Questions: Tides: Gravitational Forcing and Tidal Patterns

Question 1

If the Moon's gravity pulls ocean water toward the Moon, why is there a high tide on the OPPOSITE side of Earth from the Moon simultaneously?

• A. The Sun's gravity counteracts the Moon's pull, pushing water to the far side
• B. Earth's rotation creates centrifugal force that flings water to the far side
• C. The tidal force is a differential force: the far side experiences weaker-than-average gravitational pull, so inertia from Earth-Moon orbital motion carries water outward relative to Earth's center
• D. There is no high tide on the far side; the second daily high tide is caused by Earth rotating through the Moon-side bulge twice

Answer: (C)

This is the most misunderstood aspect of tides. The Moon pulls on Earth as a whole, but the near side is pulled more strongly and the far side more weakly than Earth's center. The tidal force is the difference in gravitational pull from one side to the other. On the far side, the gravitational pull is slightly weaker than average, and the orbital motion (inertia) 'outpaces' gravity slightly, producing a bulge pointing away from the Moon. Both bulges arise from the differential nature of the tidal force, not from two separate mechanisms.

Question 2

The Bay of Fundy has the world's largest tidal range (>16 meters). What best explains this, given that its geographic location doesn't make it face the Moon more directly than other places?

• A. It is located at a geographic pole where tidal forces are concentrated
• B. The bay's funnel shape and length create a natural resonance frequency close to the semidiurnal tidal period, amplifying the tidal signal dramatically
• C. Exceptionally strong spring tides permanently persist in that region due to local magnetic anomalies
• D. Deep water in the bay focuses tidal energy, similar to how ocean waves grow in shallow water

Answer: (B)

Basin resonance — not geography relative to the Moon — determines local tidal range. The Bay of Fundy is approximately 270 km long with a natural sloshing period close to 12.4 hours (the semidiurnal tidal period). This near-resonance amplifies each tidal cycle, much like pushing a child on a swing at exactly its natural period. This explains why a location's tidal range can be far larger or smaller than neighboring coastlines.

Question 3

Spring tides occur when the Moon is closest to Earth (at perigee), producing stronger gravitational pull.

• T. True
• F. False

Answer: False

Spring tides occur when the Sun, Earth, and Moon are aligned — at new moon and full moon — so the tidal forces of the Sun and Moon add together. This happens twice per month regardless of the Moon's distance from Earth. Perigean spring tides (when spring tides coincide with the Moon's closest orbital approach) are unusually large, but spring tides themselves are defined by alignment, not by lunar distance.

Question 4

Most coastal locations experience two high tides per day because Earth rotates through the Moon's gravitational field twice in 24 hours.

• T. True
• F. False

Answer: True

Roughly correct, with an important qualifier: the cycle is 24 hours and 50 minutes, not exactly 24 hours, because the Moon advances in its orbit while Earth rotates. Earth must rotate slightly extra to 'catch up' to the Moon's new position each day. This is why tide times shift by about 50 minutes later each day. Most locations pass through both the near-side and far-side tidal bulges once each, producing two high tides per tidal day.

Question 5

Why don't the highest tides occur when the Moon is directly overhead at a given location, and what actually determines the timing of high tide at a specific coast?

Model answer: The tidal bulges from the Moon represent a forcing signal, but each ocean basin responds like a resonant system — water sloshes within the basin at its own natural frequency. The response depends on basin geometry, depth, and how that geometry resonates with the tidal forcing period. Additionally, Coriolis forces rotate the tidal wave around amphidromic points. The result is that the timing of high tide at a specific coast reflects the basin's resonance and geography, not just the Moon's overhead position. High tide can be hours ahead or behind the Moon's transit.

This is why tidal prediction requires harmonic analysis with dozens of constituents rather than simply tracking the Moon's position. The ocean doesn't respond instantaneously — it has its own dynamics. Understanding the difference between tidal forcing (astronomical) and tidal response (oceanographic) is key to predicting real tides.