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25: 🎂 The Timeline of Timekeeping, Origins of Speech, and the Birthday of Birthdays

Let's Learn Everything!

CHAPTER

The Second of Ephemeral Time

In 1952, the International Astronomical Union redefined the second as a fraction of a year. This was known as ephemeral time and is based on the idea that 31,556,925.9747 of the tropical year of 1900 January zero at 12 hours ephemeris time is a split-second. Lewis Essen who helped develop one of the first practical usable atomic clocks called it "unusable" for scientific purposes.

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