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Roughly every four years, an extra day gets tacked onto the end of February, a time-keeping convention known as the leap year. The practice of adjusting the calendar with an extra day was established by Julius Caesar more than 2,000 years ago and modified in the 16th century by Pope Gregory XIII, bequeathing us the Julian and Gregorian calendars.
That extra day is a way of aligning the calendar year of 365 days with how long it actually takes Earth to make a trip around the sun, which is nearly one-quarter of a day longer. The added day ensures that the seasons stay put rather than shifting around the year as the mismatch lengthens.
Humanity struggles to impose order on the small end of the time scale, too. Lately the second is running into trouble. Traditionally the unit was defined in astronomical terms, as one-86,400th of the mean solar day (the time it takes Earth to rotate once on its axis). In 1967 the world’s metrologists instead began measuring time from the ground up, with atomic clocks. The official length of the basic unit, the second, was fixed at 9,192,631,770 vibrations of an atom of cesium 133. Eighty-six thousand four hundred such seconds compose one day. (see article)
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The Astronomer,” a 1668 painting by the Dutch artist Johannes Vermeer.Credit…DeAgostini/Getty Images
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