I'm trying to understand orbital inclination by looking at the planetary orbit inclination table on https://en.wikipedia.org/wiki/Orbital_inclination (there are orbital inclinations for terrestrial and gas giant planets listed there).

By inclinations to ecliptic or Sun's equator, it looks like the Earth is the most (or least) inclined from all the listed objects having the smallest (or biggest) inclination value. However, the inclination to invariable plane seems to be bigger for some planets than for the Earth, and smaller for others.

At first, I thought that displaying the absolute value (dropping the minus sign) in case of a negative angle is the case. But if those three columns were just three different reference planes, the relative difference between planet inclinations should at least be the same in all cases. But it's not the case as, for example, Mercury's and Venus' orbital planes have 3.62° between themselves as per the inclination to ecliptic, but they have 0.48° as per the inclination to the Sun's equator.

So, how should one read those numbers?


1 Answer 1


The confusing thing is that inclination is not the only thing defining an orbital plane; it also matters where the ascending node is. Only if all ascending nodes would be in the same direction, you could do the trick with the absolute values you mentioned.

I can't draw a picture right now, but imagine you have a table before you now, and use it as a reference plane (the Sun's equator). Put a sheet of paper on the table, and lift the right end a bit so that it inclines 7.155° w.r.t. the table. This is Earth's orbit, and it looks a bit like the picture in the Wikipedia article.

Now take another sheet of paper, and lift the far end a bit so that it inclines 7.155° w.r.t. the table. This is supposed to be the orbital plane of a hypothetical planet with the same inclination to the Sun's equator, but it is also inclined to the ecliptic (i.e. Earth's orbital plane).


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