Neap tide

Topic: Movements of Ocean Water: Waves, Tides and Currents · NCERT: Class 11, Ch 13 "Movements of Ocean Water"

Meaning

A neap tide is the tide with the smallest tidal range (the smallest gap between high water and low water). It comes twice a month, at the Moon's first quarter and last quarter, when the Sun and the Moon pull on the Earth at right angles (90°) and their pulls partly cancel each other [2]. It matters because it shows that the Moon controls the tides and the Sun only makes them bigger or smaller. It also explains why tidal power output and port timings change through the month.

Explanation

How a neap tide forms: cause → process → result

• Cause: the Sun and the Moon pull at right angles.

• At first quarter and last quarter (half moon), the Sun's pull on Earth runs at 90° to the Moon's pull.

• Process: the two bulges of water do not line up.

• The Moon raises a bulge of water on the side facing it and on the opposite side.
• The Sun's bulge forms where the Moon is making low water.

• So the Sun's pull partly fills in the low tide and partly takes away from the high tide.

• Result: a small tidal range.

• High tides are lower than usual and low tides are higher than usual. This gives the smallest range of the month [2].

Why tides do not stop at neap

• The Sun is about 27 million times heavier than the Moon, but it is about 390 times farther from Earth [3].
• Tide-making force gets weaker with the cube of distance, so the Sun's tidal pull is cut by about 59 million times [3].
• Result: the Sun's tide-generating force is only about half the Moon's [3]. NCERT puts it as the Moon's pull being more than twice the Sun's.
• At neap, the Sun's pull works against the Moon's, but it is too weak to cancel it.
• So the Moon's pattern still wins, and the tides only become smaller [3].

The spring–neap cycle

• Spring tide (Sun, Moon and Earth in one straight line, at new moon and full moon) → about 7 days → neap tide → about 7 days → spring tide [2].
• This follows the lunar month of about 29.5 days. So there are two neap tides and two spring tides each month.
• Conditions a neap tide needs:
• The Moon is at its first or last quarter (half moon).
• The Sun and Moon are at 90° to each other as seen from Earth.
• The pulls of the Sun and Moon partly cancel each other.

What else can change the size of a neap tide

• The Moon's path around Earth is an ellipse (a stretched circle), so its distance keeps changing.
• At perigee (Moon closest to Earth, about 363,300 km [5]) the pull is stronger and ranges get bigger.

• At apogee (Moon farthest, about 405,500 km [5]) the range is smaller than normal.

• The Earth's distance from the Sun changes too. At perihelion (around 3 January) ranges are larger. At aphelion (around 4 July) they are smaller.

• Coast shape and depth also change the local range. For example, funnel-shaped gulfs squeeze the tide and make the water pile up higher.

Where it occurs

• All oceans and all coasts: neap tides happen everywhere twice a month, because they come from the positions of the Sun, Moon and Earth, not from any one place.
• India's coast: most of the coast has a semi-diurnal tide (two highs and two lows a day of about equal height), on both the Arabian Sea and the Bay of Bengal sides. At neap, these daily highs are lower and the lows are higher.
• Tidal energy sites in India: output falls at neap tides and rises at spring tides:
• Gulf of Khambhat (Cambay), Gujarat: about 7,000 MW potential [6]
• Gulf of Kachchh (Kutch), Gujarat: about 1,200 MW [6]
• Sundarbans, West Bengal (Gangetic delta): about 100 MW [6]

• India's total estimated potential is about 8,000 MW [6][7].

• Other coasts: the same neap pattern applies on top of the local tide type. Examples are the diurnal tide in the Gulf of Mexico and the mixed tide on the west coast of North America [2].

Don't confuse with

• Spring tide: comes at new moon and full moon, when the Sun, Moon and Earth are in a straight line (syzygy). It gives the largest range, while neap gives the smallest [2]. Neither one is linked to a season.
• Apogean tide: a smaller range because the Moon is far away (apogee). Neap gives a smaller range because of the 90° angle between the Sun and Moon, not because of distance.
• Ebb tide: the falling tide that comes every day, as water moves from high to low water. A neap tide is a fortnightly (once every two weeks) phase of small range, and it still has its own flood and ebb.
• Diurnal tide: one high and one low each day [2]. This is a type by frequency. Neap is a type by Sun–Moon–Earth position.

Prelims Hooks

• Neap tide → Moon at first quarter and last quarter, with the Sun and Moon at right angles (90°) to the Earth.
• Neap tide comes about 7 days after a spring tide, and twice a month [2].
• Neap tide → lower highs and higher lows, the smallest tidal range [2].
• Tides do not stop at neap because the Sun's tide-generating force is only about half the Moon's [3].
• Trap: "Neap tides happen at full moon." Wrong. Full moon and new moon bring spring tides.
• Tidal power output falls at neap tides. Potential ranking: Gulf of Khambhat (~7,000 MW) > Gulf of Kachchh (~1,200 MW) > Sundarbans (~100 MW) [6].

Mains Points

• Astronomy explains the monthly rhythm of tides (GS-I):
• The Sun–Moon angle decides the spring–neap cycle. Neap tides at the quarters give the smallest range.

• This affects navigation, port entry for deep ships, and fishing in shallow estuaries (river mouths where fresh water meets the sea), where water depth matters.

• Tidal energy is predictable but not steady (GS-III):

• Barrage output drops at neap tides and peaks at spring tides, especially at perigee. So plants need storage or backup.
• India's potential is about 8,000 MW, mostly in the Gujarat gulfs [6]. Ocean energy has counted as renewable energy since 2019 [8].

• High cost and harm to mudflats, mangroves and fisheries are still problems.

• Coastal hazard planning (GS-III, disasters):

• Neap periods bring lower high tides. Spring and perigean spring tides ("king tides") bring the highest water [4].
• Adding tide calendars to cyclone warnings for the Bay of Bengal coast and the Sundarbans helps judge how high a storm surge may reach.

Related concepts

• Semi-diurnal tide
• Diurnal tide
• Mixed tide
• Spring tide
• Perigee
• Apogee
• Ebb and flood tide

Read more

• Types of tides: by frequency and by the sun-moon-earth positions