Re^2: calculate length of day as function of space at onset of fall

[Aldebaran]

That can happen at any odd moment day or night, not necessarily the moment when the sun rises or sets.

Agreed. The coincidence of the solar plane and the terrestrial plane is instantaneous, because we're supplying the coordinate system and numbers. There is a bias for when sunset is actually occuring, and I think they really are worth reading:

Sun Height There are a number of sun heights to choose from. The default is -0.833 because this is what most countries use. Feel free to specify it if you need to. Here is the list of values to specify the sun height with: 0 degrees Center of Sun's disk touches a mathematical horizon -0.25 degrees Sun's upper limb touches a mathematical horizon -0.583 degrees Center of Sun's disk touches the horizon; atmospheric refraction accounted for -0.833 degrees Sun's supper limb touches the horizon; atmospheric refraction accounted for -6 degrees Civil twilight (one can no longer read outside without artificial illumination) -12 degrees Nautical twilight (navigation using a sea horizon no longer possible) -15 degrees Amateur astronomical twilight (the sky is dark enough for most astronomical observations) -18 degrees Astronomical twilight (the sky is completely dark)

Let's say you add in these biases, which frankly are in the wrong direction unless you happen to be stranded on a remote island. The act of observation requires an observer and a location, so I submit that the point in space-time where autumn occurs is where a dwarf of zero height observes the sun setting in the west, which fixes the longitude in a time-zone, ready to export to others.

Tyrion is definitely my favorite Lannister anyways....