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Define Local Mean Time (LMT).

Local mean time (LMT) uses the sun as its celestial reference point and the local meridian of longitude as its terrestrial (earth) reference point. 

Therefore all points along the same meridian of longitude will have the same LMT.

Calculate the LMT at a given location.

The time difference between two meridians is directly proportional to the angular difference between them. 

The earth rotates 360° in 24 hours, giving a rotation of 15° per hour. 

Put simply, if it is 1200 hrs at the 180°W meridian then it will be an hour earlier or later 15° either side.

The 15° per hour rotation can be further broken down, as shown by the table below.

Once the time difference has been established, the difference must be applied, and this comes back to an understanding of:

  • Eastward travel  - add time
  • Westward travel - subtract time

Crossing the International Date line:

  • heading east - subtract a day, 
  • heading west - add a day.

Example 1:

If it is 1500 hrs LMT on the 5th at 45°W, what is the LMT at 90°W?


Angualar difference = 45°

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