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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallScientists do not measure Earth’s rotation with a single clock. They combine observations of distant radio sources, tracking data from satellites and ground stations, and geophysical models to estimate Earth orientation parameters (EOPs). The central quantities are UT1−UTC, which records Earth’s rotation angle relative to atomic time; length of day (LOD), the day-to-day change inferred from UT1; and polar motion, the movement of the rotation axis across Earth’s crust.
Very-long-baseline interferometry (VLBI) provides a direct astronomical reference by timing radio signals from quasars at widely separated antennas. Global navigation satellite systems (GNSS), satellite laser ranging (SLR), and atmospheric and ocean models add complementary information. The International Earth Rotation and Reference Systems Service (IERS), the U.S. Naval Observatory (USNO), NASA’s Jet Propulsion Laboratory (JPL), and other analysis centers combine these measurements into rapid estimates, final records, and predictions.
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What scientists mean by a change in Earth’s rotation
Earth’s rotation has two aspects that must be separated. First, the planet’s rotation rate changes, so the interval between successive crossings of a reference meridian is not exactly constant. Second, the direction of the rotation axis changes relative to the solid Earth. A station on the crust can therefore move with respect to the instantaneous rotation axis even when the planet’s overall spin is considered separately.
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- UT1−UTC: the difference between UT1, a time scale based on Earth’s actual rotation, and UTC, the civil time scale generated by atomic clocks.
- Length of day (LOD): the difference between a UT1 day and 86,400 SI seconds. It is normally reported in milliseconds per day.
- Polar motion: coordinates, conventionally called x and y, that describe movement of the rotation pole relative to the Earth-fixed crust.
- Celestial pole offsets: corrections associated with the orientation of Earth’s rotation axis in the celestial reference frame, including departures from modeled precession and nutation.
These parameters are estimates produced from observations and analysis, not readings from a physical dial on Earth.
The measurement chain, from quasar to published EOP
- Observe a stable reference. Radio telescopes record signals from distant quasars. Because quasars are so far away, their apparent directions provide an effectively fixed celestial reference for geodetic measurements.
- Track Earth-fixed stations. The positions and clocks of antennas, satellite receivers, and laser-ranging stations are modeled in a terrestrial reference frame.
- Estimate Earth orientation. Analysts solve for the orientation that best explains signal arrival times, station coordinates, Earth tides, atmospheric delays, and instrument-clock behavior.
- Combine independent techniques. VLBI, GNSS, SLR, and geophysical inputs constrain different parameters and time scales. A combined solution is stronger than treating any one technique as a complete measurement system.
- Publish rapid and final products. Operational services release near-real-time estimates and predictions, while later reprocessing produces more stable monthly and long-term series.
The result is a time series of EOP values. A change in one daily value is interpreted in relation to neighboring observations, model uncertainty, and the technique mix used for that product.
How VLBI measures UT1 and Earth orientation
Why quasars provide a reference
In VLBI, two or more radio telescopes observe the same quasar at the same time. The signal reaches each antenna at slightly different moments because the stations are separated by thousands of kilometers. The measured delay depends on the baseline geometry, the quasar’s direction, Earth’s orientation, atmospheric propagation, and the stations’ clocks.
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By observing many quasars from a network of antennas, analysts determine how the Earth-fixed network is oriented relative to the celestial reference frame. The rotation angle needed to align those frames is directly related to UT1. The same observations also help estimate polar motion and celestial pole offsets.
One-hour Intensive sessions
USNO describes VLBI “Intensive” sessions as short observations designed to obtain UT1−UTC quickly. A typical session lasts about one hour, uses two to five radio telescopes selected largely for wide east–west separation, and observes a few dozen quasars. Processing is prioritized so that a result can be reported within about a day of observation.
Because these sessions are optimized for UT1, they are not equivalent to a full global campaign for every EOP. Their geometry and limited duration make them a rapid rotation measurement rather than a complete 24-hour solution.
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Twenty-four-hour VLBI sessions
A 24-hour session uses a worldwide radio-telescope network to observe quasars throughout a full day. The longer duration and broader geometry support a wider set of parameters, including UT1−UTC, polar-motion coordinates x and y, and celestial pole offsets or related precession and nutation quantities.
VLBI therefore supplies a benchmark astronomical measurement of Earth rotation, while the exact network, scheduling, and analysis strategy determine which parameters are best constrained in a particular session.
How other observations contribute
| Technique or input | What is observed | Typical contribution to EOP work | Cadence or role |
|---|---|---|---|
| VLBI | Arrival-time differences for quasar radio signals at separated antennas | Direct UT1 information; 24-hour sessions also estimate polar motion and celestial pole offsets | One-hour Intensives for rapid UT1; 24-hour sessions for broader solutions |
| GNSS/GPS | Carrier-phase and code signals from navigation satellites to ground receivers | Strong input to combined polar-motion and length-of-day estimates | Continuous tracking supports frequent estimates |
| Satellite laser ranging (SLR) | Two-way travel time of laser pulses reflected by satellites | Independent geometric constraints, especially on the terrestrial reference frame and polar motion | Combined with other geodetic solutions |
| Atmospheric angular momentum and other geophysical models | Modeled exchange of mass and angular momentum in air, oceans, water, tides, mantle, and core | Physical interpretation, short-term prediction, and filtering of observed variations | Used as inputs to analysis centers’ combination and prediction systems |
This table describes complementary roles, not a universal ranking of accuracy, price, or geographic coverage. The published sources do not provide one current performance table that fairly compares all techniques under identical conditions.
JPL’s Kalman Earth Orientation Filter (KEOF), for example, combines GPS-derived polar motion and length-of-day information with VLBI UT measurements and other observations, including atmospheric angular momentum inputs. Different products can use different combinations and filtering choices, so two legitimate EOP series may not contain identical values for the same date.
How length of day is calculated
LOD is not generally read directly from a stopwatch. The IERS glossary states: “In practice this quantity is determined by differencing daily values of UT1-UTC.” In simplified form, analysts compare the change in UT1−UTC from one day to the next and express the departure from a 86,400-second SI day:
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LOD − 86,400 seconds = change in UT1−UTC from one day to the next
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Operational products usually express the result in milliseconds per day. A positive departure means that, over that interval, Earth rotated slightly more slowly than the reference 86,400-SI-second day; a negative departure means a slightly shorter rotational day. The sign convention and exact differencing or smoothing method should always be checked in the product documentation.
Because LOD is derived from changes in UT1−UTC, noisy or revised UT1 estimates can alter an early LOD value. Final series may therefore differ from rapid estimates after additional observations and reprocessing become available.
Why the rotation changes
Earth’s rotation responds to exchanges of angular momentum and to movement of mass within the Earth system. IERS geophysical-fluid data products consider contributions associated with:
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- atmospheric winds and pressure;
- ocean currents, tides, and redistribution of seawater;
- hydrology, including movement of water on land and in aquifers;
- ice and other surface mass changes;
- deformation and processes in the mantle; and
- motions and coupling involving the core.
These mechanisms can alter the spin rate, shift the center of mass, or change the orientation of the rotation axis. Their effects overlap, and the reviewed operational pages identify the processes broadly rather than assigning a fixed percentage to each process for a particular date. Consequently, a graph of LOD or polar motion should not be read as a one-to-one record of a single weather event or interior process unless a specific physical attribution study has been performed.
Is Earth’s rotation slowing down?
On long time scales, the rotation has a slowing tendency associated chiefly with tidal interactions, and the NICT VLBI overview presents a long-term series illustrating gradual slowing. That statement does not mean the day lengthens every day or at a constant rate. Atmospheric, oceanic, hydrological, tidal, mantle, and core processes produce shorter-term fluctuations around the long-term behavior.
The appropriate way to answer a question about a particular date is to inspect the relevant UT1−UTC or LOD series, identify whether it is a rapid estimate or a final value, and examine the surrounding interval. A single unusually short or long day is not evidence that the long-term trend has reversed.
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How IERS products keep civil time aligned with rotation
UTC is generated from atomic clocks, whereas UT1 follows Earth’s variable rotation. To keep civil time close to the angle of the Sun, the IERS introduces leap seconds when needed so that the absolute value of UT1−UTC remains below 0.9 seconds. The IERS announces a scheduled leap second through Bulletin C.
A leap second does not make Earth rotate suddenly faster or slower. It changes the UTC label assigned to atomic-clock time so that UTC remains within the specified relationship to UT1. UT1 itself continues to be estimated from Earth-orientation observations.
How estimates are published and revised
IERS provides rapid, monthly, and long-term Earth-orientation data, together with bulletins, downloadable files, analysis tools, and visualizations. Bulletin A is the rapid service product. The published operational description says it is available by 20:00 UTC each Thursday, contains daily estimates with roughly a one-day delay from the latest estimated EOP date, and includes predictions extending up to 360 days after the last data day. Final EOP products are published monthly when available.
These schedules describe the service at the time of publication and can change. Rapid values are useful for spacecraft operations and current monitoring, but they are provisional. For scientific analyses of historical behavior, use a final or consistently reprocessed series and record the product version and retrieval date.
How to inspect an Earth-rotation record yourself
- Choose the parameter. Use UT1−UTC to examine the rotation angle relative to atomic time, LOD for day-length departures, or polar-motion x and y for movement of the pole in the crust-fixed frame.
- Choose the product class. Select a rapid series for current conditions, a monthly or final series for historical work, or a prediction file when planning future observations.
- Check the time scale and units. Confirm whether dates are UTC, whether LOD is in milliseconds per day, and how missing or predicted values are marked.
- Separate observations from forecasts. Plot or tabulate the last observed date separately from the prediction interval; a forecast is not a later measurement.
- Record revisions. Save the product name, issue date, and version because early EOP values can be updated when new VLBI, GNSS, SLR, or model data arrive.
- Compare related parameters. A change in LOD should correspond to a change in the slope of UT1−UTC, while polar motion describes axis orientation rather than spin rate.
What these measurements are used for
Earth orientation is essential whenever a position or pointing solution must connect a spacecraft or ground station to both terrestrial and celestial coordinates. Spacecraft navigation, deep-space tracking, satellite orbit determination, geodesy, astronomy, and high-precision timing all depend on knowing how the Earth-fixed frame is oriented at the observation time.
Ignoring UT1, polar motion, or celestial pole offsets would introduce errors when converting between a station’s coordinates on Earth and a target’s coordinates in the sky. The required precision depends on the mission and observation, but the underlying need is the same: the reference frame itself is moving and rotating in a measurable, time-dependent way.
Common misconceptions
“UTC is the rotation time.”
UTC is the atomic-clock-based civil reference. UT1 is the rotation-based scale. Their difference is the quantity monitored for Earth’s changing rotation angle.
“LOD is measured independently of UT1.”
In standard IERS practice, LOD is obtained by differencing daily UT1−UTC values and comparing the result with 86,400 SI seconds.
“VLBI alone supplies every EOP value.”
VLBI is the direct astronomical route to UT1, but operational combinations also use GNSS, SLR, and geophysical inputs. Each technique constrains a different part of the solution.
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Short-term variations can make individual days longer or shorter. Long-term trend and day-to-day behavior are different questions and require different time windows.
The practical takeaway
Scientists measure changes in Earth’s rotation by tying Earth-fixed stations to a celestial reference, primarily through VLBI observations of quasars, then combining those results with GNSS, SLR, and geophysical models. UT1−UTC tracks the rotation angle against atomic time; LOD is derived from successive UT1−UTC values; and polar-motion coordinates track the axis relative to the crust. IERS and partner analysis centers publish rapid estimates, predictions, and later final products so that both real-time navigation and long-term science can use a consistent record.
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