Moon and Tides

Astronomy Brieflybrought to you by Duane Dunkerson

Home

Article Archives

Contact Me

Why No Predictions for Theoretical Tides?

by Duane Dunkerson

Why? There is the vagabond Moon, constant Sun,far Jupiter, and an inconstant Earth.

The earthly and astronomical effects on the tides areso complex that a theoretical finality is impossible.Meanwhile, periods of tidal oscillations arepredictable. At least the oscillations could benefit thebusiness interests. Seagoing commerce had to havesome approach to prediction for the tides.

The first tide predicting machine was designed byLord Kelvin. It went into operation in 1873. By1910 the US Coast and Geodetic Survey had theCoast and Geodetic Survey Tide-Predicting MachineNo.2. It was eleven feet long, two feet wide, and sixfeet high with a weight of 2,500 pounds. Thirty-sevenelements bearing on tidal prediction wererepresented by components of the machine. Thismachine was used to attempt to predict the tides.Accompanying the tides are currents. Finding thevelocities of these currents was also attempted byMachine No. 2. These velocities are a sum of aseries of harmonics of periodic elements of the tides.

By 1965 the Table of Tides was still typed by hand.Then in 1973 predictions were attempted bycomputer. This involved two walk-in closets ofpunch cards. In 1979 was inaugurated an interactivephone dial-up prediction service. PCs got into the actby 1987. Now the seven volumes of the Tide andCurrent Prediction Tables are on one CD.

The most recent water levels, within 6 minutes, canbe obtained by phone. One can also get theinformation via satellite. A tidal gauge that findsunusual water heights will automatically send suchnotice to the satellite for retransmission to variousground stations.

This is as close as it gets to knowing the tides.Michelson in 1913 got microscopically close to tidalaction. He used pipes, one set N-S, another set W-E, to measure tides through a microscope. He sawchanges as little as a variance in water level of1/1000 of an inch. These water heights were about69% of what theory would have predicted.

Copyright 2004by DuaneDunkerson

All Rights Reserved

If you wish,send me telling me whatyou think aboutthese articles,

headline;etc. Also ifyou have a

question relatingto astronomicalmatters, I can

answer itor refer you.

24/09/2010 Moon and Tides

www.thespaceguy.com/moontides.htm 1/5

Michelson observed the usual decreases in waterlevels, the ebb tides, and the increases in waterlevels, the flood tides. There are two tides per day,two high, two low. Between a high and a low there isusually six hours. The average interval between thesame high tide is about 24 hours and 51 minutes.This corresponds to two successive southernpassages of the Moon. One tide comes ahead of theMoon on the earthly side towards the Moon.Another tidal outward bulge of water is to be foundon the opposite side of the Earth.

The Moon pulls on the Earth. The water can freelyflow and does so, to and from the sublunar point.Because the earth rotates, the bulges of water movein waves. The bulge moves ahead of the Moonbecause of friction. That is, the Earth rotates fasterthan the Moon revolves. From high tide to high tide iscalled the establishment of port. For New York, thee of p is on average at 8 hours and 13 minutes.

Even with a locale pinned down, the water heights oftides varies since the Moon in its orbit is inclined tothe ecliptic. Because of this, for half a month theMoon is north of the Earth's equator and for theother half, it is south of the equator. There is then asmallish second tide. Twice a month, as the Moonappears to cross the equator, the second tide is notobserved.

There are also brief periods when the water isn'tgoing anywhere. There is not ebbing or floodingduring what is called slack water. This doesn't meanthat the Moon has stopped working gravitationally.In any event the Moon does not lift the water towardit. It can't do such lifting when its lunar force is ninemillion times weaker than the force of Earth's gravityat the Earth's surface. The tides are produced by acomponent of the tidal force of the Moon whichdraws water along the earth's surface towardsublunar and antipodal points.

If the Earth's surfaces were completely submergedby an ocean of unvarying depth and if the Earth wereto face the Moon at all times, there would be tidestwo feet high. But, of course, the oceans are ofvarying depth. Shallow water slows wave travel.Times for high water of two ports 2/10 of a mileapart may differ if one has shallower water than theother. Bigger tides arise in water that is moreshallow. The Bay of Fundy has tides of 50 feet.Other high tides have been found on the east coast ofPatagonia, the Bristol Channel, the coast ofNormandy and in the Hudson Strait. In NewBrunswick at St. John, the river, also called St. John,flows upstream during flood tide.

Current Projects

TLP Canals of Mars New telescope



In addition to rivers, lakes are also affected by thelunar tide action. These are effects usually easier todetail. Lake high tides are opposite the Moon whenthe Moon is below the horizon. If the Moon is up,the water of the lake follows the Moon around theshore. Low tide goes along the opposing shore. LakeMichigan, for example, has a tidal difference of 1and3/4 inches.

It is not only the Moon that rises above the lakes ofEarth. The Sun plays a role in the tides too. At newor full moon there are higher tides as Sun and Moonact along the same line. At first and third quarter(quadrature) of the Moon, there are neap ("neap" -from the Greek for "scanty") tides.

Neaped or not, the Moon comes in closer at perigee.It is 10 per cent closer and then the tides can be20% greater. Now, by this time this should beenough to show that the tides don't allow forprediction. Overcome these various factors and thenthere is Jupiter. Yes, Jupiter causes tides. They aretiny. Nevertheless we are speaking of tides. Tidesare tides. Jovian tides are 10,000,000 less effectivethan the familiar lunar causation. The solarcontribution, by the way, is 5/11 of the lunar one.The Moon, Sun, Jupiter, and other contributors tocelestial mechanics for accurately locating all thegenerating forces are simply not enough. You need toconduct observations per locale for at least morethan 18 years.

In some matters the past provides a comfort zone ofpredictability. We know sometimes what happenedfirst then what came next. Unfortunately the lack ofprediction involving tides reaches back to ancienttimes. Actually it is a subsidiary effect of the tides thathas thrown off the calculations for the occurrences ofancient eclipses. The eclipses happen later than wewould suppose. This is because the earth's rotation isslowing due to tidal friction. At the same time(s) theMoon is increasing in speed in its orbit. The tidaldrag is two billion horsepower.

For more uncertainty as to the when or what tideswere or could do, one can go much, much furtherback in time as Asimov did in his speculation that thetides played the key role in the evolution of life. Thetides challenged some of sea life by tossing then uponto land twice daily. To change you need change -like no credit extended unless you have credit. Theagents of change became changed. With the Mooncloser such a long time ago, the tides were morepronounced. Asimov speculated that perhaps oneneeds a sea and Moon and tides. Without these inexistence, evolution like ours could not come about.



It is like us, contrarily, to look ahead and not so farbehind. One can attempt to predict when there willbe a new moon by gathering your own tidal data.During new and full moon high tides get higher andlow tides get lower. Measurement of your local tidescan lead to a prediction as to when the next newmoon should appear. If you could stick with themeasurement for around 18 years, you couldbecome fairly accurate in those predictions.

.



Documents

Moon and Tides