  We use rapidly cooled achondrites to test the assumption of 26Al homogeneity
in the solar nebula, by checking if there is a single value of tSS, the
absolute "Pb-Pb" age of the Solar System's t=0, that makes concordant their
ages from the Al-Mg and Pb-Pb systems. We find that values tSS = 4568.42
+/-0.24 Myr do make these ages concordant, and therefore the hypothesis of
homogeneous 26Al is not falsified. This age, defined to be when the solar
nebula had (26Al/27Al) = 5.23 x 10^-5, is significantly older than the ~4567.3
Myr inferred from direct measurements of Pb-Pb ages in CAIs. Discrepancies
between the Al-Mg and Pb-Pb chronometers in chondrules and CAIs have previously
been interpreted as arising from heterogeneities in 26Al, under the presumption
that the Al-Mg and Pb-Pb systems in CAIs closed simultaneously. We examine this
assumption and show that resetting is to be expected in CAIs. In particular, we
quantitatively demonstrate that it is plausible that Pb-Pb ages of CAIs were
reset at late times, without resetting the earlier Al-Mg ages, if they were
transiently heated in the same manner as chondrules. We critically examine
Pb-Pb isochrons, refining data and suggesting best practices for their
calculation and reporting. We advocate reporting chronometry as times of
formation after t=0 rather than absolute ages, as only the former is useful for
astrophysical models of the solar nebula. We strongly advocate averaging of
multiple samples, rather than anchoring to individual meteorites, to improve
precision.
