Why are there 2 tides a day?

For us fresh water sailors, it can come as a surprise that there are two tideas a day - thinking that there should be only one, and that would correspond with the position of the moon relative to the earth.

But some of us have discovered that there are two tides daily. Why twice a day? Could it be that the moon rotates around the earth twice a day, but we can't see the rotation during the day? There should be a simple answer to this fundamental question about nature that all you ocean salts should know instinctively. It would seem to be easier to answer than "What is gravity? .

Any ideas about this bit of nautical trivia?

Theis -

Not all places have 2, but I think that is most common. In the Gulf of Mexico I have friends who only have 1 per day.

Anyway, if I understand it correctly, water bulges both towards *and* away from the moon. Moon goes around Earth once every day (more or less), so you get 2 tides. I've noticed here that one tide is usually a bit lower than the other that day - maybe it's the "backside" tide? Dunno for sure...

But why are there two bulges (and there are two)?

The reason that there is only one tide in some places, and why the tide is so huge in other places (Bay of Fundy, for example), is supposed to be because of wave theory/harmonics/shape of basins. But the premise is correct that the moon only rotates once a day, and that there are two tides a day. I believe you are correct (but don't know for sure -where are the ocean salts to hel us answer one of life's persistent questions?) that one tide is larger than the second. Does the larger one correspond with the moon?

The reason for all of this tidal variation is that the Sun and the Moon both have an effect on the tides. And both the Sun and the Moon exert a gravitation pull on both the Ocean and on the Earth itself, thus pulling both the ocean away from the Earth on the nearest side and the Earth away from the Ocean on the far side.

Grab your old US Coast Guard Auxiliary Advanced Coastal Navigation (ACN) text and you will discover that the moon is more influential than the Sun on the tides due to its proximity. The relative distance of the moon from the Earth and the relative distance of the sun from the Earth play a role.

Thus we have diurnal with one high and one low tide per lunar cycle (~one earth day at approximately 24 hours and 50 minutes); semi-diurnal with two nearly equal high and two nearly equal low tides per day (Boston and New York); and mixed pattern tides like those in Seattle and San Francisco where sailors, surfers, and clam diggers experience two daily unequal high and low tides. Because of the fifty (50) minute difference between the lunar and earth day the times for high and low tide are fifty (50) minutes later each day.

Tidal ranges are smaller when the moon is in apogee (furthest from the Earth, and larger when the moon is in perigee (closest to the Earth).

When the moon and sun are in conjunction (pulling together) i.e. a new or full moon, a phenomenon called syzygy with larger tidal ranges occur. These tides are termed spring tides. When the moon is in quadrature (at right angles to one another), then we experience tides with a lower range termed neap tides.

So you can more-or-less predict the tides in your local area by knowing something about the tidal patterns in your area, and looking at the moon.

Don't throw away you tide tables and go out and howl at the moon, but you might follow the first new moon in the autumn or winter to your local USCG flotilla and take their spiffy ACN class where you will learn all sorts of interesting factoids. At around $50 for a twelve to fourteen week course, it’s probably the best deal that you will ever find in the world of marine education. Learn all about the world of coastal navigation and piloting before GPS, and you might learn the secrets of the tides as well.

OK, I confess. I am a USCG AUX instructor.

Such a simple question and such a complicated answer. The sun, although having a small effect on the amount of the tide, isn't a significant factor for their being two tides a day. The phenomenon of two tides daily is caused by the moon.

I think the Coast Guard has some more work to do, so hopefully, when the real answer surfaces, the ACN can be updated.

Hey Scott,

As an Aux instructor, perhaps you can answer a question about the course for me. First off, I'd recommend the ACN course to most everyone. It was both enjoyable and educational. I already had a pretty good informal background in coastal nav, but I found that I learned quite a bit of new stuff anyway, and it was helpful to go through the whole process in a focussed way.

Ok, now for the question: Why is there still such a stong emphasis on Loran and RDF in the ACN course while so little mention of GPS nav? We spent a full 3 hour class on Loran while GPS got little more than a mention. This parallels the course book pretty closely, too.

Just curious.

Nathan,

I should say that I do not personally teach ACN, and I have no official response to that question as an Auxiliarist, but my own personal perspective is that these courses are developed over time by groups of volunteers and they are revised from time to time by groups of volunteers. Since books are developed over time and printed in ink, they take some time to change. The challenge is to keep the texts current with developments in the marine electronics industry. The Auxiliary has over the past two years or so been promoting the use of computer-based training aids. The use of lap top computers, and programs like Microsoft PowerPoint make it possible to change our training programs more rapidly and at less expense. Each Flotilla must pay for the books and course materials that we use in our Public Education courses, and therefore there is a direct relationship between the cost of the texts to the Auxiliary and the cost of the courses to the students.

The Auxiliary is now offering a new short course usually titled "GPS for Mariners", based upon a new book, "GPS for Mariners", from McGraw-Hill authored by Robert Sweet of the Power Squadron, a Power Squadron navigation instructor and former radar and communications system engineer who helped develop GPS for the U. S. Air Force. My Flotilla has taught this course twice this year. I took his course for trainers. He is an inspirational speaker and a good instructor.

Bob Sweet’s book is available from a number of sources and is useful with or without the Auxiliary or Power Squadron course. Our Flotilla also incorporates a PowerPoint presentation based on that book into our ACN course. This will vary from Flotilla to Flotilla. The ACN exam is standard nation-wide, and is based on the text, so the ACN course necessarily still focuses on the use of Loran and RDF. Otherwise the students would not be able to pass that section of the exam. Loran is still operational but is fading slowly away. Three hours on Loran in 2004 does seem a bit excessive to me, if that excludes a fair treatment of GPS, although I do find Loran to be a somewhat interesting subject.

If you are in the Search and Rescue field, RDF is more than academically interesting. I am also a boat crewmember, and last week we were on a SAR mission in conjunction with a Vessel Assist unit. Vessel Assist has one of the bi-directional RDF units that can establish a bearing on any VHF signal. We were able to locate and get to a May Day vessel with minutes of receiving his rather vague and sporadic Mayday message. Some Auxiliary land and vessel stations are equipped with these units.

I remember sailing down the fog-shrouded coast of California with my father when an RDF unit, a depth sounder and a compass were our only means of navigation, but you are correct. My Father, who was a WWII Naval officer was darned good with an RDF despite the sparse stations on the Central Coast between Monterey and Santa Barbara even in those days. I remember sailing in the fog for an entire day without seeing any land and then coming out not more than 300 feet from his intended waypoint, a dinky little buoy off Morro Bay.

The ACN course still focuses on the use of RDF and Loran for coastal navigation purposes, while GPS has become the more common and more useful standard.

I argue that without a background in coastal piloting, masterful use, or even safe use of a GPS is much more difficult. Problems like running into shoals and islands between waypoints while using non chart-plotting GPS units could easily be avoided if the rudiments of coastal piloting are mastered before the GPS class. It is also helpful to understand marine publications like the notice to mariner’s publications, light lists, tide and current tables etc. Failure to keep an accurate paper chart is a more common mistake among those who don’t know how to use dead reckoning and those who don’t know how to mark the chart properly and consistently.

Bob Sweet's book and the Auxiliary course patterned after it endeavors to provide a rudimentary education in both the use of a GPS and in coastal piloting, but the background provided in the ACN course is much more thorough in the coastal piloting area.

I do think that it is unfortunate that it is becoming more difficult to obtain any level of formal education in celestial navigation these days. I understand that the Power Squadron still does teach Celestial Navigation in some regions, and that some Auxiliaryflotillas may also.

So, the short answer to your question is that there are currently two couses, ACN and GPS for Navigators, with some flotillas combining elements of both within ACN, but without the benefit of Bob Sweet's wonderful little book. You have to take the GPS for Navigators Course to get that.

Thanks for the response Scott.

I did, actually, find the Loran and RDF section of the class quite interesting, as I had (and still have, actually) no hands on experience with those types of electronics. Yes, I have been sailing exclusively since the advent of GPS!

I agree with your comments about the fundamentals of coastal nav being important BEFORE the addition of electronics such as GPS. Though, I do think this logic really applies to all electronics including RDF, Loran, and radar, which we did spend a fair amount of time with.

For me, the best part of the class was working nav problems on paper charts, which is what we did spend most of the class time working on.

And, I learned that my wife is both faster and more accurate than I am doing chart work! A good thing to know!

If your Triton sails in heavy weather with her rail down as far as my Ariel, then having someone else below who knows what they are doing putting pencil marks on the paper charts is a very sweet thing. I am still waiting for someone to invent navigation tools with Velcro on one side.

One cool thing that I do have is a GPS that mounts on the bulkhead behind my chart table below the skylight that covers the space that was once covered by the icebox lid. I can mount the GPS so that it is readable from the settee in front of the chart table, where I sit to do my plotting, or I can rotate it so that the screen faces upwards. It is a bit hard to read through the sky light from the cockpit above under many light conditions, but in heavy no-glare weather when the GPS is best left below I can make out the essentials. That is one way to produce an all weather GPS.

I think that Carl Alberg probably had that in mind when he designed that oft-hated top-loading icebox. That icebox was merely intended as an interim use for the space until a GPS could be invented. A GPS is better than cold beer in the cockpit any day. :D

By the way, both my Garmin 76 GPS and Garmin 176 GPS/chart plotter work very well below that skylight with their integral antennas. I have had no need thus far to install a remote antenna. In as much as the 176 will operate on rechargeable AA batteries or the house battery and can be integrated with my digital selective call VHF, but is also transportable to my cockpit, car, etc. it is very versatile and oh so functional....however it's pretty hard to go anywhere with the mast down boat as mine is presently.

And now for the answer on why there are two tides - if anyone cares. The answer as you will see, is still a lot of goochy fatchy, but hopefully it is spelled out below.

The mistake conventional wisdom makes is is to believe that the moon draws the water towards it. But if this were so, boat speeds on the open ocean would be impacted on the open water by the currents and rushing of water as the moon passes overhead - and no one to my knowledge has suggested that.

It is not the water that is drawn to the moon but the earth itself. The water, being more flexible, however, is distorted more. The gravitational pull of the moon is strongest against the densest material - which happens to be the center of the earth not the water closest to the moon.

A couple assumptions:

The gravitational force between two bodies is a function of the mass of the bodies. The center of the earth is much denser than the surface water so the gravitational forces on the center are much stronger than on the surface. (The moon, the other body, is constant for the purposes of the explanation).

The earth is a plastic - its shape changes (Only the outer crust is relatively rigid).

The earth, because of the gravitational pull of the moon, is a nominal ellipse, with the extremeties being in line with the moon. The center of gravity of the earth, however, is fixed relative to the sun, so the overall position of the earth can not change. The two ends have to bulge out. The water being more flexible than the land surface, deflects more.

Because any point on the earth becomes an extremity of the ellipse twice a day is why there are two high tides a day.

Those are my thoughts. Below is what the Adler Planatarium directed me to for you to interpret as you wish.

http://www.badastronomy.com/bad/misc/tides.html