Tag: international atomic time

How to measure time

International Atomic Time

(TAI)

Coordinated Universal Time

(UTC)

Universal Time

(UT)

  • Fixed-length seconds and days
  • Fixed-length seconds
  • Seconds vary in length to match Earth’s rotation
  • Not in sync with solar time
  • Doesn’t drift more than half a second from solar time
  • Perfectly in sync with solar time
  • All days have 86400 SI seconds
  • Some days can have 86401 SI seconds
  • All days have 86400 sun and not SI seconds (different length)

Historically, time standards were often based on the Earth’s rotational period. From the late 18th to the 19th century it was assumed that the Earth’s daily rotational rate was constant. Astronomical observations of several kinds, including eclipse records, studied in the 19th century, raised suspicions that the rate at which Earth rotates is gradually slowing and also shows small-scale irregularities, and this was confirmed in the early twentieth century (decreasing roughly by 0,002s per century).

For this reason, there are several different ways to measure time:

  • based on Earth rotation around axis,
  • based on atomic clocks and the SI second,
  • based on Earth rotation around sun.
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