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A Day Is Not Exactly 24 Hours

Star trails over an observatory.

The Earth's rotation slows down over time.

Startrails lining the sky over an observatory. The Earth's rotation slows down over time.

©iStockphoto.com/nukleerkedi

Modern timekeeping defines a day as the sum of 24 hours – but that is not quite correct. The Earth's rotation slows down over time. So in terms of solar time, most days are a little longer than 24 hours.

How Long Is Today?

Average Day Lengths & Leap Seconds

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Why Are the Days Getting Longer?

The speed of the Earth's rotation decreases over time, but it also varies from day to day. One of the main factors are the celestial bodies surrounding us. For example, the Moon's gravitational pull causes tides and changes the Earth's shape, ultimately resulting in a lower rotational speed. The distance between Earth and Moon changes constantly, which makes for daily variations in the speed our planet rotates around its own axis.

Find Day Length for any Date

(*) Based on mean solar day. Numbers provided by the International Earth Rotation and Reference System Service (IERS).

Topics: Earth, Timekeeping, Astronomy

Coordinated Universal Time (UTC)

01:02:59

Wednesday, July 26, 2017

UTC is the common time standard across the world

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Keeping Time

  1. UTC – Coordinated Universal Time
  2. International Atomic Time (TAI)
  3. Universal Time (UT)
  4. Local Mean Time (LMT)
  5. A Day Is Not Exactly 24 Hours
  6. AM and PM: What Do They Mean?
  7. Atomic Clocks

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