Are tides bigger on full moon days?
Higher
Larger tidal ranges around full and new moons appear in almanacs as spring tides.
- Average (whole period)
- 1.73 m
- Tidal range on full moon days (±24h)
- 2.08 m
- Tidal range on new moon days (±24h)
- 2.06 m
- Data
- 2015–2024, San Francisco tide gauge (NOAA), 3,447 days
Act II: If not the moon, then what—
+20%
The tidal range on full moon days is about 20% larger than average. That is the gravity of the moon and sun itself — known to every almanac as spring tides, the oldest verified lunar effect there is.
No. This one really is the moon. The moon, too, can do things.
Why this topic exists
Yes, we know: the moon causes tides. Nobody needed a fact-check for that. And yet this site ran tides through exactly the same statistical machinery and verdict criteria as every other topic.
The reason is simple. A site that prints "No difference" on every page invites the suspicion that it was built to say "no difference." So we feed the machine a phenomenon where the effect is unquestionably real, and show that the verdict swings properly to "Higher." It is an instrument check — in statistics, a positive control.
How this verdict is computed
From verified water-level data at the San Francisco tide gauge (NOAA, recording since 1854), we compute each day's tidal range (the difference between the day's highest and lowest tide). The rest is identical to earthquakes and births: days within ±24 hours of the full moon instant are compared with the all-period average, judged with a 95% confidence interval.
- Larger ranges around full and new moons are spring tides; smaller ranges near the quarters are neap tides — in almanacs for centuries
- Strictly speaking, spring tides peak one or two days after the syzygy (the "age of the tide"), so our ±24h window slightly understates the real effect. The verdict doesn't budge anyway — that is how strong a real lunar effect looks
The moon is not innocent of everything
This site clears the moon of false accusations — it does not claim innocence on all counts. Lunar gravity moves entire oceans. The "No difference" on the earthquake and births pages and the "Higher" on this page come from the same calculation. That is exactly why you can trust the verdicts.
Sources
- NOAA CO-OPS Tides & Currents (San Francisco tide gauge, station 9414290, public domain)
- Moon phase instants computed in-house from Jean Meeus, "Astronomical Algorithms" (UTC)
Last updated: June 11, 2026 13:31 UTC (rebuilt daily)