  Einstein's static model is the first relativistic cosmological model. The
model is static, finite and of spherical spatial symmetry. I use the solution
of Einstein's field equations in a homogeneous and isotropic universe --
Friedmann's equation -- to calculate the radius of curvature of the model (also
known as "Einstein's universe"). Furthermore, I show, using a Newtonian
analogy, the model's mostly known feature, namely, its instability under small
perturbations on the state of equilibrium.
