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What “length of day” means
In this context, a day is not simply the fixed 24-hour interval shown by a clock. The International Earth Rotation and Reference Systems Service (IERS) defines length of day as the difference between the duration of a day measured using UT1 and 86,400 SI seconds. LOD is commonly expressed in milliseconds per day. IERS: UT1 and length of day
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That difference is small, but it matters in precise timekeeping and in describing Earth’s orientation. A day of 86,400 SI seconds is an exact reference interval; the time Earth takes to rotate through the corresponding angle is not perfectly constant.
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| Time scale | What it represents | Why it matters here |
|---|---|---|
| UT1 | Time based on Earth’s rotational position. | It provides the rotation-based measure used to determine LOD. |
| UTC | The uniform civil time scale based on atomic clocks, with each day defined as exactly 86,400 seconds. | Comparing UTC with UT1 reveals how Earth’s rotation differs from the uniform atomic-time reference. |
The difference between the two scales is written UT1−UTC. It is an accumulated time-scale difference, not LOD itself. Scientists derive LOD from the changes in successive daily UT1−UTC values: the change indicates whether the rotation-based day was longer or shorter than 86,400 SI seconds. U.S. Naval Observatory: UT1−UTC
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How scientists measure Earth’s rotation
1. Observe distant radio sources with VLBI
Very-long-baseline interferometry (VLBI) links observations from radio telescopes at widely separated locations. The telescopes observe distant radio sources, including quasars. By analyzing when the signals arrive at different stations, scientists determine Earth’s orientation and the UT1−UTC value. VLBI is a key method for accurately determining UT1. U.S. Naval Observatory: 24-hour sessions NASA Earthdata: Very Long Baseline Interferometry
2. Track the rotation day by day
The USNO describes daily one-hour “Intensive” VLBI sessions that determine how much Earth has rotated over a day. Measurements of Earth orientation are used to establish daily UT1−UTC values; the differences between those values provide LOD. In other words, telescopes help measure Earth’s orientation, while LOD is calculated from the resulting time series.
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NASA explains that UT1 correction changes especially rapidly among the Earth orientation parameters, making daily measurement necessary. NASA Earthdata: Very Long Baseline Interferometry
Why Earth’s rotation changes
Earth’s rotation is affected by motion and redistribution of mass. Large tides, ocean currents, earthquakes, and melting ice are among the factors that can influence the rotation axis or spin velocity. Their effects combine, so the rate does not follow a simple, perfectly predictable pattern. NIST: Leap seconds FAQs NASA Earthdata: Very Long Baseline Interferometry
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As one specific example—not a typical earthquake effect—NIST reports a NASA estimate that the 2004 Indonesian earthquake shortened the day by 2.68 microseconds. NIST: Leap seconds FAQs
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 adjusts UTC to keep it close to Earth-rotation time; it does not change Earth’s spin. IERS says leap seconds are introduced so the absolute difference between UT1 and UTC remains below 0.9 seconds, and IERS announces whether one is needed. IERS: UT1 and length of day
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How to read an LOD value
- LOD compares a UT1-based day with 86,400 SI seconds; it is a difference, not a separate civil clock.
- UT1−UTC is the time-scale difference from which daily LOD is derived. Changes between consecutive values reveal the day’s length difference.
- A value reported in milliseconds per day describes a small deviation relative to the fixed 86,400-second reference. Its date matters because Earth’s rotation varies.
Sources defining the measurement do not establish a current LOD reading, so a numerical “today” value should be taken from a dated Earth-orientation data release rather than inferred from the definition.
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