Time: LMT, UTC and the date line
Navigation · Chapter 15 · DGCA CPL and ATPL
Why clocks differ round the world, how to turn longitude into time, and how to work out the local time anywhere, on any date.
Remember
- The Sun is highest at 1200 LMT. East is ahead, west is behind. LMT at Greenwich = UTC.
- 15° = 1 h, 1° = 4 min, 15′ = 1 min.
- East: LMT = UTC + arc time. West: LMT = UTC − arc time. Carry the date.
- Two places or a flight: always go through UTC.
- Westbound across the date line, date + 1; eastbound, date − 1. Standard time = UTC ± a fixed offset.
Why is it a different time in different places?
Why east is ahead
- Noon is where the Sun is highest • Looking down on the North Pole • The Sun is over 060°E: there it is 1200 local mean time • Greenwich at the bottom, east to the right
- East is ahead, west is behind • The Earth turns west to east • East of the Sun has had its noon: clocks ahead (090°E: 1400) • West has not: Greenwich only 0800
- The Sun moves west, 15° each hour • 360° in 24 h = 15° every hour
How can one picture show the time everywhere at once?
The time strip
- The world as a strip • The world laid out flat: west on the left, east on the right • Each tile is the local time on one meridian, every 15° • Light tiles: day · dark tiles: night · gold: noon, where the Sun is overhead
- One meridian, one time • Every place on a meridian has the same LMT, whatever its latitude • Greenwich's LMT is the world clock: utc
- Turn the clock • Drag the UTC slider • The Sun moves west; every tile's time moves on with it
How do you turn degrees of longitude into hours and minutes?
Arc into time
This lesson is part of the Navigation plan. See plans
How do you get the local time from UTC?
UTC and LMT at one place
This lesson is part of the Navigation plan. See plans
Solve it: UTC and local mean time
Solve it: UTC and LMT
This lesson is part of the Navigation plan. See plans
How do you find the time at one place from the time at another?
Two places and flights
This lesson is part of the Navigation plan. See plans
Worked question 1: B is 15° east of A
Worked question 1: two places
This lesson is part of the Navigation plan. See plans
Worked question 2: local time at 75°W to UTC
Worked question 2: LMT to UTC
This lesson is part of the Navigation plan. See plans
Worked question 3: UTC from 84°37′W, then the LMT at 12°52′W
Worked question 3: in two parts
This lesson is part of the Navigation plan. See plans
Worked question 4: the LMT at 007°15′E
Worked question 4: UTC to LMT
This lesson is part of the Navigation plan. See plans
Worked question 5: the LMT at landing in Manila
Worked question 5: landing time
This lesson is part of the Navigation plan. See plans
What happens to the date when you cross 180°?
The date line
This lesson is part of the Navigation plan. See plans
What time do the clocks in a country actually show?
Standard time and zone time
This lesson is part of the Navigation plan. See plans
Test yourself on chapter 15
Test yourself
This lesson is part of the Navigation plan. See plans