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Saturday, October 27. 2012
Tides are not all that easy to understand. For example, can you explain why there are no tides at the equator? It's complicated because the earth revolves and rotates, the moon revolves on an angle to the equator, and then there's the sun too. Lots of moving parts. Plus local factors.
My need to refresh my understanding arises because hurricane Sandy, who will likely be visiting the northeast somewhere between Chesapeake Bay and Boston at the end of this weekend, is coming during a full moon - the Hunter's Moon as the redskin Injuns termed it.
(The latest tracking guesses show it mostly missing New England and turning inland somewhere between Delmarva and Jersey. Meteorologists love this sort of unusual weather event. I heard one on the radio saying that he had always wondered why conditions were so rare for a storm to make a left turn due to a low cold front sucking in a storm. The Weather Nerd is following things, and posted this pic showing the Clash of Titans - the polar cold front and Sandy:)
Full Moons indicate that the moon and the sun are on the same side of the earth, illuminating the moon, and New Moons when they are pulling from opposite sides. Thus both of these phases exaggerate the tidal effect by combining lunar with solar gravity. They're called Spring Tides.
In Maggie's Farm's part of the world, high tide and low tide vary from 6-10' in each tide change, and increase around 20% during Spring Tides. Thus a storm surge during a Spring Tide is likely to cause more damage and flooding to structures (ie, structures which are built where they don't belong in the first place - on coastal beaches, waterfronts, flood plains, and on old marshes covered with a thin layer of fill back before the laws were changed).
Storm surges in New England, whether from Nor'easters or the occasional hurricane, can be 5-10 feet above high tide. A very large or slow-moving storm can effect more than one tide cycle.
Here's a simple, straightforward piece on tides.
Re storm prep, I fetched a fresh bottle of Dalwhinnie, some smokes, charcoal for cooking, good veggies to grill, and I topped off my gas can for the chain saw so I don't get trapped here by fallen trees and limbs. Firewood? Check. My ancient, second-hand diesel generator seems to be on the blink, but we can cope. Sometimes it will start if I kick it in the kidneys while pushing the button.
Meanwhile, I eagerly await the first lunatic to blame the storm on
Bring it on! Give us your best shot, Gaia.
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Now hold on there, Bucky. I almost did a post on Sandy a few days ago because, from where I sit, this is proof positive that global warming is nigh and we're all going to die within years months days unless we immediately donate $100 to Greenpeace.
Hasn't the global warming crowd been telling us that AGW encompasses both hot and cold extremes at the same time?
Are you calling the Washington Post a liar??
Hurricane Sandy is likely to be one of those rare tropical beasts that produce snow. In the Mid-Atlantic region, southwest Virginia and West Virginia have the best chance of receiving accumulating snow from this storm.
A tropical storm -- producing snow? Proof positive, indeed!
Oh, the reason I didn't write the post is because I didn't want to get laughed off the site.
As a landlubber I have a stupid question to ask about tides;
If it is high tide in New England, is it low tide in the UK, or does the low tide just make the ocean deeper?
High tide moves around the earth as it spins, pulling towards wherever the moon is in relation to where you are.
Read the link.
Being near the Jersey shore, I'm as ready as can be for yet another "storm of the century" (funny, we've had three this century so far, how many more are there?). I've got all the supplies and plenty of books to read by candlelight.
Seriously though, thanks for the link to the article - I learned something new, I had no idea that there were no tides at the equator. And one of these days I will make that trip up to the Bay of Fundy.
Full Moons indicate that the moon and the sun are on the same side of the earth, illuminating the moon, and New Moons when they are pulling from opposite sides
Ah do believe suh, that you may have gotten this back'ards. Fo the Sun to light up the full moon to an Earthbound point-of-view, the Sun would need be located such that the POV is betwixt and between the Sun and the Moon.
If the Sun were a basketball, the Earth would be a BB, and it would be 100 feet away, the heighth of a ten storey building.
The moon would be a quarter the size of the BB, whatever that would be, i guess think 'pinhead' tho that's a disc not a ball.
The circumference of Earth, the tape measure around its belly, is about the same distance as the earth is to the moon (very harmonious), so the pinhead is in very tight orbit around the BB.
If you are in town, standing on the sidewalk, and in front of your eyes the little BB is spinning and the little pinhead is rotating around the spin always facing the BB, ten storeys up the white-hot basketball is throwing a LOT of light!
--or if you want to visualize in the level, the burning basketball is 6 or 7 car-lengths away.
Good post. But, actually there are tides at the equator. Here's a tide table for a location in Ecuador:
Up to 3 meter amplitude.
for an example of a global map of tidal amplitude.
7,926 miles = Earth diameter
93,000,000 miles = earth to Sun
= 11,734 Earth diameters to Sun
.177 cal BB = 5.65 BB/inch x 100' (1200 '') = 6,780 BB/100'
6,780 = 58% of 11,734, so add 42 feet;
... and the fiery basketball should be 142 feet away (the 100' was a try for a memorable number good enough to re-create a scaled mental image, but yep it IS pretty far off --have to add three more cars or four more storeys)
If you wanted to get away from the flat pinhead, and stay with familiar and ball-shaped objects, you could make the BB the moon, and next thing up would have to be a marble, which is too big for the BB but nevermind that, because the ratio BB to marble, estimating at 1:100, means the sun would have to be some sort of grand-opening special-made beachball with a diameter almost the width of a football field, and out there in the inky darkness almost three miles (2.7) from the giant beachball, you would find, if you were lucky, the little blue spinning marble, and, about an inch away, locked Tango-like forever facing while rotating 'round the marble, the BB.
Earth circumference ~ 40,000 km
Earth-moon distance ~ 384,000 km (average)
Tides are linked with the moon but they're not in complete sync with it. There'is a two hour difference in the time of high tide from day to day, just as with the time of moonrise. Moonrise and hight tide do not necessarily coincide. The difference in time between moonrise and high tide in any place is known as the Vulgar Establishment of the Port. I love that expression.
--what i did wuz --using miles --24k for earth circumference, and (should've been) 240k for earth-moon distance --but i dropped the zero in my fogged up old memory, and used 24k for both. But, just put the BB 10 inches from the marble, instead of 1 inch. The beachball and 2.7 miles are ok, and the little tiny binary system way out there, is still little tiny, just opened up. That Tango falls away as an image, tho, the dancers arms are now probably too short.
OTOH, the image of the Moon being being 10 times closer to the Earth sure makes a pretty picture in the mind --tho the Bay of Fundy would be the Bay of RUN!dy
--i'll be glad to shoot off my mouth again and make another --well, re-make the same --fool of myself and try to put your info into a visualization:
One full spin of Earth takes 24 hours.
One full lunar orbit takes about 30 days.
So earth is having to drag moon along behind it, by the difference between a 24 day month and what we have (see Robespierre's calendar, he tried to fix it, including ridiculizing the nomenclature).
If earth was fine upstanding fellow, those high tides and moon rises would normalize and we wouldn't need the tables. But earth is tilted (by the diff between true and magnetic North), and moon orbits the center of the combined mass, its orbital plane always tangential to earth's sun-orbital plane.
Furthermore, that tilted attitude means that, at whatever angle the sun rays strike you today, it will be exactly one full trip around the sun before you ever feel that angle again.
(if you stand on the same north/south spot --the same 'parallel' --all year, but have moved east/west, it will be one full year to the day, rather than one full year to the second)
(if you move along a Meridian, all bets are off as to when you will feel that sun ray angle again --you'd have to locate the new 'incident' in space, and only then, in time)
Hey, i forgot what my point was.