A disco ball, the bottom half of which rotates as normal and the top half of which is divided horizontally into thirds, rotating around the vertical axis once a day, once an hour and once a minute respectively. The disco ball is covered, as normal in small reflective tiles except that in each of these day/hour/minute sections of the ball there is one red tile. You can thus tell the time by setting up your light sources pointing at the disco ball, looking for the reflected red dots on your walls and marking the hour/minute/second marks around your room. After you have done this you can use these marks and the moving red dots to tell what the time is.
-- hippo, Aug 21 2009

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