Earth rotation period
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Earth Rotation Period: Mean Solar Day and Sidereal Day
The Earth completes one full rotation relative to distant stars, known as a sidereal day, in about 23 hours, 56 minutes, and 4 seconds. However, the period most familiar to us—the mean solar day—is about 24 hours. The difference arises because the Earth is also orbiting the Sun, so it must rotate a bit more each day for the Sun to return to the same position in the sky. This distinction is well established in astronomy, though the exact cause of the difference is still discussed in scientific literature, with some explanations focusing on the relationship between Earth's rotation and its orbit around the Sun rather than distant stars .
Variations in Earth's Rotation Period
Long-Term Changes in Length of Day
The Earth's rotation period is not perfectly constant. Over centuries, the length of the mean solar day (LOD) has been increasing. Historical records and modern measurements show that the LOD increases by about 1.8 milliseconds per century, which is less than the 2.3 milliseconds per century predicted by tidal friction alone. This suggests that other factors, such as post-glacial rebound and core-mantle coupling, also play a role in Earth's rotational changes 16.
Short-Term and Decadal Fluctuations
In addition to the long-term trend, there are fluctuations in Earth's rotation on timescales from years to decades. Analyses have identified periodicities such as a prominent 6-year cycle, an 18.6-year tidal oscillation, and a quasi-periodic 13-year signal. These variations are linked to exchanges of angular momentum between the solid Earth and its fluid envelopes (atmosphere, oceans, and core), though the exact mechanisms are still being studied 39. Decadal fluctuations, with periods around 30 years, have also been observed and are thought to be related to changes in the torques acting on Earth's mantle .
Polar Motion and Nutation
The position of Earth's rotation axis also changes over time. These changes are classified as "polar motion" (movement of the axis relative to Earth's surface) and "nutation" (movement relative to inertial space). Polar motion is generally caused by processes within the Earth and occurs over longer timescales, while nutation is mainly driven by gravitational forces from the Sun and Moon and occurs over shorter, nearly diurnal periods 210.
Measurement and Analysis of Earth's Rotation
Advances in observational techniques have greatly improved the accuracy and resolution of Earth rotation measurements. Modern analyses use historical eclipse records, lunar occultations, and atomic time scales to track changes in Earth's rotation from ancient times to the present. These data provide important constraints for understanding the physical processes affecting Earth's rotation 146.
Conclusion
The Earth's rotation period is not fixed but varies due to a combination of tidal friction, internal geophysical processes, and interactions with the atmosphere and oceans. The mean solar day is gradually lengthening, and periodic fluctuations occur on timescales from years to centuries. Ongoing research continues to refine our understanding of these variations and the mechanisms behind them 1369.
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