What is a tidal force? How do tidal forces produce tides in the Earth's oceans?

Understanding the gravitational field gradient is essential when studying tidal forces. The concept refers to the variation in gravitational pull experienced across an object's length or volume. In simpler terms, imagine an invisible tug of war happening across the planet's body due to varying forces of gravity.

For instance, when the Moon is overhead, its gravity pulls on the Earth's surface. But as gravity's strength decreases with distance, the Earth's side closer to the Moon feels a stronger pull than the side farther away. This difference in strength across the Earth's body—that's the gravitational field gradient. Now, because the Earth is not a uniform sphere of rigid matter but rather has oceans that can flow and deform, this uneven gravitational tug results in the movement of the water, culminating in what we experience as tides.

The Moon's gravitational pull is not just responsible for romantic nights; it actually shapes the Earth's water bodies. This pull is a critical part of the dance that leads to the formation of tides on our planet. The Moon's gravity doesn't affect the Earth uniformly; instead, the nearest point to the Moon experiences the most significant attractable force.

This is more noticeable in bodies of water, as opposed to the solid ground, due to water's fluidity—it succumbs to this force, causing the water to bulge towards the Moon. Imagine a child pulling a blanket towards themselves to feel cozy; the nearest side of the blanket lifts to follow the pull—similarly, the Moon 'cozies up' with the water beneath it, creating what we call the tidal bulge.

The creation of tides is like a slow-moving wave circling our planet, influenced by not only the Moon's gravitational pull but also by the Sun to a lesser extent. When the Moon's pull causes water to bulge towards it, we have what is known as a high tide. Conversely, areas perpendicular to the line connecting the Earth's center to the Moon experience a decrease in water levels, leading to low tides.

Interestingly, the bulge isn't limited to the area beneath the Moon. Since the Earth and the water are pulling at each other, there's a counter-bulge on the opposite side. This is why, at any given time, there are two high tides and two low tides occurring somewhere on Earth. Tides are the result of the water trying to reach equilibrium in the continuously changing gravitational field it's immersed in.

Now let's consider the Earth's rotation—our planet's daily spin on its axis. It's not just a cool fact that gives us day and night; it is also instrumental in the tide's ebb and flow rhythm. As the Earth rotates, the bulges caused by the Moon's gravitational pull shift around the globe. This means tides are constantly on the move.

The Earth spins eastward, so high tides travel westward. This is why coastal areas experience cyclic changes in tidal levels throughout the day. Typically, most places will see two high tides and two low tides, roughly six hours apart, as a result of this celestial choreography. Earth's consistent rotation ensures that the tidal processes maintain a predictable schedule, which has influenced human activity and biological rhythms for millennia.