rule-based calendars Earth's rotation sky observations drift

Rule-based calendars can slowly drift from the sky

A calendar that uses fixed rules makes dates easy to calculate. But changes in Earth’s rotation slowly put those dates out of step with the sky, so the rules eventually need changing. A calendar based on watching the sky can stay in step, but future dates are harder to predict.

The library-burning story appeared centuries late

Accounts blaming Arab conquerors for burning Alexandria’s Great Library first appeared roughly five centuries after the 642 conquest. By then, the library and its research center, the Mouseion, had probably already disappeared. Historians doubt the story partly because the accounts came so late.

A lunar crater can be colder than Pluto

26 K (−247 °C)

A spacecraft measured 26 K (−247 °C) near the winter solstice in Hermite, a permanently shadowed crater near the Moon’s north pole. It was the lowest temperature the spacecraft had measured anywhere in the Solar System.

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rule-based calendars Earth's rotation sky observations drift

Rule-based calendars can slowly drift from the sky

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