Assertion: mercury's period | /r/HasProperty | about 59 days
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Source sentences:
text: Three years later, radar observations by Americans Gordon H. Pettengill and Rolf B. Dyce, using the 300-meter Arecibo Observatory radio telescope in Puerto Rico, showed conclusively that the planet's rotational period was about 59 days.
text: Their observations demonstrated conclusively that the planet’s rotational period was about 59 days and the planet did not have a synchronous rotation (which was widely believed at the time).