The Night Sky: The tides of the Hudson

Coastal civilizations forever noticed that the ocean’s rhythmic rise and fall mostly followed the Moon’s position. But how could this be? We orbit the huge massive Sun, not the lightweight Moon. Why should the Sun have the dominant gravitational effect on us — and yet the Moon boasts the greater tidal pull? 

Answer: Tides are caused not by simple gravity, but by the difference between the Moon’s pull on one side of Earth compared to the other. Tides happen only because Earth’s diameter is large compared with the Moon’s distance, creating inequality on the two sides of our planet. When you stand under the Moon, however, there’s no real difference between the Moon’s distance to your head compared to its distance to your feet. So there is no tidal effect within human or animal bodies. None. Zero. That’s why your body fluids don’t try to change position when the Moon passes overhead. And why tea never tries to climb the sides of the cup.

The solar tidal influence is 46% as strong as the Moon’s. It’s not nothing. So when the Moon and Sun are either together in the sky or on opposite sides of the heavens — that is, either new Moon or a full Moon like the one this Sunday, the 22nd — they act along the same plane and we get the highest tides. These are called spring tides because the seas maximally spring out and then spring back in. 

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Tides, of course, are not just felt on the coast. Those on the Hudson are not paltry, and typically range by four or five vertical feet with each in-and-out breath of the waters. When an Atlantic storm raises the seas at the mouth of the Hudson, sensors at Battery Park sound the alarm, and we know an additional two feet may arrive here six hours later.

All this happens because the river is barely higher than the Atlantic as it courses past our area. Although listed on maps as three feet above Mean Sea Level (MSL), the daily tides push the Hudson’s surface from one foot below MSL to 5 1/2 feet above and much higher during storms. The river can thus be visualized as a dynamic organism whose perpetually undulating sine wave pattern moves from Manhattan all the way up to Troy, where it ends ingloriously at the dam. From Poughkeepsie north, no salt water remains, and the Hudson is never brackish in Kingston or Saugerties.

Once high tide hits lower Manhattan, it progresses upriver at 17 miles per hour. It takes six hours to reach Saugerties, which means when it is high tide up there, it is exactly the next low tide down in the city. Of course, while the swell of the river moves at 17 mph, the water itself does not. Anyone watching floating debris will see it progress northward with the incoming tide, and then later flow south. It may repeat the zigzag process before it finally clears one’s location for keeps. This is why it would take someone in an inner tube 126 days to float from here to lower Manhattan — about four months. Few commuters choose this inexpensive travel option.

As is the case nearly everywhere in the world, our highest Hudson tides usually happen a day or two after new Moon and full Moon. And while two high tides happen each full day, they’re typically quite unequal. Within a month of the equinox, however — right now — the primary and secondary highs are similar. That’s because the Sun, and both new and full moons all hover more-or-less over the equator, and thus pull evenly.

Times and strengths of high and low tides are posted at places like the Saugerties lighthouse, or you can get Kingston’s tide schedule at https://www.usharbors.com/harbor/new-york/kingston-ny/tides/.

No rush. Whenever you’re ready. They’ll be there.