Geometric Considerations About Seemingly Wrong Tilt of Crescent Moon

Sa？etak The following phenomenon is well-known and again and again appears as an unanswered question in literature and on internet platforms: If you see moon and sun in the sky at the same time, then the (bisector of the) crescent moon in most cases does not seem to be precisely directed at the sun. Particularly at sunset, when you would expect the bisector of the crescent moon to be horizontal, it mostly points upwards. To “prove” that, photos that seem to support this view are displayed. In this paper it is shown by means of geometry what the “wrong moon tilt” is all about and that an explanation is to be found in the nature of central or normal projections (photography is basically a central projection, at an extremely long focal length it is approximately a normal projection). The paper also deals with the reason why the seemingly wrong tilt is subjectively felt. The path of the light from the sun to the moon is in any case displayed straight (apart from minor deviations due to refraction close to the horizon), except one takes photos with a fish-eye lens