Coarse moduli spaces of Weierstrass fibrations over the (unparameterized) projective line were constructed by the classical work of [Miranda] using Geometric Invariant Theory. In our paper, we extend this treatment by using results of [Romagny] regarding group actions on stacks to give an explicit construction of the moduli stack W n of Weierstrass fibrations over an unparameterized P 1 with discriminant degree 12n and a section. We show that it is a smooth algebraic stack and prove that for n ≥ 2, the open substack W min,n of minimal Weierstrass fibrations is a separated Deligne-Mumford stack over any base field K with char(K) = 2, 3 and ∤ n. Arithmetically, for the moduli stack W sf,n of stable Weierstrass fibrations, we determine its motive in the Grothendieck ring of stacks to be {W sf,n } = L 10n−2 in the case that n is odd, which results in its weighted point count to be # q (W sf,n ) = q 10n−2 over F q . In the appendix, we show how our methods can be applied similarly to the classical work of [Silverman] on coarse moduli spaces of self-maps of the projective line, allowing us to construct the natural moduli stack and to compute its motive.