Dear Esteemed Astronomers, could you kindly advise how to find/derive the location of the Martian vernal equinox? Is it simply located at the cross product of Mars's north pole and the normal to Mars's orbital plane? It feels like an introductory problem, and I missed the first page! Is there a textbook I should read?

I believe Mars's north pole can be found on the NASA fact sheet (blue pole), or alternatively via the IAU's recommendations on cartographic coordinates and rotational elements (cyan trace). Plotted onto a celestial sphere, these two seem in close agreement with the conventional/googleable answer that the pole lies near Cygnus, see below. I expect the IAU answer would lie closer if I added in the full corrections for the ~0.24 odd Julian century that has passed.

The normal to the orbit (orange/gold) I think can be located by rotating earth's celestial pole by Mars's inclination to orbit (~25.2°)

The cross product of these two vectors aligns with the Martian ecliptic/orbit and sits between Sagittarius and Scorpio. Is that the slam dunk?

Celestial sphere showing constellations around zodiac/ecliptic and unit vectors for quantites described in question


1 Answer 1


I believe parts of your answer may be found by considering Astronomical Algorithms by Jean Meeus, the Belgian Metrologist and amateur astronomer who took on the IAU and defied their definitions on the geographical longitudes, great reading.

In my edition, Chapter 30 outlines "Elements of the Planetary Orbits", provides a handy diagram showing the direction of definitions of said elements, and provides tables of said elements including Mars. Plotting these will show you that you were wrong in your interpretation of the location of the earth's periapsis, whoops.

celestial sphere showing major planetary orbit elements of venus, earth, and mars, with orbits scaled such that mars's orbit coincides with the celesital sphere of radius 1

Curiously, Jupiter's path to periapsis is defined such that the last leg of the path goes clockwise, while Mars's path to periapsis only goes counterclockwise, see below.

celestial sphere as above, but zoomed out, showing orbit of Jupiter with Jupiter's ascending node and location of periapsis marked. Similarly marked are Mars's elements

I sure hope this is right, and I sure wonder why people are so insistent on producing solar system visualizations that completely lacking in visual representations of orbital information.


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