Astronomical Theory Section - Planetary Phenomena


Why doesn't the crescent Moon point at the Sun?   (3)

  • Introduction
  • Discussion
    • The Sphere of the Heavens
    • Curved trajectories
    • 3 points don't always define a plane
    • The view from outer space
    • Viewports and spherical distortion
    • Photographic evidence
    • Wide-angle vs. Closeup
    • Summary
    • Advanced topics
      • Very different sky-paths
      • An incorrect hypothesis
      • Geometric derivation
Given that the Moon is lit by the Sun, one would intuitively expect its illuminated part to directly face the Sun i.e. if one imagined the crescent Moon as an archery bow, its "arrow" would be aimed straight at the Sun. However, simple visual observation shows this is not the case - the crescent always points above the Sun. Why should this be so?

I examine the conundrum from many points of view, considering aspects of human perception, the effect of changing viewpoint, and photographic experiments & distortions, before showing how they can all come together to produce a self-consistent but misleading picture. I then examine some of the incorrect ideas put forward and finally attempt a geometric derivation of why lines which are straight in 3D are seen as curved when seen from a rotating frame of reference - a key part of the explanation.

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