Because your eyes are relatively slow a fast moving light is seen as a stripe, while a fast moving flashing light is seen as a dotted line. Now all you have to do is move a series of leds very fast in the darkness outside, and make them flash in such a way that a massage is written in the sky.
http://members.home.nl/b.vandam/lonely/pagina52.html
Spoke POV for bicycles using 64 LEDs
http://www.ianpaterson.org/projects/spokepov20050704/index.htm
A little bit improved from Tiny POV project.
http://www.electric-clothing.com/megapov.html
This project is a second revision of the MiniPOV by Ladyada.
http://www.ladyada.net/make/minipov2/index.html
An Inexpensive Persistance of Vision Toy
http://www.ladyada.net/make/minipov/index.html
A double sided MiniPOV for displaying text in a bike wheel.
http://drewish.com/blogger/archives/2005/03/17/2sided_pov_toy.html
Spoke POV is an easy-to-make electronic kit toy that turns your bicycle wheel into a customized display!
http://www.ladyada.net/make/spokepov/index.html