As nouns the difference between psis and apsis
is that psis
is while apsis
is apse (a semicircular projection from a building).
Other Comparisons: What's the difference?
(architecture) A recess or projection, with a dome or vault, at the east end of a church; an apse.
(astronomy) Either of the points in the elliptical orbit of a planet or comet where it is closest or furthest from the sun; perihelion or aphelion; an apside