On Fri, May 18, 2018 at 11:26:17PM -0400, Cheng Tseng wrote:
> Now back to determining how powerful a nuclear weapon has to be so
> that the detonation on the Moon (And any after effects.) can be seen
> from Earth...

The first part is trivial.  Pretty much any nuclear detonation above
the surface on the near side will be visible from Earth by anyone
looking in the right direction at night.  A large one would likely be
visible during the day.  It would require an enormously larger weapon
than anything we have now to leave a crater visible from Earth with
even good telescopes, though.  Perhaps one detonating well above the
surface might discolour it over a wide enough area to be visible.

If a detonation just needs to be visible at all, without needing to be
visible to naked eye, even a non-nuclear explosion would do.  There
are thousands of telescopes of various sizes pointed at the Moon at
any given time.  Some of them are even specifically watching for
explosions, though they would be expecting meteorite impacts rather
than nuclear detonations and most are not monitored in real time.
Flashes due to impacts on the lunar surface are recorded frequently,
all of them less than a hundredth of the energy of even small nuclear
detonations.

- Tim