What Is the Lunitidal Interval?
The lunitidal interval is the time difference between the Moon’s transit over your local meridian and the following high tide at a specific location. Mariners and coastal observers use this lag to estimate high water times once they know when the Moon transits.
In simple terms: if the Moon transits at 1:00 PM and high tide tends to occur at 4:40 PM, your local lunitidal interval is roughly 3 hours 40 minutes. This value is location-specific and can vary slightly with weather and tidal cycles.
How to Use This Calculator
Step-by-step
- Enter one known Moon transit time.
- Enter the corresponding observed high tide time.
- Optionally enter a future Moon transit to predict a future high tide.
- Select whether your coast is typically semidiurnal or diurnal.
- Click Calculate Interval.
Calculator Method
The tool computes:
Lunitidal Interval = High Tide Time − Moon Transit Time
The result is normalized to one lunar day (about 24 hours 50 minutes), so it still works when the high tide occurs after midnight relative to the transit time.
If you provide a future transit, the calculator estimates:
Predicted High Tide = Future Transit + Lunitidal Interval
For semidiurnal coasts, it also estimates a second high tide about 12 hours 25 minutes later.
Worked Example
Suppose your tide station shows:
- Moon transit: March 10, 2026 at 11:20
- Observed high tide: March 10, 2026 at 15:05
The calculated lunitidal interval is 3 hours 45 minutes. If tomorrow’s Moon transit is 12:02, your first predicted high tide is near 15:47 local time.
Important Accuracy Notes
1) Weather can shift tide times
Wind setup, atmospheric pressure, river discharge, and storms can move actual high tide earlier or later than a simple astronomical estimate.
2) Spring-neap effects matter
Around new and full moon, tidal ranges change. Timing can also drift slightly compared with a fixed interval.
3) Local geography dominates
Bays, estuaries, shelves, and harbor resonances strongly affect lag and amplitude. Always calibrate with local data whenever possible.
Best Practices for Real-World Use
- Use recent local tide gauge observations to update your interval.
- Re-check after major weather events.
- For navigation, rely on official tide tables and marine forecasts first.
- Use this calculator as a quick planning aid, not a safety-critical authority.
Frequently Asked Questions
Is lunitidal interval the same everywhere?
No. It varies by location and can differ significantly even between nearby harbors.
Why does this calculator use a lunar day?
Tides are tied to the Moon’s position relative to Earth. A lunar day is approximately 24 hours 50 minutes, which is why tide times shift each day.
Can I use this for fishing or boating?
Yes, as a quick estimate. For operational decisions, always verify with official hydrographic and meteorological sources.