1. Is "0 cm" on the Tide Table the Same as Zero Sea Height?
The Japan Meteorological Agency’s tide tables list the times of high and low tides every day, along with the tide level. The times are shown in 24-hour format, and the unit for tide level is centimeters [1]. But the 0 cm mark on the table is not the sea surface. It is a specific height called the "Tide Table Datum." This datum is set at a certain distance below the average sea level, depending on the location [1]. Therefore, when a table says "Low Tide 20 cm," it does not mean the water depth is 20 cm. It means the water is 20 cm higher than that specific datum line.
This raises an important question: Is this datum line the same everywhere in the country?
2. Do Harbor Tide Tables and Sea Charts Use the Same Baseline?
No, they do not. For example, the tide table for Mikawa Port, published by the Chubu Regional Development Bureau, uses the "Mikawa Port Port Management Datum." This line is 1.239 m (about 4.07 ft) below T.P., which stands for Tokyo Bay Mean Sea Level [4]. Because this is a different line from the one used in the Japan Meteorological Agency’s tables, you must check if the baselines match before comparing numbers.
Sea charts also use a different baseline called "Lowest Astronomical Tide" or simply the "lowest water level" for reading depth. This line is calculated by taking the average sea level and subtracting the full range of four major tidal waves (called M2, S2, K1, and O1) [5]. The idea is to choose a level that is rarely reached, so that depth readings are safe for ships [5]. However, "lowest" is just a convenient name. It is not rare for the sea to drop below this line. In many places along the Japanese coast, this happens every year around the time of large winter tides. This is called a "negative tide level" [5]. If you see a minus sign (-) in the table, that is the kind of day it is.
Note: This baseline is based on the idea that subtracting the sum of the amplitudes of the four main tidal components from the average sea level creates a line that is rarely crossed, ensuring depth is read safely.
3. Why Are Some High or Low Tide Times Left Blank in the Table?
The times of high and low tides get about 50 minutes later each day. This happens because the moon orbits the Earth in a cycle of about one month [2]. The 50-minute delay is a rough guide; it varies depending on the day and location.
When these daily delays add up, the high tide or low tide for a certain day can pass midnight and happen the next day. As a result, that day might only have one high tide (or one low tide) listed. The Mikawa Port table fills these missing columns with a dash (-) [4]. In the Japan Meteorological Agency’s tables, the columns for days with no prediction are simply left blank [1]. If you find a blank day, look at the times on the days before and after. You should be able to see how the delay is slowly moving forward.
4. How Does the Moon's Shape Show Big and Small Tides?
Around the new moon and full moon, the forces of the moon and the sun overlap. This creates a "spring tide," where the difference between high and low tide (the tidal range) is large. Around the first and last quarter moons, the difference is small, creating a "neap tide." Both types occur roughly twice between one new moon and the next [2].
The Mikawa Port tide table includes a column for the moon, showing symbols for new moon, full moon, first quarter, and last quarter [4]. Not all tide tables have this, but if yours does, you can estimate the size of the tide just by looking at the date row. By now, you can read the contents of the table. But how accurate are these numbers when you look at the actual sea on that day?
5. Why Do the Table's Predicted Tide Numbers Differ from the Actual Sea?
The tide tables from the Japan Meteorological Agency are predictions of "astronomical tides," caused by the gravitational pull of the moon and sun. They are not measurements of the actual tide, and the agency warns that they may differ from observed levels [3]. The predictions are made by breaking down recorded tide data into waves with different periods (called tidal constituents), finding the specific values for each location (harmonic constants), and adding them together. In Japan, when dividing more than one year of tide data, 60 tidal constituents are used [5].
The Mikawa Port table also states that it does not account for abnormal weather like storm surges from typhoons or tsunamis, so significant errors can occur [4]. In the open ocean, if the atmospheric pressure is 1 hPa lower, the sea surface rises by about 1 cm [6]. If the pressure is 20 hPa lower, a simple calculation suggests the sea rises by about 20 cm (this is a rough guide for the open ocean).
The table is a schedule of tides caused by the moon and sun. When looking at the actual sea, you must also check the weather and atmospheric pressure.
6. How Can You Investigate Tide Tables Further on Your Own?
You can try this with an adult. Open the Japan Meteorological Agency’s "Tide Table" page and look at one month of data for a nearby location. Subtract the low tide height from the high tide height for each day and list them. You will see the pattern of spring and neap tides. If there are blank days, you can also check the time delays on the days before and after.
If you are going to the sea, always go with an adult and check the weather and wave information for that day. Since the table is a prediction, it is safer to stay away from the water if a typhoon or low-pressure system is nearby.
For official information, the Japan Meteorological Agency’s tide table page has a section on "How to Read Tide Tables." Try reading how the datum (the height of 0 cm) is shown for each location [1].
Safety note: When checking at the seaside, confirm the weather, waves, and footing in advance, and always act with an adult.