(a) Eliminate the parameter to find a Cartesian equation of the curve.
(b) Sketch the curve and indicate with an arrow the direction in which the curve is traced as parameter increases.
$ x = \sin\dfrac{1}{2}\theta $, $ \quad y = \cos\dfrac{1}{2}\theta $, $ \quad -\pi \leqslant \theta \leqslant \pi $