Timekeeping & Delta-T
Earth’s rotation is not constant. It speeds up and slows down in cycles, causing the Length of Day (LOD) to vary. This variation creates the difference between atomic time and observed time - known as Delta-T (ΔT).
This page builds on concepts from Days & Years, which explains how day and year lengths are interconnected through Earth’s precession cycles.
Earth’s Rotation and Length of Day
Scientific measurements show Earth’s rotation has gradually been slowing down over the past century, resulting in a slight increase in the Length of Day - on average around 1.7 milliseconds per century.
However, this slowing is not uniform. Earth’s rotation varies in cycles across different timescales, sometimes speeding up and sometimes slowing down relative to the long-term trend.
Two Timekeeping Systems
Two major definitions of time exist:
| System | Definition | Behavior |
|---|---|---|
| Terrestrial Time (TT) | Fixed at exactly 86,400 SI seconds per day | Constant, based on atomic clocks |
| Universal Time (UT) | Based on Earth’s actual observed rotation | Varies with Earth’s rotation speed |
Terrestrial Time (TT) is a theoretical uniform time scale that coincidentally aligned with the mean solar day around 1820 AD. It’s independent of Earth’s actual rotation and is used for astronomical predictions.
Universal Time (UT) represents the observed rotation angle of Earth relative to an inertial reference frame - it’s time as actually experienced on Earth.
What is Delta-T?
Delta-T (ΔT) is the difference between these two time systems:
ΔT = TT − UT- When Earth rotates slower (LOD > 86,400 seconds): UT falls behind TT, and ΔT increases
- When Earth rotates faster (LOD < 86,400 seconds): UT gains on TT, and ΔT decreases
Historical Values
| Era | ΔT Value |
|---|---|
| ~1900 AD | ~0 seconds |
| 2020 AD | ~69 seconds |
The International Earth Rotation and Reference Systems Service (IERS) maintains the official ΔT values. Leap seconds are occasionally added to UTC to keep it synchronized with atomic time.
Note: All estimated values of ΔT before 1955 AD depend on observations of the Moon, either via eclipses or occultations. Only after 1955 AD are values directly measured.
The Delta-T theory
The historical Delta-T graph from 1650 AD shows a distinctive V-shape:
This V-shape occurs because:
- Before ~1820 AD: The Length of Day was less than 86,400 seconds, so UT was ahead of TT (ΔT was negative/decreasing)
- Around 1820 AD: LOD crossed exactly 86,400 seconds (ΔT reached minimum)
- After ~1820 AD: LOD exceeded 86,400 seconds, so TT pulled ahead of UT (ΔT increasing)
The underlying trend is simple: the Length of Day has been slowly increasing over centuries, crossing the 86,400-second mark around 1820-1850 AD.
The Model’s Interpretation
In the Holistic Universe Model, the Length of Day cycle follows the perihelion precession cycle of 20,868 years:
- LOD reaches maximum when Earth is at one extreme of the perihelion cycle
- LOD reaches minimum at the opposite extreme
- The 1246 AD perihelion alignment marks a key reference point in this cycle
Important: The perihelion precession determines the long-term trend of LOD, but actual LOD values also fluctuate around this trend on shorter timescales. These fluctuations - sometimes above the trend, sometimes below - may correlate with climate patterns (warmer/colder periods). The model predicts the trend; the fluctuations require additional study.
Model Prediction
The model makes a specific testable prediction about Earth’s rotation:
Earth’s rotation will begin speeding up again, with LOD decreasing until approximately 11,680 AD (perihelion at aphelion point), then slowing down again.
The Tidal Friction Question
Important distinction: The model’s prediction involves the long-term trend over millennia, not short-term variations.
Established physics (tidal friction):
- Lunar laser ranging measures the Moon receding at ~3.8 cm/year
- This directly implies angular momentum transfer from Earth to Moon
- Earth’s rotation must be slowing on average (~2.3 ms/century over geological time)
- Eclipse records over 2,700 years confirm long-term slowing
Short-term variations (observed):
- Earth’s rotation fluctuates on timescales of days to decades
- Causes include: core-mantle coupling, atmospheric angular momentum, post-glacial rebound
- Recent years (2020-2021) saw unexpected speedup
- These variations are around the long-term slowing trend
The model’s claim:
- The model proposes an additional millennial-scale cycle superimposed on the tidal slowing
- This cycle has ~20,868-year period linked to perihelion precession
- Currently we are past the maximum (LOD decreasing phase)
- The model does NOT claim tidal friction doesn’t exist
Key question: Can a millennial cycle exist alongside (or partially counteract) tidal slowing? The model predicts yes; standard theory says tidal slowing dominates over all timescales. This is testable with continued precision measurement over centuries.
Recent Observations (2020–2022)
Starting in 2020, Earth unexpectedly began rotating faster — a trend the model predicts but standard theory does not:
| Observation | Detail |
|---|---|
| 2020 | 28 shortest days since atomic clocks were invented |
| June 29, 2022 | Shortest day ever recorded — 1.59 milliseconds under 24 hours |
| 2019–2022 trend | Average LOD shifted from +0.39 ms to −0.25 ms relative to 86,400 seconds |
Scientists at the International Earth Rotation and Reference Systems Service (IERS) acknowledge difficulty predicting LOD more than six months ahead. The multi-year speedup trend remains poorly explained by standard models, which attribute short-term LOD variations to ENSO cycles, core-mantle coupling, and atmospheric angular momentum.
How the model aligns: The model predicts that we are past the LOD maximum (1246 AD) and that Earth’s rotation should be gradually speeding up over the coming millennia, reaching a minimum LOD around 11,680 AD. The 2020–2022 speedup is qualitatively consistent with this prediction.
The model predicts the long-term trend of LOD, not short-term fluctuations from ENSO, volcanic events, or core dynamics. The 2020–2022 data is consistent with the trend but does not prove it — continued observation over decades is needed.
Day Length Stalled for 1 Billion Years
A landmark 2023 paper further challenges the assumption that Earth’s rotation has slowed monotonically:
Mitchell & Kirscher (2023, Nature Geoscience 16, 567) found that Earth’s day length stalled at approximately 19 hours for roughly 1 billion years during the mid-Proterozoic (2.0–1.0 Ga). They proposed that atmospheric thermal tides from solar heating balanced the decelerative torque of lunar oceanic tides, temporarily stabilizing Earth’s rotation.
This finding proves two things:
- LOD dynamics are more complex than simple tidal deceleration — additional mechanisms can influence or even reverse the tidal slowing
- Cyclical LOD behavior is physically possible — if atmospheric tides could halt rotational slowing for a billion years, other mechanisms could create cyclical variations
The model proposes a 20,868-year LOD cycle superimposed on the long-term tidal trend. The Mitchell & Kirscher finding establishes that such complex rotational dynamics are not unprecedented.
Summary
- Two time systems exist: TT (fixed atomic time) and UT (observed Earth rotation)
- Delta-T (ΔT) measures the difference: ΔT = TT − UT
- LOD varies cyclically - not in a straight line, but with fluctuations around a trend
- The V-shape in ΔT occurred because LOD crossed 86,400 seconds around 1820 AD
- The model connects LOD cycles to the 20,868-year perihelion precession
- The model predicts Earth’s rotation will speed up until ~11,680 AD
For the complete Delta-T and LOD formulas, see Formulas. For derivations, see Formula Derivation.
Continue to Moon & Planets to learn about lunar and planetary movements in the model.