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Length of day (LOD) is the difference between a day measured from Earth’s rotation and a fixed interval of 86,400 SI seconds. Because Earth’s rotation varies slightly, that difference changes over time. Scientists determine LOD from daily measurements of UT1−UTC, using very-long-baseline interferometry (VLBI) to track Earth’s orientation against distant radio sources.
What length of day means
The International Earth Rotation and Reference Systems Service (IERS) defines LOD as the difference between the duration of a day measured using UT1 and 86,400 SI seconds. It is commonly expressed in milliseconds per day. In plain terms, LOD tells you how much a rotation-based day differs from a fixed 24-hour interval.
That distinction matters because 86,400 SI seconds is an exact unit of time, while Earth does not rotate at a perfectly uniform rate. A civil clock’s 24-hour day is therefore not a guarantee that Earth completed exactly one rotation in that interval.
UT1 and UTC measure different things
| Time scale | What it tracks | How it relates to LOD |
|---|---|---|
| UT1 | Earth’s rotational position | Provides the rotation-based measure used to determine day length. |
| UTC | A uniform civil time scale based on atomic clocks, with a day of exactly 86,400 SI seconds | Provides the fixed-time reference against which Earth-rotation time is compared. |
The difference between these scales, written UT1−UTC, shows how Earth’s rotation time relates to atomic time. It is not the same quantity as LOD: UT1−UTC is the accumulated time-scale difference, while LOD describes the day-to-day change in rotation rate. See the U.S. Naval Observatory’s explanation of UT1−UTC and its Earth-orientation information.
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How scientists determine LOD
1. Observe distant radio sources with VLBI
Very-long-baseline interferometry uses radio telescopes at widely separated locations to observe the same distant radio sources, including quasars. By comparing when the signals reach different telescopes, scientists determine Earth’s orientation and calculate UT1. VLBI is a key method for accurately determining UT1 and other Earth orientation parameters, as described by NASA Earthdata.
2. Compare daily UT1−UTC values
The IERS says LOD is determined in practice by differencing daily UT1−UTC values. The change between consecutive daily values reveals whether Earth’s rotation-based day was longer or shorter than 86,400 SI seconds. The resulting LOD difference is typically stated in milliseconds per day; it is a calculated quantity derived from the observations, not a separate reading from a household clock.
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3. Use daily sessions to monitor rotation
The U.S. Naval Observatory describes daily one-hour VLBI “Intensive” sessions that determine how much Earth has rotated in a day. The process relies on coordinated radio-telescope observations and analysis, not on consumer telescopes or clocks. Its 24-hour sessions overview explains the role of quasars and VLBI in these measurements.
Why Earth’s rotation changes
Earth’s spin is affected by motion and redistribution of mass. NASA Earthdata identifies tides, ocean currents, earthquakes, and melting ice as influences that can affect the rotation axis or spin velocity. These effects combine, so the rotation rate does not follow a perfectly steady pattern.
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As one specific example—not a typical earthquake effect—NIST reported in 2025 that NASA scientists estimated the 2004 Indonesian earthquake shortened the day by 2.68 microseconds. The figure illustrates how an individual event can affect rotation, but it should not be generalized to earthquakes as a class. NIST discusses this estimate and the complexity of the influences in its explanation of changing day length.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What leap seconds do—and do not do
A leap second is an adjustment to UTC, not a change to Earth’s spin. The IERS introduces leap seconds to keep the absolute value of UT1−UTC below 0.9 seconds, and it announces whether an adjustment is needed. The adjustment helps civil time stay close to Earth-rotation time; it does not make the planet rotate faster or slower. See the IERS glossary.
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Why the measurement is useful
Tracking UT1 and other Earth orientation parameters provides a precise account of how the planet is oriented and rotating. NASA Earthdata notes that UT1 changes more rapidly than the other Earth orientation parameters, making daily measurement necessary. The measurements also let scientists calculate LOD consistently rather than treating every nominal 24-hour day as an identical amount of physical rotation.
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