Copernician
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The Copernician Period in the lunar geologic timescale runs from approximately 1100 million years ago to the present day. It encompasses the era during which the Moon's bright-rayed craters were formed, of which the crater Copernicus is the most prominent example. The date of this crater's formation marks the beginning of the Copernician Period.
The bright streaks that radiate from young Lunar craters such as these are formed from fresh ejecta blasted out of the crater by the impact that formed it. Almost all craters likely have such rays immediately after they form, but over time additional impacts and micrometeor "weathering" cause the rays to fade. Thus, only relatively recent craters still have visible rays; the older craters' rays having eroded away. The division between Copernician era craters and earlier craters is thus somewhat arbitrary.