Time: UTC, LMT, Zone Time and Sunrise/Sunset
Time Scales
Coordinated Universal Time (UTC) is the reference for all flight planning and ATC. Local mean time (LMT) is the time at a specific longitude, with the sun crossing the meridian at 1200 LMT. Zone time divides the Earth into 24 zones, each 15° of longitude wide, with zone time generally equal to LMT of the zone's central meridian.
Conversions
Longitude to time: 15° = 1 hour, 1° = 4 minutes, 1' = 4 seconds. East is ahead of UTC, west behind. To convert LMT to UTC, add or subtract the longitude in time: LMT = UTC ± longitude/15 (east plus).
Sunrise and Sunset
Sunrise/sunset tables are tabulated against latitude and date in UTC or LMT. Twilight (civil, nautical, astronomical) depends on the sun's depression below the horizon (6°, 12°, 18°). The equation of time is the difference between apparent and mean solar time and can reach about ±15 minutes.
Traps
Examiners mix zone time, LMT and UTC in one question and expect the direction of the correction to be right. Remember the date line: crossing eastbound you gain a day when going westbound you lose one - check carefully which way the date changes.
Worked Example
An event at 1000 UTC at longitude 075°E: 75/15 = 5 hours ahead, so LMT = 1500. At 120°W, 120/15 = 8 hours behind, so LMT = 0200.