Calculate Date of Spring Equinox, possibly using JPL's Horizons website?

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?

• To get the equinox you need the planet's equator which means you need the orientation of its pole. The orientation of the planets are in Horizons somewhere because you can specify a geographic position on another planet and view another body from it and calculate altitude/azimuth etc. But I'm not sure how to get things out of there in such a form that you would get the JD of the planet's equinox. I have a hunch someone will be able to answer this though. Great first question! +1 – uhoh Mar 28 at 23:21