Leap seconds are used to adjust the time scale UTC to the apparent motion of the Sun, i.e. to Earth rotation. They are introduced in such a way that UT1-UTC stays smaller than 0.9 s in absolute value.

The decision to introduce a leap second in UTC to meet this condition is the responsibility of the IERS. Leap seconds are announced in IERS Bulletin C.

Related terms:

• Coordinated Universal Time
• Universal Time (UT1)

Related links:

• IERS Bulletin C
• Earth orientation data
• Leap seconds

common term for the difference in the duration of a day as measured by UT1 and 86,400 SI seconds. In practice this quantity is determined by differencing daily values of UT1-UTC. Units are generally given as ms day^-1.

Source: IERS Technical Note 36: Glossary (PDF, 244KB, Not barrier-free file.) of the IERS Conventions (2010)

LLR (Lunar Laser Ranging) is a space geodetic technique that measures the round-trip travel times of light pulses between stations on the Earth and five retroreflectors (ca. 2010) on the surface of the Moon.

Source: IERS Technical Note 36: Glossary (PDF, 244KB, Not barrier-free file.) of the IERS Conventions (2010)