When a periodical phenomenon can be described by a sinusoid, you can find the corresponding function by determining:
the equilibrium $y=d$;
the amplitude $a$(maximum deviation from the equlibrium);
the period $p$;
the horizontal translation (w.r.t. the standard graph) $c$.
Two function rules are possible:
$f\left(x\right)=asin\left(b\right(x-{c}_{1}\left)\right)+d$ where $b=\frac{2\pi}{p}$
$f\left(x\right)=acos\left(b\right(x-{c}_{2}\left)\right)+d$ where $b=\frac{2\pi}{p}$
Note that the values for $a$, $b$ and $d$ are the same for both, but that the values of $c$ are not. The translation with respect to the standard sine is different from the one with respect to the standard cosine.