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Point A is at Sea Level and point B is at 10 000 ft. Which one of the following statements is true?
  • A
    The sunrise is earlier at point A because of the greater diffraction of sunlight at point B.
  • B
    The sunrise is earlier at point B because of the greater horizontal visibility at point B.
  • C
    The sunrise is earlier at point A because of the greater reflection of sunlight at point A.
  • D
    The sunrise is earlier at point B because of the greater refraction of sunlight at point A.

Refer to figure.
Although an altitude of 10 000 ft is extremely small compared to the Earth’s radius, its influence cannot be overlooked.

  • The sensible horizon is the horizon you would measure with a flat ruler at a tangent to the Earth’s surface.
  • The visible horizon is the horizon that we can see.
  • Refraction of light due to the Earth’s atmosphere is 34’ of arc or 16’ from the edge to the centre of the Sun.
The Sunrise or Sunset is the Local Mean Time (LMT) when:
  • The top lip of the Sun just rises above, or disappears below the visible horizon or
  • The centre of the Sun is 16’ below the visible horizon or
  • The centre of the Sun is: 16’ + 34’ = 50’ of arc below the sensible horizon.

The Air Almanac takes into account the above and the times of sunrise and sunset given are what an observer at the surface would note against the visible horizon at LMT.

With an increase in the observer’s altitude the sunrise will occur earlier and sunset later, because of the improved horizontal visibility, as shown in the figure.

Your Notes (not visible to others)



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