Question

Explain why two high tides occur every day, even though the Moon lies directly above any portion of Earth only once a day.

Short answer

Two high tides occur due to the Moon's gravitational pull and Earth's rotation, causing two bulges in the water daily.

Step-by-step solution

Understanding the Moon's Gravitational Pull

The Moon exerts a gravitational pull on the Earth that causes the ocean water to bulge out on the side of Earth nearest to the Moon. This bulge is what we perceive as a high tide.

Analyzing the Inertia of Ocean Waters

On the side of Earth opposite to the Moon, inertia causes water to form a second bulge. This happens because the Earth is pulled toward the Moon more than this ocean water, causing another high tide.

Explaining Earth's Rotation

As Earth rotates, different geographic locations pass through these bulges of water. Since the Earth makes a full rotation approximately every 24 hours, each location experiences two high tides.

Interpreting the Tidal Cycle

Due to Earth's rotation and the fact that the Moon orbits the Earth, each location typically experiences two high tides roughly 12 hours apart. This results in two high and two low tides every day.

Key concepts

Moon's Gravitational Pull

The Moon's gravitational pull is a powerful force that plays a significant role in creating tides on Earth. Imagine the Moon as a giant magnet, attracting the waters of the Earth toward it. On the side of Earth closest to the Moon, this gravitational pull causes ocean water to bulge outward. This bulge creates what we know as a high tide.
As the Moon moves in its orbit, it continuously pulls on the Earth's water, creating these bulges. It's this consistent pull that makes the tide levels change throughout the day as the Moon wields its gravitational influence.
When the Moon's gravity pulls on the oceans, it not only affects the side facing it but also causes effects around the entire globe, leading to phenomena like tidal ranges and changes in water levels.

Earth's Rotation

Earth's rotation is a significant factor in the way tides work. The Earth spins on its axis once every 24 hours, which means any given point on the Earth's surface passes through both high and low tide areas during this period. As the Earth rotates, different parts of the world move past the bulges created by the Moon’s gravitational pull.
This rotation means that in approximately a 24-hour span, a given location will experience the bulge (high tide) twice, as well as move through the low points (low tide) in between.
- The rotational movement of Earth distributes the effect of tidal forces over the entire planet, creating patterns of high and low tides as locations rotate into and away from the tidal bulges. - This rhythmic passage of high and low tide is a natural result of Earth’s regular and consistent rotational motion.

Inertia of Ocean Waters

Inertia is the tendency of objects to resist changes in their state of motion. For ocean waters on Earth, inertia has an intriguing effect when combined with the Moon's gravitational forces. On the opposite side of the Earth from the Moon, the ocean waters are somewhat "left behind" as the solid body of the Earth moves towards the Moon.
This is due to the inertia of the water, which tends to remain in its original place. As a result, the ocean becomes stretched out and forms another bulge similar to the one on the side facing the Moon.
- This secondary bulge happens due purely to inertia, creating a second high tide despite the absence of a direct gravitational pull from the Moon. - Inertia and gravity work together to produce the two high tides observed in most places each day.

Tidal Cycle

The tidal cycle consists of the regular rise and fall of sea level caused by the gravitational interactions with the Moon as well as the Sun. As the Earth rotates and the Moon orbits, this cycle repeats approximately every 12 hours. Each location on Earth typically experiences two complete tidal cycles every lunar day.
During a tidal cycle: - Two high tides and two low tides occur approximately every 24 hours. - High tides are spaced roughly 12 hours apart. - The predictability of the tidal cycle allows for accurate forecasts of tide times, which is crucial for maritime activities and coastal living.
The consistency of this cycle is mainly due to the combined effects of Earth’s rotation and the Moon's gravitational pull, while the intricate dance of Earth, Moon, and Sun creates slight variations in the cycle depending on positions and distances at any given time.