Earth Rotation & Universal Time
An interactive exploration of Earth's rotation, time standards, and the relationship between atomic time (UTC) and astronomical time (UT1). Use the 3D globe and calculator to understand how Earth's rotation defines our timekeeping systems.
What Are UTC and UT1?
UTC (Coordinated Universal Time) is the global time standard used by computers, networks, and civil timekeeping worldwide. It is based on extremely precise atomic clocks (TAI - International Atomic Time) that maintain uniform seconds independent of Earth's rotation.
UT1 (Universal Time) is a time scale directly tied to Earth's actual rotation. It measures the rotation angle of Earth relative to distant celestial objects. Unlike UTC's uniform atomic seconds, UT1 reflects the slightly irregular rotation of our planet.
DUT1 (Delta UT1) is the difference: DUT1 = UT1 − UTC. The International Earth Rotation and Reference Systems Service (IERS) publishes DUT1 values to keep the two time scales synchronized. When |DUT1| approaches ±0.9 seconds, a leap second is added to or removed from UTC to bring it back in alignment with UT1.
3D Globe (Earth rotation)
A 3D wireframe globe showing Earth's rotation around its axis. The prime meridian (gold line) and equator (gold ring) rotate with the Earth. The rotation axis (gray dashed line) with N and S labels remains fixed in space. Six major cities are marked with subtle colored dots to provide geographic reference. The Sun (yellow sphere) shows the direction of your calculated solar angle and updates as you change the time or longitude.
UT1 Calculator
This is an illustrative calculation: it applies DUT1 directly to UTC and estimates solar angle from UT1 and longitude. Solar angle here is a simplified local solar angle, not GMST/GAST. For precise work, use full IERS conventions (GMST/GAST, EOP, and nutation models).
Why only the solar angle moves when you change longitude: UT1 is the same everywhere (it's Earth-rotation time), so changing longitude shifts your local hour angle relative to Greenwich—hence the solar angle changes, but UTC/UT1 values do not.
Time Relationships
The mathematical relationships used in this demonstration:
- UT1 = UTC + DUT1 where DUT1 is published by IERS (typically within ±0.9 s)
- Mean solar day = 86,400 SI seconds - UT1 tracks Earth's rotation relative to the Sun
- Sidereal day = 86,164.09 seconds - Earth's true rotation period relative to stars
- Local solar angle ≈ (UT1 hours × 15°/hour) + longitude - simplified calculation for illustration
Note: This page uses simplified solar angle calculations for educational purposes. For precise astronomical work, use GMST/GAST (Greenwich Mean/Apparent Sidereal Time) with full IERS Earth Orientation Parameters (EOP) and nutation models.
Historical Evolution of Time Standards
Local Mean Time
Each city used its own local solar time based on the sun's position. No global coordination existed.
Greenwich Mean Time (GMT)
International Meridian Conference established GMT as the world's time reference, based on mean solar time at Greenwich Observatory. The prime meridian (0° longitude) was set at Greenwich.
Universal Time (UT1)
UT1 was defined as a precise measure of Earth's rotation angle, replacing GMT. Based on astronomical observations rather than the sun's apparent motion.
Coordinated Universal Time (UTC)
UTC became the global time standard, based on atomic clocks (TAI) with leap seconds added to keep it synchronized with UT1. This allows for stable, precise timekeeping while staying aligned with Earth's rotation.
Sidereal vs Solar Day
Sidereal Day
Time for Earth to rotate 360° relative to distant stars. This is Earth's true rotation period.
Solar Day
Time for the Sun to return to the same position in the sky. Longer because Earth moves in its orbit around the Sun.
The solar day is approximately 3 minutes 56 seconds longer than the sidereal day. This difference accumulates: after one year (365.25 solar days), Earth completes 366.25 sidereal rotations. The extra rotation is why we have one more sidereal day than solar days per year.
DUT1 History & Variations
DUT1 (the difference between UT1 and UTC) varies over time as Earth's rotation fluctuates. The IERS publishes DUT1 values, and when |DUT1| approaches 0.9 seconds, a leap second is added or removed from UTC.
This chart shows approximate DUT1 values over recent years. Earth's rotation is slowing due to tidal friction, causing UT1 to gradually drift behind UTC. Leap seconds are inserted to keep them synchronized.
Leap Seconds Timeline
Since 1972, 27 leap seconds have been added to UTC to keep it within 0.9 seconds of UT1. All have been positive (UTC paused for one second), and they typically occur on June 30 or December 31.
Leap seconds are controversial in computing and telecommunications because they create irregularities in otherwise uniform time. In 2022, the International Telecommunication Union voted to eliminate leap seconds by 2035, allowing UTC and UT1 to drift apart by up to one minute.
Earth's Rotation Variations
Earth's rotation is not constant. The length of day (LOD) varies due to multiple factors:
Tidal Friction
Effect: +1.7 ms/century
Moon's gravity creates tidal bulges that slow Earth's rotation. Over geological time, days are getting longer.
Glacial Rebound
Effect: -0.6 ms/century
Post-ice-age crustal rebound redistributes Earth's mass, slightly speeding up rotation (like a spinning ice skater pulling in their arms).
Core-Mantle Coupling
Effect: ±1-2 ms/decade
Electromagnetic and gravitational interactions between Earth's liquid outer core and solid mantle cause irregular variations.
Atmospheric Winds
Effect: ±0.5 ms/season
Seasonal changes in atmospheric circulation transfer angular momentum, causing daily variations in Earth's rotation rate.
Current measurements show Earth's rotation is gradually slowing by about 1.1 milliseconds per century on average. However, recent data (2020-2024) shows Earth has been rotating slightly faster than usual, making days shorter by fractions of a millisecond.