Seasons “Cheat Sheet”

WARNING: THIS POST IS OVERWHELMING. There is too much information in one place, but I promise there are fun and interesting gems hidden within. It is meant as a reference, and a starting point for discussion of each of these topics.
There are a few numbers and references I am often asked for, especially at the West Seattle solstice and equinox sunset watches. I thought I’d collect these things in one place for “easy” reference.
Strict Definitions (from the USNO)
Equinox – The time at which the Sun passes through one of the two points on the celestial sphere where the celestial equator and the ecliptic intersect. (Yup, pretty technical – the non-technical version is “when the Earth’s axis is pointed neither towards nor away from the Sun.”)
Solstice - The time at which the Sun is at either of the two points on the ecliptic at which the longitude of the Sun is 90° or 270° (halfway between the equinoxes). The non-technical version is “when the Earth’s axis is pointed exactly towards or exactly away from the Sun.”
Horizon - Your eye is considered to be the surface of the Earth, so the horizon is the line exactly straight ...