Sun coins

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Sun coins under trees

The circular to elliptical light spots that can often be observed under trees and bushes in sunny weather are called sun thalers or sun rings . This phenomenon occurs wherever sunlight falls through small, narrow openings, for example when the blind is down. The shape of the opening is irrelevant, the spots are almost always perfect circles or ellipses - except in the case of a partial solar eclipse .

The bright spots are images of the solar disk , because the small openings act like a natural pinhole camera . The sun is projected through the openings onto the corresponding surface. The curls will be a bit blurred at the edge if the gaps between the leaves are too big.

When the sun is low, a keyhole on the opposite wall can create an image of the sun, which sometimes even shows sunspots .

A phenomenon similar to the sun talers can also be created artificially, for example by preparing a brightly shining flashlight in the following way: a template with a - for example - triangular opening is stuck over the opening, which in turn is covered with a transparent, but highly scattering film (e.g. laminating film, envelope window). If you shine with this lamp through one or more small openings (the leaves of a houseplant can also be used for this), triangular light spots are created behind them.

Ludwig Bechstein uses the phenomenon in Sonnenkringel as a fairy tale motif.


  • Hans Joachim Schlichting: Sun coins don't fall from the sky. In: Mathematics and science lessons. 48/4, ISSN  0025-5866 , 1995, pp. 199-207 ( PDF ).
  • Hans Joachim Schlichting: Sonnentaler - images of the sun. In: Practice of Natural Sciences - Physics. 43/4, No. 19, ISSN  0177-8374 , 1994, pp. 2-6 ( PDF ).

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