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Tides: why there are two a day, and spring vs. neap tides

Summary

Tides are very long waves caused by the pull of the moon and the sun. Most coasts get two high tides and two low tides in each lunar day of about 24 hours 50 minutes, so tide times shift later each day. Spring tides, with a bigger range, happen near new and full moon. Neap tides, with a smaller range, happen near the quarter moons.

Draft. License: CC BY 4.0 (this summary). Sources: [S1] NOAA National Ocean Service, "Tides and Water Levels: The lunar day" (public domain (US government work)); [S2] NOAA National Ocean Service, "What is a spring tide?" (public domain (US government work)).

Facts

• id: F1 claim: Tides are long-period waves driven by the gravitational pull of the moon and sun on the ocean. confidence: medium date: 2026-11-02 sources: [S2] license: CC BY 4.0 checked_on: 2026-11-02
• id: F2 claim: Most coastal areas, with some exceptions, have two high tides and two low tides a day. confidence: medium date: 2026-11-02 sources: [S1] license: CC BY 4.0 checked_on: 2026-11-02
• id: F3 claim: A lunar (tidal) day is 24 hours and 50 minutes, 50 minutes longer than a solar day, because the moon orbits in the same direction Earth rotates. confidence: medium date: 2026-11-02 sources: [S1] license: CC BY 4.0 checked_on: 2026-11-02
• id: F4 claim: High tides are about 12 hours 25 minutes apart, and water takes about 6 hours 12.5 minutes to go from high to low or low to high. confidence: medium date: 2026-11-02 sources: [S1] license: CC BY 4.0 checked_on: 2026-11-02
• id: F5 claim: Spring tides happen at new and full moon, when Earth, sun and moon are nearly aligned; high tides are a little higher and low tides a little lower than average. confidence: medium date: 2026-11-02 sources: [S2] license: CC BY 4.0 checked_on: 2026-11-02
• id: F6 claim: The name 'spring tide' comes from the tide 'springing forth' and has nothing to do with the season; they happen twice each lunar month all year. confidence: medium date: 2026-11-02 sources: [S2] license: CC BY 4.0 checked_on: 2026-11-02
• id: F7 claim: Neap tides happen about seven days after spring tides, at first- and third-quarter moon, when the sun and moon are at right angles; the tidal range is smaller than average. confidence: medium date: 2026-11-02 sources: [S2] license: CC BY 4.0 checked_on: 2026-11-02

Caveats

• Local geography changes tide patterns a lot; some places get one tide a day or mixed tides.
• Use official tide predictions for safety and boating, not this rule of thumb.

Pointers

• id: P1 answers: Tide predictions for a US station where: https://tidesandcurrents.noaa.gov/tide_predictions.html access: free, public web page

@misc{iil_note_vywem7qm,
  author = {{iinventedlight Library}},
  title = {{Tides: why there are two a day, and spring vs. neap tides}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-02},
  url = {https://iinventedlight.com/library/reference/us-gov/tides-spring-neap/},
  note = {Permanent ID iil:note:vywem7qm. License: CC-BY-4.0. Summary of https://oceanservice.noaa.gov/education/tutorial\_tides/tides05\_lunarday.html.},
}

For language models and agents: start at /llms.txt. This page in Markdown: /library/reference/us-gov/tides-spring-neap.md.