Why do Sky View grid lines have a small bump to them sometimes?
Because of atmospheric refraction. Refraction raises everything to a greater extent as you look lower and lower, increasing rapidly near the end to about 32′ at horizontal. This effect is evident when displaying in one (ideal) coordinate system and overlaying (refracted) grid lines in the opposite coordinate system. Suppose you are pointing near the horizon, are using Alt-Az display mode and are displaying RA-Dec grid lines. When projected onto the celestial sphere as seen from your location under all that air, this has the effect of curving the grid lines a little higher in the vicinity of the horizon than one would see without refraction. Indeed, because of refraction you actually see about 32′ more than 90° down from the zenith — bonus! Demonstrate this to yourself by going to the Main window and setting an Air Pressure value of 0 to effectively eliminate refraction. You will see the grid lines straighten out, and objects that were visible slightly above the horizon will drop below
