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).

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:

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

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.

The Delta-T theory

The historical Delta-T graph from 1650 AD shows a distinctive V-shape:

This V-shape occurs because:

1. Before ~1820 AD: The Length of Day was less than 86,400 seconds, so UT was ahead of TT (ΔT was negative/decreasing)
2. Around 1820 AD: LOD crossed exactly 86,400 seconds (ΔT reached minimum)
3. 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

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

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.

Summary

1. Two time systems exist: TT (fixed atomic time) and UT (observed Earth rotation)
2. Delta-T (ΔT) measures the difference: ΔT = TT − UT
3. LOD varies cyclically - not in a straight line, but with fluctuations around a trend
4. The V-shape in ΔT occurred because LOD crossed 86,400 seconds around 1820 AD
5. The model connects LOD cycles to the 20,868-year perihelion precession
6. The model predicts Earth’s rotation will speed up until ~11,680 AD

Continue to Moon & Planets to learn about lunar and planetary movements in the model.