  Quintessence models based on a scalar field, phi, with an inverse power law
potential display simple tracking behavior at early times, when the
quintessence energy density, rho_phi, is sub-dominant. At late times, when
rho_phi becomes comparable to the matter density, the evolution of phi diverges
from its scaling behavior. We calculate the first order departure of phi from
its tracker solution at low redshift. Our results for the evolution of phi,
rho_phi, Omega_phi, and w are suprisingly accurate even down to z=0. We find
that w and Omega_phi are related linearly to first order. We also derive a
semi-analytic expression for w(z) which is accurate to within a few percent.
Our analytic techniques are potentially applicable to any quintessence model in
which the quintessence component comes to dominate at late times.
