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Slowing Rotation Markers

Sharpen a nail. Pound through a bit of plywood that is flat, no warps. Place nail in middle of wood.

To observe. Put a sheet of paper over nail. align paper with north via compass. Make reference line on paper. Date the paper. Pin the paper to the board.

Then at periodic times go a put a pen point on the very tip of the shadow being cast by the sun and the nail. Get as close to the point as possible. Then for each point, on the paper, and near the mark made, note the time from manual watch.

Every day change the paper and keep them handy. To compare, merely line the holes in the paper up, then the north pointing reference points.

Such solar observation stations were used for centuries to good effect prior to telescopes with solar filters.