Wednesday, June 13, 2007

Vexillology and the Moon Hare

A friend of mine recently asked me a question regarding the moon, and I thought it might be good to share the answer with my readers.

The question was whether the fact that Europeans tend to see the face of a man in the moon (with the Mare Imbrium and Mare Serenitatis forming the eyes, and the Mare Nubium and/or Mare Humorum forming the Mouth), whereas Asians tend to see a hare or rabbit (with the Mare Foecunditatis and Mare Nectaris forming the ears), had any astronomical basis.

Now, longitude is of course a largely arbitrary quantity (for which reason it was historically so hard to determine that a large prize was offered for the development of a method to determine it reliably while at sea) and should not have any effect on the appearance of celestial bodies (except for the time at which they transit, some parallax and effects following from those, such as visibility of eclipses etc., but certainly not the size or orientation of a disc). Latitude, on the other hand, has a true astronomical meaning, and a moment's thought should show you that the angle that the crescent moon (which always points towards the sun, which moves accross the sky at different angles to the horizon at different latitudes) forms with the horizon varies with latitude -- in fact, it is something of a cliché that this is reflected in the flags of islamic countries at different latitudes: compare the flags of Turkey, Pakistan and Mauritania.

Since the part of the moon turned towards the sun is of course independent of the observer's latitude, it follows that from the point of view of an observer close to the equator the orientation of the moon disk is such that the rabbit in the moon is quite clearly visible, whereas an observer in the temperate zone sees the moon under an angle at which he would likely prefer the face, unless being told about the rabbit (which, at least for me, easily supersedes the face). I therefore hypothesize that the tradition of the moon rabbit spread into East Asia from South Asia, whereas the tradition of the face in the moon comes from Nothern Europe. Does anyone know whether that would appear to agree with the historical record? It seems rather plausible to me.