UPSC Prelims 2015 Q70: Geography – Oceanography
Tides occur in the oceans and seas due to which among the following?
1. Gravitational force of the Sun
2. Gravitational force of the Moon
3. Centrifugal force of the Earth
- (a) 1 only
- (b) 2 and 3 only
- (c) 1 and 3 only
- (d) 1, 2 and 3
Answer: (d) 1, 2 and 3
Explanation
Causes of tides
Tides are the regular rise and fall of sea level, once or twice a day.
| Force | Causes tides? | Role |
|---|---|---|
| 1. Gravitational force of the Sun | Yes | Weaker effect, because the Sun is far away |
| 2. Gravitational force of the Moon | Yes | The main cause, because the Moon is near |
| 3. Centrifugal force of the Earth | Yes | Makes the second bulge on the side away from the Moon |
Types of tides:
| Tide | When | Position |
|---|---|---|
| Spring tide | New moon and full moon | Sun, Earth and Moon in a straight line; highest tides |
| Neap tide | First and last quarter | Sun and Moon at right angles; lowest range |
The highest tides in the world occur in the Bay of Fundy, Canada.
Answer: (d) 1, 2 and 3.
Why each option is right or wrong
- (a) Incorrect. The Moon's gravity and the centrifugal force also cause tides.
- (b) Incorrect. The Sun's gravitational force also plays a part.
- (c) Incorrect. The Moon's gravitational force is the main cause.
- (d) CORRECT. Tides are due to the Sun, the Moon and the centrifugal force together.
Key point
Tides: Moon's gravity (main), Sun's gravity and centrifugal force. Spring tides at new and full moon; neap tides at the quarters.
Remember
Tides = Moon + Sun + centrifugal force.
Common mistakes
- Leaving out the Sun, whose pull is smaller but real.
- Not knowing why there are two tidal bulges; centrifugal force makes the second.
- Mixing up spring (highest) and neap (lowest range) tides.
Exam tip
Moon first, Sun second, centrifugal force for the far-side bulge.
Sources
- NCERT Class XI — Fundamentals of Physical Geography, Ch 14 (Movements of Ocean Water)
- G.C. Leong — Certificate Physical and Human Geography (The Oceans)