NDA Geography · Oceanography

Tides and Ocean Movements

Tides are the daily rise and fall of the sea raised by the gravitational pull of the Moon and Sun — large when the three bodies line up (spring), small when the Moon and Sun pull at right angles (neap).

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Why this matters

5 PYQs, mostly EASY–MODERATE. The marks come from the mechanism: tides are gravitational; two tides arrive 12 h 26 min apart; SPRING tides happen at new and full Moon (Sun-Earth-Moon in a line) and have the biggest range; NEAP tides happen at the quarter Moons (Sun and Moon at right angles) and have the smallest range. Learn the geometry once and four of the five questions answer themselves.

Concept 1 of 2

What tides are and why two arrive each day

Intuition

A tide is the periodic rise and fall of the whole ocean, pulled up by the GRAVITY of the Moon (mainly) and the Sun. As the Earth spins, the bulge of raised water passes a coast roughly twice a day. But the Moon also moves forward in its orbit each day, so the place 'catches up' a little later each time — which is why two successive high tides are about 12 hours 26 minutes apart, not exactly 12 hours.

Definition

  • A tide = the periodic rise and fall of ocean water in response to the gravitational pull of the Moon and the Sun.
  • The Moon's pull dominates (it is far closer than the Sun), raising a bulge of water on the near side (and another on the far side).
  • Two successive tides at a place are about 12 hours 26 minutes apart — half of a 'lunar day' (~24 h 52 min). The extra ~26 minutes is because the Moon has moved ahead in its orbit and the Earth must rotate a little further to face it again.
  • Do NOT confuse tides with currents (steady horizontal flows), waves (wind-driven surface energy), or a tsunami (a seismic sea wave).

Worked example

Why are two successive high tides about 12 h 26 min apart rather than exactly 12 hours?
Practice this conceptself-check · 3 quick reps

From the bank · past-year question

Example 1OceanographyEASY
The periodic rise and fall of ocean water in response to gravitational forces is called

[Q78 · Sep · 2021]

12 h 26 min, not 12 h

The intuitive answer '12 hours' is the trap. The correct gap is 12 hours 26 minutes, because the Moon advances in its orbit and the Earth must spin a little further to face it again.

Tide is not a current or a wave

A tide is the GRAVITY-driven vertical rise/fall of the sea. A current is a steady horizontal flow, a wave is wind-driven, and a tsunami is a seismic sea wave — different phenomena.
Drill 1 more on what tides are and why two arrive each day

Concept 2 of 2

Spring tides and neap tides

Intuition

How big a tide gets depends on whether the Sun and the Moon pull together or against each other. When the Sun, Earth and Moon line up (new or full Moon — a 'syzygy'), their pulls ADD, stretching the ocean into the biggest tides: SPRING tides. When the Moon is at right angles to the Sun (the quarter Moons — 'quadrature'), the two pulls partly CANCEL, giving the smallest tides: NEAP tides. (Spring here means 'to spring up', nothing to do with the season.)

Definition

  • Spring tide — Sun, Earth and Moon in a STRAIGHT LINE (syzygy: conjunction = new Moon, OR opposition = full Moon). The solar and lunar pulls add → the GREATEST difference between high and low water (largest tidal range).
  • Neap tide — Moon at RIGHT ANGLES to the Sun as seen from Earth (quadrature, at the first and third quarter Moon). The pulls partly cancel → the SMALLEST tidal range. Neap tides occur every ~14–15 days, coinciding with the quarter Moons.
  • So spring tides need syzygy (conjunction OR opposition), NOT quadrature; neap tides need quadrature.
SPRING TIDESun, Earth, Moon in a line (syzygy)SunEarthlarge bulgesMoonNEAP TIDEMoon at right angles to Sun (quadrature)SunEarthsmall bulgesMoon

Worked example

Of these positions of Sun, Earth and Moon, which produce a SPRING tide: (1) syzygy conjunction, (2) syzygy opposition, (3) quadrature?
Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 2OceanographyMODERATE
Which of the following positions of Sun, Earth and Moon is/are suitable for Spring Tide? 1. SYZYGY Conjunction 2. SYZYGY Opposition 3. Quadrature Select the correct answer using the code given below :

[Q137 · Apr · 2023]

'Spring' has nothing to do with the season

Spring tide means the water 'springs up' — it is the BIGGEST tide and happens twice a month (new + full Moon), in every season. Don't read it as a springtime tide.

Spring needs a LINE; neap needs a RIGHT ANGLE

Spring tide = Sun-Earth-Moon in a straight line (syzygy, pulls ADD). Neap tide = Moon at right angles to the Sun (quadrature, pulls CANCEL). Swapping the two is the classic error.
Drill 2 more on spring tides and neap tides

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

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