Digital sundial
A digital sundial is a clock that indicates the current time with numerals formed by the sunlight striking it. Like a classical sundial, the device contains no moving parts. It uses no electricity nor other manufactured sources of energy. The digital display changes as the sun advances in its daily course. Technique. There are two basic types of digital sundials. One type uses optical waveguides, while the other is inspired by fractal geometry. Optical fiber sundial. Sunlight enters into the device through a slit and moves as the sun advances. The sun's rays shine on ten linearly  distributed sockets of optical waveguides that transport the light to a seven-segment display. Each socket fiber is connected to a few segments forming the digit corresponding to the position of the sun. Fractal sundial. The theoretical basis for the other construction comes from fractal geometry. For the sake of simplicity, we describe a two-dimensional (planar) version. Let denote a straight line passing through the origin of a Cartesian coordinate system and making angle with the -axis. For any define to be the perpendicular projection of on the line . Theorem. Let , be a family of any sets such that  formula_1  is a measurable set in the plane. Then there exists a set such that There exists a set with prescribed projections in "almost" all directions. This theorem can be generalized to three-dimensional space. For a non-trivial choice of the family , the set described above is a fractal. Application. Theoretically, it is possible to build a set of masks that produce shadows in the form of digits, such that the display changes as the sun moves. This is the fractal sundial. The theorem was proved in 1987 by Kenneth Falconer. Four years later it was described in "Scientific  American" by Ian Stewart. The first prototype of a digital sundial was constructed in 1994; it writes the numbers with light instead of shadow, as Falconer proved. In 1998 a digital sundial was installed for the first time in a public place (Genk, Belgium). There exist window and tabletop versions as well. Julldozer in October 2015 published an open-source 3D printed model sundial.