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In conventional sundials the gnomon projects the shadow on a reference frame. The negative clock is the one that projects the rays of light through a crack.

In the graph we show the domes of the cosmic room. Through these domes the rays of light pass over an inner wall during most of the year, let's say, ten out of twelve months.

Now shortly after twelve....

Twelve twenty-five....

Shortly before one in the afternoon, and so on, the clock points the course of time on the south wall.