Animate[circle=ContourPlot[x^2+y^2==1,{x,-1.45,1.1},{y,-1.1,1.1},ContourStyle->{Gray,Thickness[0.01]},Frame->False,AspectRatio->6/7];
triangle=Graphics[{EdgeForm[Directive[{Thick,Black}]],LightGray,Polygon[{ {0,0},{Cos[t],Sin[t]},{Cos[t],0}}]}];
triangle2=Graphics[{EdgeForm[Directive[{Thick,Black}]],Blue,Polygon[{ {0,0},{Cos[t],Sin[t]},{1,0}}]}];line=Graphics[{Black,Thick,Line[{{0,0},{Cos[t],0}}],Line[{{Cos[t],0},{Cos[t],Sin[t]}}]}];
letters={Graphics[Text[Style["O","Subsubtitle"],{0,0},{0,-Sign[t-Pi]1.5}]],Graphics[Text[Style[A,"Subsubtitle"],{Cos[t],Sin[t]},{-Sign[t-Pi/2]Sign[t-3Pi/2]1.5,Sign[t-Pi]1.5}]],Graphics[Text[Style[B,"Subsubtitle"],{Cos[t],0},{1.5,-Sign[t-Pi]1.5}]],Graphics[Text[Style[C,"Subsubtitle"],{1,0},{-1.5,-Sign[t-Pi]1.5}]]};
legend=Graphics[Text[Style[Abs[Sin[t]/2] ,"Subsubtitle"],{-1,0.75},{1,-1.5}]];
points=Graphics[{Black,PointSize[0.03],Point[{{0,0}}],Point[{{Cos[t],Sin[t]}}],Point[{{Cos[t],0}}],Point[{{1,0}}]}];
sector=Graphics[{LightPink,Disk[{0,0},1,{0,t}]}];Show[circle,sector,circle,legend, triangle2,line,points,letters],{t,0,2Pi,.00001},AnimationRate->10000]