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?

  • $\begingroup$ 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 $\endgroup$ – uhoh Mar 28 at 23:21

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