Why the ground's tide doesn't match the sea's
The same Moon and Sun pull on the rock under your feet and on the ocean beside it. Yet if you stand on a Maine dock and watch our earth-tide clock, the two won't line up. The sea can reach high water just as the ground is near its lowest. That isn't a mistake. They are two different tides, and they keep different clocks.
One day in Rockland, Maine
Here is Friday, 18 September 2026, in Rockland on Penobscot Bay. Water times are NOAA's predictions for the Rockland harbour station. Ground heights are the earth tide computed for the same spot. Times are Eastern Daylight Time.
| Time | The sea | The ground |
|---|---|---|
| 4:52 AM | High water (8.5 ft) | +4.4 in, near its high |
| 5:50 AM | Falling | +4.6 in, its high for the day |
| 10:41 AM | Low water (2.2 ft) | +0.6 in, near the middle |
| 4:58 PM | High water (9.3 ft) | −4.3 in, near its low |
| 6:50 PM | Falling | −4.5 in, its low for the day |
| 11:28 PM | Low water (1.4 ft) | −2.8 in, rising |
That afternoon, the sea reached its highest point of the day just as the ground was nearly at its lowest. The sea had two highs and two lows. The ground had only one of each. The sizes are nothing alike, either: the water moved about 8 feet, and the ground moved about 9 inches from top to bottom.
The rock answers right away
Rock is stiff and elastic, like a very hard rubber ball. When the Moon's pull changes, the whole planet changes shape within minutes. So the ground tide follows the sky closely. That morning the ground peaked at 5:50 AM, about a quarter of an hour before the Moon reached its lowest point below the horizon, at 6:04 AM.
The sea has to travel
Water has to actually move. To rise at your harbour it has to flow in from somewhere, and ocean basins are bounded by continents, shaped by the sea floor and turned by the Earth's spin. So the tide travels around each basin as a slow wave, and the timing you see depends on where you stand along it.
The Gulf of Maine is a special case. Together with the Bay of Fundy, it sloshes back and forth on its own natural rhythm of roughly 13 hours. That is close to the Moon's 12.4-hour beat, so the Gulf amplifies the twice-a-day tide. That is why Fundy has the biggest tides in the world. It also means Maine's high water comes hours after the Moon passes overhead, on a schedule set by the shape of the Gulf, not by the sky alone.
Why the ground had one high that day
The Moon stretches the Earth into two bulges, one under the Moon and one on the opposite side. On 18 September 2026 the Moon was about 28° south of the equator. So the bulge under it sat far to the south, and the opposite bulge sat 28° north, not far from Maine's 44°. Rockland passed close to the northern bulge once a day, around dawn, and far from the southern one in the evening. The result was one big high and one low a day.
About twice a month the Moon crosses the equator, and the two bulges become even. Then the ground gets two nearly equal highs a day. Drag the time slider on the tide page and you can watch the pattern change over the month.
The ocean in the Gulf of Maine keeps its two tides a day all month long. Its resonance favours the twice-a-day rhythm, so that part always wins there. Elsewhere, the sea does its own thing. The Gulf of Mexico often gets just one tide a day, and the Mediterranean hardly has a tide at all.
So which one is right?
Both are. They measure different things. A tide table tells you where the water line will be against the land. The earth-tide clock tells you how far the land itself has been lifted, compared with where it would sit if there were no Moon or Sun. On the dock, you rise and fall with the ground, so you only ever see the water move.
The two do touch in one way. Near the coast, the weight of all that incoming water presses the crust down a little, and it rebounds as the water leaves. This “ocean loading” follows the sea's schedule, not the sky's. It is a separate, smaller effect, and our clock does not include it yet.