The light curve of a distant Type Ia supernova acts like a clock that can be used to test the expansion of the Universe. SN 1995K, at a spectroscopic redshift of z = 0.479, provides one of the first meaningful data sets for this test. We find that all aspects of SN 1995K resemble local supernova Ia events when the light curve is dilated by (1+z), as prescribed by cosmological expansion. In a static, non-expanding universe SN 1995K would represent a unique object with a spectrum identifying it

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... a regular Type Ia supernova but a light curve shape and luminosity which do not follow the well-established relations for local events. We conclude that SN 1995K provides strong evidence for an interpretation of cosmological redshifts as due to universal expansion. Theories in which photons dissipate their energy during travel are excluded as are age-redshift dependencies.