  In 2019 the International System of units (SI) conceptually re-invented
itself. This was necessary because quantum-electronic devices had become so
precise that the old SI could no longer calibrate them. The new system defines
values of fundamental constants (including $c,h,k,e$ but not $G$) and allows
units to be realized from the defined constants through any applicable equation
of physics. In this new and more abstract SI, units can take on new guises ---
for example, the kilogram is at present best implemented as a derived
electrical unit. Relevant to astronomy, however, is that several formerly
non-SI units, such as electron-volts, light-seconds, and what we may call
"gravity seconds" $GM/c^3$, can now be interpreted not as themselves units, but
as shorthand for volts and seconds being used with particular equations of
physics. Moreover, the classical astronomical units have exact and rather
convenient equivalents in the new SI: zero AB magnitude amounts to
$\simeq5\times10^{10}$ photons $\rm m^{-2}\,s^{-1}$ per logarithmic frequency
or wavelength interval, $\rm 1\,au\simeq 500$ light-seconds, $\rm 1\,pc\simeq
10^8$ light-seconds, while a solar mass $\simeq5$ gravity-seconds. As a result,
the unit conversions ubiquitous in astrophysics can now be eliminated, without
introducing other problems, as the old-style SI would have done. We review a
variety of astrophysical processes illustrating the simplifications possible
with the new-style SI, with special attention to gravitational dynamics, where
care is needed to avoid propagating the uncertainty in $G$. Well-known systems
(GPS satellites, GW170817, and the M87 black hole) are used as examples
wherever possible.
