I'm familiar with orbit ephemerides and the ways to use those to plot orbits and positions. However, in all the charts I find, I can't seem to find a way to determine when an equinox on another planet occurs.
Take for an example the data one can generate from JPL HORIZONS. I can get the ephemerides and osculating for Earth and Mars. The spring equinox on Earth is easy, it's defined as the X axis. But within HORIZONS, I can't seem to find anywhere that determines the "local X axis" for Mars, to determine the first day of spring.
Now, I can easily Google "Mars seasons" to determine the answer, but if I wanted to extend this exercise for arbitrary planets, where's that data stored? What's that "offset" (angle between reference X and local X) commonly called?