Ocean tides are primarily caused by the gravitational pull of the Moon and, to a lesser extent, the Sun. The Moon's gravity creates bulges of water on both the side of Earth facing the Moon and the opposite side. As Earth rotates, these bulges move around the planet, causing the cyclical rise and fall of sea levels we experience as tides. The Sun's gravity also influences tides, creating smaller bulges. When the Sun, Earth, and Moon align (during new and full moons), these bulges combine to produce larger spring tides. When the Sun and Moon are at right angles to each other (during first and third quarter moons), their gravitational forces partially cancel, resulting in smaller neap tides. The complex shapes of ocean basins and coastlines also affect the timing and height of tides at specific locations. Friction between the tides and the ocean floor gradually slows Earth's rotation, lengthening the day by a very small amount over time.
The National Aeronautics and Space Administration (NASA) publication, "Ocean Tides and the Earth's Rotation (2001)," elucidates the intricate relationship between the cyclical rising and falling of Earth's oceanic tides and the gradual deceleration of the planet's rotation. This deceleration, while subtle, accumulates over vast stretches of geological time, measurably lengthening the duration of a day. The primary causal agent of this phenomenon is identified as the frictional forces generated by the relentless movement of tidal waters across the ocean floor. This friction, acting as a sort of natural brake, dissipates the Earth's rotational energy, slowing its spin.
The document meticulously explains how the gravitational influence of both the Moon and the Sun, the celestial bodies primarily responsible for tidal activity, generates bulges of water on opposing sides of the Earth. These bulges, effectively high tide points, are not static but rather travel across the ocean's surface, following the Moon as it orbits our planet and, to a lesser extent, the Sun. As these tidal bulges traverse the globe, they encounter resistance from the ocean bottom, particularly in shallow coastal regions and over irregular seabed topography. This constant interaction between the tidal waters and the solid Earth creates friction, which converts some of the planet's rotational kinetic energy into heat, gradually diminishing the Earth's angular momentum and consequently its rate of rotation.
Furthermore, the document elaborates on the complexity of this interaction, acknowledging that the actual tidal bulges do not perfectly align with the Earth-Moon axis due to the continents obstructing the free flow of water. This misalignment introduces a torque, a rotational force, that acts against the Earth's rotation, further contributing to the deceleration effect. The document also details how this tidal braking effect has observable consequences beyond the lengthening of a day, influencing the Moon's orbital characteristics. Specifically, the angular momentum lost by the Earth is transferred to the Moon, causing it to slowly spiral outwards, increasing its orbital distance from our planet.
In conclusion, "Ocean Tides and the Earth's Rotation" provides a detailed explanation of the complex interplay between tidal forces and the Earth's rotational dynamics. It establishes the frictional forces generated by tidal movement as the primary mechanism for the ongoing, albeit gradual, slowing of Earth's rotation and links this deceleration to observable changes in the Moon's orbit. This deceleration, while imperceptible on a human timescale, accumulates over geological epochs, offering a fascinating example of the interconnectedness of celestial mechanics and terrestrial processes.
Summary of Comments ( 3 )
https://news.ycombinator.com/item?id=43697252
HN users discuss the complexities of tidal forces and their effects on Earth's rotation. Several highlight that the simplified explanation in the linked NASA article omits crucial details, such as the role of ocean basin resonances in amplifying tides and the delayed response of water to gravitational forces. One commenter points out the significant impact of the Moon's gravity on Earth's angular momentum, while another mentions the long-term slowing of Earth's rotation and the Moon's increasing orbital distance. The importance of considering tidal forces in satellite orbit calculations is also noted. Several commenters share additional resources for further exploration of the topic, including links to university lectures and scientific papers.
The Hacker News post titled "Ocean Tides and the Earth's Rotation (2001)" linking to a NASA article about tides has a moderate number of comments, exploring various aspects of the topic.
Several commenters discuss the complexity of tidal forces and the factors influencing them. One points out that the simplified explanation presented in the linked NASA article doesn't capture the full picture, as the actual tidal bulge is significantly offset from the direct line between the Earth and the Moon due to the Earth's rotation and the inertia of the oceans. This leads to a discussion about the lag in the tidal bulge and its effect on the Earth's rotation, with one user explaining how this lag creates a torque that gradually slows down the Earth's spin and transfers angular momentum to the Moon, causing it to recede from Earth.
Another commenter dives into the impact of continents on tides, noting that they complicate the picture further by obstructing the free movement of water and creating different tidal patterns in various locations. A subsequent reply elaborates on how the shape of ocean basins and resonances can amplify or diminish tidal effects.
Some comments focus on the long-term consequences of tidal forces. One user discusses the eventual tidal locking scenario, where the Earth's rotation would synchronize with the Moon's orbit, leading to a situation where the same side of the Earth always faces the Moon. Another commenter mentions the impact of solar tides, although acknowledging they are weaker than lunar tides.
A couple of commenters offer additional resources, such as links to websites with tide predictions and a Wikipedia page on tidal acceleration. One user humorously suggests that the slowing of Earth's rotation is a good thing, as it gives us all slightly longer lifespans.
While there isn't a single overwhelmingly compelling comment, the discussion as a whole provides valuable insights into the intricacies of tides and their effects on the Earth-Moon system, going beyond the simplified explanation provided in the linked NASA article. The comments highlight the importance of factors like the Earth's rotation, the inertia of the oceans, the shape of continents and ocean basins, and the gravitational influence of the Sun.