It is a massive cathedral and inside there is a hole in the roof through which a slowly (but perceptibly) moving spot of light shines illustrating the movement of the earth round the sun and the exact time of the solstices and astrological periods on a huge linear sundial on the marble floor. Arrive shortly before 12:30 (probably 13:30 in the Summer?) and you can follow the spot of light as it crosses the floor to reach its destination. Great for kids and free!
A side chapel also contains a Foucault's pendulum kept in motion by magnets.

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