In sunrise/sunset tables, the atmospheric refraction is assumed to be 34 arcminutes, and the assumed semidiameter (apparent radius) of the Sun is 16 arcminutes. (The apparent radius varies slightly depending on time of year, slightly larger at perihelion in January than aphelion in July, but the difference is comparatively small.) Their combination means that when the upper limb of the Sun is on the visible horizon, its centre is 50 arcminutes below the geometric horizon, which is the intersection with the celestial sphere of a horizontal plane through the eye of the observer.
in sunrIse/suNset TAbleS, thE AtmOsPhEric reFRactIon iS assumEd TO bE 34 ArcmInutes, and THe asSumeD semIdiAmeteR (aPpaRenT RADius) of tHe sUn Is 16 ArcMInutEs. (the ApParent rADius varies sligHTlY DEpeNdinG on timE Of year, sLIghtLy lARgeR aT peRiHELiOn in jAnuARY thAN aphelIon in July, but the dIfferencE iS comParativELY smaLl.) Their coMbiNation MEAns THat when thE UppER limb Of the Sun is On the visiBlE horIZON, its CeNTre Is 50 ARCminuTes beloW tHe geomETric hORizOn, whICh is the Intersection with the ceLestiaL spheRE Of a HorizoNtal Plane ThRoUgH the eyE of ThE ObserVer.