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Light effects with short/long line alignments

Light in a passage, viewing along stone rows or light from a remote horizon marker can be important measures for the time of year. The light can be on the back wall, on the side walls or possibly targets (lines, figures, etc.) to signal a specific date/event.
A few points can determine the reproducibility of these light effects:
  1. The disk size of the sun/moon (0.5) will make the difference between light and dark less obvious (penumbral region). Certainly on the side walls which are in almost the same direction as the sun/moon.
  2. The variation of refraction due to weather changes a apparent altitudes lower then say 0.5 (some variation of 0.18) and the variation of the lunar parallax (0.1) due to distance between moon and earth, can change the reproducibility. Certainly on the side walls when they are in almost the same direction as the Sun/Moon.
  3. An uneven surface can make it more difficult to see where a shadows stops and starts. Certainly on the side walls which are in almost the same direction as the Sun/Moon.
  4. Horizon markers or side stones can sometimes not be repositioned to the same location/slant as 5000 years ago. So erosion, vegetation growth, dilapidation and reconstruction factors (taphonomy in archaeology).
The below tries to evaluate these on the reproducibility (accuracy) for certain physical configurations of stones.
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Last major content related changes: Feb. 2, 2001 and 2008