One of the debates about reading ARK is whether one should look up the source of references one is not familiar wih. I think it can be interesting and enjoyable and Google usually makes it possible. I also look up words when I'm not sure of their definition of. I found the title phrase in "BEAM 5" and thought Johnson had invented because he liked the sound so I'm glad I looked it up.
Obliquity of the ecliptic is the term used by astronomers for the inclination of Earth's equator with respect to the ecliptic, or of Earth's rotation axis to a perpendicular to the ecliptic. It is about 23.4° and is currently decreasing 0.013 degrees (47 arcseconds) per hundred years because of planetary perturbations.
Image By I, Dennis Nilsson, CC BY 3.0
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