Is there a way to improve this fitting?
dat={{0, 0.183453}, {10, 0.18317}, {20, 0.182312}, {30, 0.180844}, {40,
0.178713}, {50, 0.175843}, {60, 0.172141}, {70, 0.167506}, {80,
0.161841}, {90, 0.155074}, {100, 0.147198}, {110, 0.138308}, {120,
0.128652}, {130, 0.118654}, {140, 0.108915}, {150, 0.100168}, {160,
0.093188}, {170, 0.0886728}, {180, 0.0871093}, {190,
0.0886728}, {200, 0.093188}, {210, 0.100168}, {220, 0.108915}, {230,
0.118654}, {240, 0.128652}, {250, 0.138308}, {260, 0.147198}, {270,
0.155074}, {280, 0.161841}, {290, 0.167506}, {300, 0.172141}, {310,
0.175843}, {320, 0.178713}, {330, 0.180844}, {340, 0.182312}, {350,
0.18317}, {360, 0.183453}};
Block[{\[Theta] = 40},
fit = NonlinearModelFit[
dat, {(a A Cos[\[Theta] Degree] +
B Sin[\[Theta] Degree] Cos[\[Theta] Degree] Cos[\[Phi] Degree]),
A > 0, B > 0 },
{{A, 0}, {B, 0}, {a, 0}}, \[Phi]];
ListLinePlot[{Table[{\[Phi], ((a A Cos[\[Theta] Degree] +
B Sin[\[Theta] Degree] Cos[\[Theta] Degree] Cos[\[Phi] \
Degree])) /. fit["BestFitParameters"]}, {\[Phi], 0, 360, 10}], dat},
PlotStyle -> {Gray, Red}, PlotLegends -> {"fit", "dat"}]]
