Gather 10 balls of yarn of the shades of the sky in the daytime (blue, gray, white). Each day, knit two rows of the color which is closest to the average color of the sky. Repeat for a year.
That's a summary-- there's some more detail on the web site, but the text isn't cut and pastable.
You could get a fancier scarf by choosing colors at sunrise or sunset.
Link thanks to shadesong.