Ring dials

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The ring dial makes use of light, not a shadow to indicate the time. Sunlight falls through a tiny hole onto the hour lines marked on the inside of a ring. The ring has to be turned so that the light spot falls on the correct date line.
Traditionally ring dials were very small, less than 25 mm in diameter; this made them difficult to read. There is no reason against making them bigger. The one in the photo has a diameter of 10 cm.

Ring dials are altitude dials.

Types of Sundials

Reflected ceiling dials
Vertical dials
© Helga Nordhoff