What Is A Neap Tide?
QUICK ANSWER
A neap tide is the smallest tidal range, occurring at first and third quarter moons. At these times, the sun and moon are at right angles to Earth, so their gravitational pulls partially cancel rather than reinforce. The result is lower high tides and higher low tides than usual.
A neap tide is the smallest tidal range of the lunar cycle, the opposite extreme from spring tides. Neap tides occur twice each month when the sun and moon are at right angles relative to Earth. Understanding neap tides reveals how the combined effects of sun and moon gravity produce the rich variation in tidal ranges throughout each month.
When do neap tides occur?
Neap tides occur twice each lunar month at the first quarter (waxing half-moon) and third quarter (waning half-moon) phases. At these times, the moon is at right angles to the sun as viewed from Earth. So while the moon pulls in one direction, the sun pulls perpendicular to that. The opposing geometric effect produces the smallest tidal ranges of the cycle. Neap tides alternate with spring tides on a roughly 14-day cycle, so any given week of the month has either spring or neap conditions.
Why do they have smaller ranges?
Neap tides have smaller ranges because the sun's gravitational pull partially counteracts the moon's during quarter moon phases. The moon creates its tidal bulges along the Earth-moon axis. The sun creates its own bulges along the Earth-sun axis. When these axes are perpendicular (at quarter moons), the sun is essentially pulling water toward the low tide regions of the lunar tide and away from the lunar high tide regions. The net effect is reduced tide range: lower high tides and higher low tides than average.
How are neap tides different from spring tides?
Neap and spring tides are at opposite ends of the tide range spectrum. Spring tides have the largest range; neap tides have the smallest. Spring tides occur at new and full moons (when sun and moon align); neap tides occur at quarter moons (when sun and moon are at right angles). Both still have the same basic semi-diurnal cycle of two highs and two lows per day, separated by the same 12 hours 25 minutes. They alternate, with each type occurring twice per month, two weeks apart.
How does the alternation work?
Spring and neap tides alternate on a 14-day cycle through the lunar month. At new moon (day 0), spring tides occur. About 7 days later at first quarter, neap tides occur. Around 7 days after that at full moon (day 14-15), spring tides return. Another 7 days brings third quarter and another neap tide. The cycle repeats with each lunar month. Each phase of the cycle has gradually changing tidal range, with spring and neap tides representing the peaks and valleys of tidal range variation throughout the month.
A neap tide is the smallest tidal range of the lunar cycle, occurring at first and third quarter moons when sun and moon are at right angles. The perpendicular gravitational pulls partially cancel, producing lower high tides and higher low tides than usual. Neap tides alternate with spring tides on a 14-day cycle, with each type occurring twice per lunar month.
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