  The Gaia spacecraft has just released a large set of parallaxes, including 41
novae for which the fractional error is <30%. I have used these to evaluate the
accuracy and bias of the many prior methods for getting nova-distances. The
best of the prior methods is the geometrical parallaxes from HST for just four
novae, although the real error bars are 3X larger than stated. The canonical
method for prior nova-distances has been the expansion parallaxes from the nova
shells, but this method is found to have real 1-sigma uncertainty of 0.95 mag
in the distance modulus, and the prior quoted error bars are on average 3.6X
worse than advertised. The many variations on the
maximum-magnitude-rate-of-decline (MMRD) relation are all found to be poor, too
poor to be useable, and even to be non-applicable for 5-out-of-7 samples of
nova, so the MMRD should no longer be used. The prior method of using various
measures of the extinction from the interstellar medium have been notoriously
bad, but now a new version by Ozdonmez and coworkers has improved this to an
unbiased method with 1-sigma uncertainty of 1.14 mag in the distance modulus.
For the future, I recommend in order (1) using the Gaia parallax, (2) using the
catalog of Ozdonmez, (3) using M_max = -7.0 +- 1.4 mag as an empirical method
of poor accuracy, and (4) if none of these methods is available, then to not
use the nova for purposes where a distance is needed.
