal<\/li><\/ul>\n\n\n\n<h3>Change of Date<\/h3>\n\n\n\n<ul><li>Local date changes when mean sun transits anti-meridian of any place<\/li><li>If mean sun at 45 East Longitude, it is 1200 LMT on 16th May at 45 East<\/li><li>At its anti-meridian of 135 West Longitude, LMT is 0000 LMT on the 16th May<\/li><li>We know that 0000 LMT on the 16th May is also 2400 LMT on the 15th May<\/li><\/ul>\n\n\n\n<h3>Flight across International Date Line<\/h3>\n\n\n\n<ul><li>Flight across the International Date Line would result in gaining or losing a day<\/li><li>Westward flight across the local anti-meridian, increases local date by one day<\/li><li>Flight from west to east would end up losing a day<\/li><li>Eastward flight across the local anti-meridian, decreases local date by one day<\/li><li>Flight from east to west would end up gaining a day<\/li><\/ul>\n\n\n\n<h3>Zone Time<\/h3>\n\n\n\n<ul><li>Zone times were used by ships for time keeping to align with periods of sunlight and darkness<\/li><li>Greenwich meridian is taken as datum reference for zone time<\/li><li>Earth divided into 25 zones of 15\u00c2\u00b0 longitudes with time difference of 1 hour<\/li><li>Zone time varies from UTC by increasing or decreasing periods of 1 hour<\/li><\/ul>\n\n\n\n<h3>Zone Time Calculation<\/h3>\n\n\n\n<ul><li>Knowledge of zone number would enable us to convert ZT to UTC and UTC to ZT<\/li><li>For westerly longitudes zone number is positive<\/li><li>For easterly longitudes zone number is negative<\/li><li>UTC = Zone Time + Zone Number<\/li><li>A zone is 7.5\u00c2\u00b0 about a central meridian, like Zulu time is 7.5 E to 7.5 W<\/li><li>Near International Date Line there are two semi-zones of M and Y<\/li><\/ul>\n\n\n\n<h3>Standard Time<\/h3>\n\n\n\n<ul><li>Standard times are official times established by law for each country or part of a country<\/li><li>Generally, it is the mid-longitude for a particular region<\/li><li>Standard times would be close but may not be equal to LMT or ZT<\/li><li>Standard Time corrections for most countries are listed in the Air Almanac<\/li><li>Some countries with large east west extent use Daylight saving time<\/li><li>Daylight saving time is normally 1 hour in advance of Standard Time<\/li><li>International date line is based on standard time hence not a straight line<\/li><\/ul>\n\n\n\n<h2>Best of luck<\/h2>\n","protected":false},"excerpt":{"rendered":"<p>Measurement of Time Hour Angle Hour angle is the arc of the equinoctial which is equivalent of longitude Hour angle is measured westwards from 0\u00c2\u00b0 to 360\u00c2\u00b0 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&hellip;<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"disable_featured_image":false},"categories":[324],"tags":[],"_links":{"self":[{"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/posts\/14247"}],"collection":[{"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/comments?post=14247"}],"version-history":[{"count":0,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/posts\/14247\/revisions"}],"wp:attachment":[{"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/media?parent=14247"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/categories?post=14247"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/ibexaviation.com\/pilot-training\/wp-json\/wp\/v2\/tags?post=14247"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}