Consider the following statements with reference to neap tides: 1. Normally, there is a seven-day interval between two neap tides. 2. At this time, the Sun and the Moon are at right angles to each other. 3. The Moon's attraction is diminished by the counteracting force of the Sun's gravitational pull. Which of the statements given above is/are correct?
- (a)1 only
- (b)2 only
- (c)1 and 2
- (d)2 and 3
Correct — D, 2 and 3. Statements 2 and 3 are the standard description of a neap tide and are two halves of one explanation: at the first and third quarters of the moon the Sun and Moon are at right angles as seen from the Earth, so the Sun's tide-raising pull works across the Moon's instead of with it, and the Moon's effect is cut back by that counteracting force. The result is the small tidal range that defines a neap. Statement 1 is the one that has been altered. Neap tides come at the two quarter moons of a lunar month, so one neap follows another after about a fortnight. Seven days is the interval between a spring tide and the neap tide that follows it, and moving that figure from the spring-to-neap gap to the neap-to-neap gap is exactly the change the examiner has made.
- (a)1 only — Keeps the one false statement and rejects both true ones. The right-angle geometry at the quarter moons is the definition of a neap tide.
- (b)2 only — Statement 2 is true, but so is statement 3 — the two are the same fact expressed as geometry and then as force. The Sun's pull opposing the Moon's is why the range is small.
- (c)1 and 2 — The most tempting option, because it pairs the altered interval with a correct statement. A fortnight, not a week, separates two neap tides.
Tidal range depends on how the Sun's tide-raising force lines up with the Moon's. At new moon and full moon the three bodies are in line, the two forces add, and the tide is a spring — high tides higher and low tides lower than average. At the first and third quarters the Sun and Moon stand at right angles from the Earth, the forces partly cancel, and the tide is a neap, with a reduced range. Springs and neaps alternate about every seven days, so each type recurs about every fourteen or fifteen days.
This is a favourite way of building a wrong statement: keep a genuine number from the textbook and move it to a neighbouring quantity. Seven days is real, but it belongs to the spring-to-neap interval, not to the gap between two neaps. Draw the lunar month as four quarters and the arithmetic becomes obvious — spring at new moon, neap at first quarter, spring at full moon, neap at third quarter, each step about a week, so the same kind of tide comes round in about a fortnight. The Moon's tide-raising influence is a little over twice the Sun's, which is why the Sun can weaken a tide but never reverse it.
- Neap tides occur at the first and third quarters of the Moon, when the Sun and Moon are at right angles from the Earth.
- At neap tide the Sun's pull counteracts the Moon's, so the tidal range is smaller than average.
- Successive neap tides are about a fortnight apart; the seven-day interval is between a spring tide and a neap tide.
- Spring tides occur at new moon and full moon, when the Sun and Moon are in line and their pulls reinforce.
- The Moon's tide-raising force is a little over twice the Sun's, so the Sun modifies the tide rather than dominating it.
- New moon — Sun, Moon and Earth in line: SPRING tide, largest range
- About seven days later, first quarter — Sun and Moon at right angles: NEAP tide
- About seven days later, full moon — in line again: SPRING tide
- About seven days later, third quarter — right angles again: NEAP tide
- So neap to neap is about a fortnight, not a week
The seven-day figure in statement 1 is genuine, but it belongs to the step from a spring tide to a neap tide.
- Accepting a genuine number that has been attached to the wrong interval.
- Treating the Sun as irrelevant to tides because the Moon dominates.
- Reversing spring and neap, and placing neaps at the full moon.
A three-statement item in which the false sentence is made by moving a real figure from one interval to an adjacent one.
Assertion (A): During the Neap Tides, the high tide is lower and the low tide is higher than usual. Reason (R): The Neap Tide, unlike the Spring Tide, occurs on the New Moon instead of on the Full Moon.
- (a) Both A and R are individually true, and R is the correct explanation of A
- (b) Both A and R are individually true, but R is NOT a correct explanation of A
- (c) A is true, but R is false
- (d) A is false, but R is true
Answer(c) A is true, but R is false
The same topic with the error placed in the lunar phase instead of the interval. The reduced range is right; the claim that a neap comes at new moon is wrong, since new moon is when the Sun and Moon line up and produce a spring tide.
Which one of the following statements is correct about Spring tides?
- (a) High tide is higher than average high tide and Low tide is higher than average low tide.
- (b) High tide is lower than average high tide and Low tide is higher than average low tide.
- (c) High tide is higher than average high tide and Low tide is lower than average low tide.
- (d) High tide is lower than average high tide and Low tide is lower than average low tide.
Answer(c) High tide is higher than average high tide and Low tide is lower than average low tide.
CAPF's own officially keyed statement of the other half of the cycle. Spring tide means a bigger range in both directions; neap tide, described in this item, means a smaller one.
- practice — not a real PYQ
Spring tides occur when the Moon is in which phase?
- (a)First quarter only
- (b)Third quarter only
- (c)New moon and full moon
- (d)First and third quarters
Answer(c) New moon and full moon — the Sun and Moon are in line and their tide-raising forces add.
- practice — not a real PYQ
The approximate interval between a spring tide and the neap tide that follows it is
- (a)one day
- (b)seven days
- (c)fourteen days
- (d)twenty-eight days
Answer(b) seven days — a quarter of the lunar month.