tracing a local worldline
across the last five billion years
over spans where the atom
was exposed to sunlight
let its worldline shine
where it was buried
or in shadow
leave it black
(atoms that have never
broken Earth's surface
we can ignore
in this visualisation)
now colorize those 5B years
of shining spans
with a full spectrum
so recent spans shine red
and the most ancient spans
blue-violet
individual atoms
may have longer or shorter spans
of mostly shining
or mostly not
shorter spans of a single hue
longer spans crossing multicolored stripes
now consider the surface of Earth
at any given moment
with all exposed atoms
colored by that moment's hue
currently red
but if we roll back the clock
shifting thru orange yellow green
to blue and finally violet
now since each atom
'remembers' its colors
for each moment the Sun shone
we can command
that only atoms that are
currently lit red
but that also
in the far distant past
were lit, at some points
yellow
now light, say
in a pattern
of yellow and red
so that geological layers
laid down in the yellow era
but exposed again now
are selectively lit
and geologists' maps
can and do show
where those yellow-red patterned
layers lie
