One incidental item I needed to prepare for play on a planet's surface was determination of the seasons. On the face of it, that's pretty easy:
I compute the angle between the (normalized) direction vector to the sun and the direction vector of the axis of planet rotation and compute the angle between them (arccos( dot(sunvec, axisvec) ). The year on this planet [Home] happens to be 405.346 days long, the sidereal rotation period is exactly 24 hours, and as mentioned a few times already the tilt of the rotation axis relative to the ecliptic plane is 31 degrees. Astronomical seasons quite obviously fall out of it: if the angle between the rotation axis and direction to the sun is less than 90 degrees and the sign of the first derivative is negative, it's spring. If the angle is less than 90 degrees and the sign is positive, it's summer, > 90 degrees and sign positive it's fall, and > 90 and sign negative it's winter. So in the above plot, the first 101 days are spring, the next are summer, and so on.
That works for earth in astronomical seasons as well and many people will say, for example, that (northern hemisphere) winter starts on the winter solstice, usually around December 21 or so. However, you'll run into to people that say that winter starts on December 1st, spring starts on March 1st and so on.
Looking through wikipedia as to why this would be I came across seasonal lag, noting that due to the large amount of water on earth, that mass of water acting as something like a thermal capacitor. In Traveller world generation needs, this means the higher the hydrographics of the world, the longer the delay will be between the astronomical seasons and the thermometric seasons. Adding to the difficulty in generating a simple model to represent this, the lag isn't symmetric for summer and winter. A single location can have a short wintertime lag and a long summertime lag. As far as I can tell, this isn't mentioned or represented in traveller rules and I imagine is generally left to referee whim, or not dealt with at all.
Home has a different orbit situation than Earth. Aphelion occurs 5 days after the fall equinox, rather than mid summer. If, like earth, the continents are arranged in such a way that more of the land mass is in the northern hemisphere that will probably amplify the effect of northern hemisphere winter in addition to the greater axial tilt and further distance from the system star (which is the same luminosity as the sun), but that's not modeled at all yet.
This still leaves the question as to why December 1 marks the start of winter for many people. Thermometric seasonal lag works the other direction, if anything, January 1 should mark the start of winter rather than December. It seems like this definition came about arbitrarily in 1780, and I suspect that part of the reasons that December got counted as winter is that there was generally snow on the ground by december 1 in northern europe back then.
On a world like Home, it's quite likely that snow would appear on the ground far earlier than the equivalent of December 1 on europe-equivalent latitudes, at a guess. I also suspect that the population would have to cluster much more around the equator, due to the more intense winter and (possibly?) truncated growing season thanks to the slightly increased orbital distance.
This all is certainly overkill in a gaming environment in which even a UWP is considered too much information, but it's fun.
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