What’s the difference between axial tilt and orbital inclination?

[EmilyVG]

I've been trying to get a handle on the difference between a planet's axial tilt and its orbital inclination, but I keep mixing them up when reading about climate models. For instance, when they talk about Mars having a more variable tilt over long timescales, is that strictly about its spin axis wobble relative to its orbital plane around the Sun, or is the planet's whole orbital path shifting?

[EvelynWD]

It's about obliquity. The tilt of Mars' spin axis relative to its orbital plane around the Sun changes a lot over millions of years. The orbital plane itself doesn't flip around; its orientation drifts, and that drift is separate from the axis tilt, which is what climate papers usually mean when they talk about a variable tilt.

[Joshua12]

I used to picture it like a spinning top on a table. The lean is the tilt, the table surface is the orbital plane. The climate swings come from the axis tilt changing, not from the planet suddenly taking a different path around the Sun.

[CharlesL]

From a dynamics angle, axial tilt is the angle between the spin axis and the normal to the orbital plane. It can wander because solar torques on Mars' equator pull the axis around and because the entire orbital plane itself precesses due to planetary gravity. The long term chaos mostly shows up in obliquity, not a wholesale shift of the orbit itself.

[JosephHH]

I tried a tiny model once: hold the orbital plane fixed and let the axis precess; insolation patterns changed a lot, which clicked for me. Then I tried letting the orbit tilt too and it got messy fast, so I backed off.

[FrankMD]

Do you think the statement about Mars' tilt being more variable is really about obliquity rather than the orbit's inclination?