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Space Topics: Planetary AnalogsStars Above, Earth BelowAstronomy and Space Exploration in America's National Parks
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Tidepools along Acadia National Park
Eleven-foot (3.5-meter) tides leave pools and sea creatures high upon on the rocky coast at low tide. Credit: Tyler Nordgren |
Acadia National Park, Maine -- Today is a beautiful fall day along the coast of Maine. There isn’t a cloud in the sky, the temperature’s cool and the wind in the trees rustles leaves that are just starting to turn. Up until today the days had been overcast and fog would roll in early in the afternoon. Not an auspicious place for an astronomer. But I didn’t really choose this park for its stargazing, although this is perhaps the darkest national park east of the Mississippi (more on that next time). Rather I came here for the sea.
I’ve always lived near the sea. My grandmother owned a house on the
Oregon coast when I was young. My memories are vivid of walking along the cold
and empty beaches when I was small. Sometimes the sand would stretch forever.
Sometimes logs blown in by a storm created a worldwide jungle-gym just for
kids. Other times the tide was so far out that tidepools filled with alien
creatures were left behind just ripe for tiny fingers to poke and play with.
Where did the water go when the tide went out? I’d look out to sea and
wonder if China was over there and maybe they had all the water now.
Tides are one of the big draws here in Acadia. There’s an island off the northern coast of Bar Harbor that at low tide becomes a periodic peninsula. Visitors are warned that if they make the hike out to the newest part of town, they have only about an hour and a half on either side of low tide before they are stuck there until the next low tide lets them back. The tides here in Acadia fluctuate by about 11 feet in height between low and high. Farther up the Gulf of Maine, in the Canada’s Bay of Fundy the tides are about twice as high.
What causes tides? Galileo was perhaps the first to attribute them to an astronomical source. He thought they were proof that Earth actually turned and devoted his final chapter of his Dialogue on the Two Chief World Systems describing how they supported the Copernican Model. He argued that a perfectly stationary Earth couldn’t give rise to tides and that instead it was the motion of the Earth that gave rise to tides like waves in a whirling bucket.
Moon above Acadia Fog
A nearly full Moon hovers above the coastal fog of Maine. Credit: Tyler Nordgren |
Model tides
A ranger in Acadia National Park draws on visitors to help explain tides during a hike to see tidal pools along the coast. Note the fellow with the hat that says "Moon." Credit: Tyler Nordgren |
OK, so it doesn’t really work that way. One way we can see for ourselves the celestial influence on the tide is found in the tide tables available for free in the markets here in town. Each day the times of high and low tides come about 50 minutes later than they did the day before. Check an almanac for the times of Moon rise (or scan the sky if you are lucky enough to have two consecutive clear evenings) and you will find that the Moon rises 50 minutes later each day too.
The Moon must therefore have something to do with the tides and yesterday while hiking out to some tide pools at the southern end of the park I saw this effect in action. A park ranger leading a hike out to the same tidepools had drawn his guests onto a shelf of rock to give a demonstration. Designating one person the Moon and one the Sun he described the gravitational interactions between them and the Earth that gave rise to the twin tidal bulges of water (one on either side of the Earth). We can imagine that the Moon, and to a lesser extent the Sun, could raise a bulge on one side of the Earth. After all, the gravitational pull between two objects is stronger the closer they are to one another. Therefore, the side of the Earth closest to the Moon should feel a stronger pull than the side farther away. In this way water, which is a bit more fluid than solid rock, should be ever so slightly pulled up on one side of the Earth causing a bulge. As the Earth turns under this bulge, water first rises up along the shore and then is pulled away from it as the bulge passes on. But high tide comes every 12 hours, not every 24 so there must be two bulges the Earth turns under each day. Where does the second one come from?
If the portion of the Earth nearest the Moon feels a stronger gravitational pull than the center of the Earth, then similarly the center of the Earth should feel a stronger pull than the side of the Earth farthest from the Moon. In this way, while water on the near side of the planet is pulled up off of the center of the planet, the center of the planet is pulled out from underneath the water on the farside. Two bulges appear to form, one opposite the other and we get two sets of tides each day, and each day and they come 50 minutes later as the Moon slowly makes its way around the Earth.
Ocean world
Tidal forces from Jupiter create internal heat on its moon Europa. Deep beneath a frozen layer of ice awaits a global ocean with more water than all the oceans of Earth. Credit: NASA/JPL/DLR |
Tides aren’t a unique function of the Earth and its Moon. Tidal forces like those seen here are at work on many of the moons in the outer solar system where the giant planets they orbit cause extreme stretching of the comparatively tiny moons. Volcanoes on Io, frozen over oceans on Europa and possibly Ganymede are all due to heat generated by tides coming from the giant planet Jupiter. Geysers of water on Enceladus are thanks to internal heating and stretching of the surface caused by enormous Saturn.
Walking along the seashore I cannot help but think of Newton and the majesty of astronomy made manifest in the simple beauty of a tidepool.
I am here for three more weeks during which I will be working with a number of locals on astronomy here in Acadia and Mount Desert Island. As one example, while I have been typing this entry a fellow next to me has set up a solar telescope while his wife sells paintings here on the village green Sunday art show. In a couple weeks I have to make a jaunt down to Orlando for the annual Division of Planetary Sciences meeting. I should be back in time to see the leaves change and then I am off to the Great Smoky Mountains.
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