Dr. SCHAWINSKI: The reason we call it a ghost is because we believe it is something called a light echo.

NPR: What Makes Our Solar System Special?

It's what we call a light echo, an effect that we've seen around supernovas - stars that explode - and because of the vast scales involved, the speed of light is actually very slow.

NPR: What Makes Our Solar System Special?

Instead, it crests, fades to a light linger, then builds up to a second echo of the first bold attack.

FORBES: Making 46: Maker's Mark Produces First New Bourbon In Half A Century

And so, you can actually, like, in an echo in sound, you can see the light moving outwards.

NPR: What Makes Our Solar System Special?

First presented on 11 March, 2002, and shown annually on 11 September ever since, the tribute consists of 88 7, 000-watt xenon light bulbs placed on a roof near the WTC site in two 48-foot squares that echo the shape and orientation of the twin towers.

BBC: 11 September history tour of New York