  We propose a completely new parametrisation of the dark energy equation of
state, which uses the dark energy density, $\Omega_e$ as a cosmic clock. We
expand the equation of state in a series of orthogonal polynomials, with
$\Omega_e$ as the expansion parameter and determine the expansion coefficients
by fitting to SNIa and $H(z)$ data. Assuming that $\Omega_e$ is a monotonic
function of time, we show that our parametrisation performs better than the
popular Chevallier--Polarski--Linder (CPL) and Gerke and Efstathiou (GE)
parametrisations, and we demonstrate that it is robust to the choice of prior.
Expanding in orthogonal polynomials allows us to relate models of dark energy
directly to our parametrisation, which we illustrate by placing constraints on
the expansion coefficients extracted from two popular quintessence models.
Finally, we comment on how this parametrisation could be modified to
accommodate high redshift data, where any non--monotonicity of $\Omega_e$ would
need to be accounted for.
