The First Derivative of the Solar Signal
At the equinox, the rate of change of daylight peaks. At the solstice, it falls to zero — sol sistere, the sun stands still. The Terran Calendar anchors where the signal is strongest.
The rate at which daylight changes at your latitude — the first derivative of the solar signal. Maximum at both equinoxes. Zero at both solstices. The year’s turning, made visible.