"out" emphasises the sense of light being emitted. g. g.
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"out" emphasises the sense of light being emitted. Certain emitters of light, such as e.g. a torch or lighthouse, can be said to "shine out", and the phrase is also used figurately, e.g. "his honesty shone out". However, to say that a jacket is "shining out" seems unusual, and it is possible that your sentence may be interpreted instead as "My jacket is shining / out in the sun", i.e. "out" g