Equinox (celestial coordinates)
In modern astronomy the ecliptic and the equinox are reducing in importance as needed, or even convenient, reference ideas. (The equinox remains important in ordinary civil use, in defining the seasons, however.)  This is for many reasons. One important reason is that it is very hard to be exact what the ecliptic is, and there is even some confusion in the books about it.  Should it be centered on the Earth's center of mass, or on the Earth-Moon barycenter?