-- Sailor's aid or sailor's nightmare - the tides explained

Sailor's aid or sailor's nightmare - the tides explained    

'Moon and tide - forever interlinked'    .

As sailors, we all know that tides come twice a day, vary according to the moon, and, depending on where you are sailing are either unimportant, reasonably important, or critically important to a successful completion of your voyage. But why the moon? and if the moon only circles the earth once a day, why are there two tides? Here, Grant Headifen of Nauticed, explains:

It's not surprising if you don't exactly understand how tides work - Tides took a lot of figuring out by some very smart people over the ages - including Galileo - but here's the story:

Gravitational pull:

Waiting for the tide - by Kevin Temple, from Pixdaus -  .. .  
But now that the principle causes of tides are understood and quite well mathematically modeled, they are pretty easy to understand by the average Joe when explained properly.

Tides exist solely because of the celestial bodies the moon and the sun. The moon is relatively close but comparatively small. The sun is far away but a giant when it comes to mass. Gravitational pull increases linearly with mass but decreases inversely with the square of the distance. So when you balance these out and do the calculations, the sun’s effect is 46% that of the moon.

So the moon has the dominant and most noticeable effect on the earth’s tides. However the Sun still has a significant effect.

There are a couple of tricks to understanding the tides that took the physicists quite a while to figure out in the old days. The moon circles the earth about every 24 hours and 50.4 minutes. But for most of us it seems that the tide takes about 6 and a bit hours to go out and 6 and a bit hours to come back in. That’s a frequency of 12 and a half hours not 24:50 hours. What’s up with that?

Tide is actually a gravitational concept rather than specifically related to the movement of our oceans. All celestial objects are susceptible to tidal effects from other 'relatively close' bodies. Consequently a planet with in gravitational range of another body will experience pull leading to stresses acting to deform the shape of the planet.

That means that as the moon circles the earth, the earth is deformed by the moon. Similarly, the moon also experiences deformation by the earth and thus is suffers a tide effect. The effect is called tidal force. Fortunately for life on earth, the deformation is small but it does lead to the rising water that we experience. Therefore, with out the moon, life on earth would be vastly different.

As non celestial body and gravitational experts we’ll probably struggle to understand the resultant effect because it happens in a way that we might not initially expect.

Double Bulge Phenomenom:

Neap tide -  .. .  
It is suffice for this description of tides to state that the planet doesn’t just bulge towards the distant body but actually bulges on both sides in a line between the center of the two bodies because of the volumetric stress imparted upon the planet. Various arguments still exist on why, some are flat out wrong and some are right. As sailors we really don’t care too much other than knowing the resultant.

To gain a visual appreciation for this, although not in anyway accurate in science, take a calamari ring and hold in two points opposite each other. Now hold one side still and pull the other side. You’ll see the ring bulge on both sides in a line of the direction of pull.

Spring tide -  .. .  
That is a similar resultant action to what the earth is experiencing. The bulge occurs on both sides of the planet diametrically opposite each other and in the plane of a line between the moon and the earth. So following this, we have a high tide on the same side of the planet as the moon and simultaneously on the opposite side. Thus leading to a theoretical tidal period twice for every rotation of the moon about the earth.

The double bulge concept has nothing to do with the spin and inertia of water on the earth, although you will find some arguments and textbooks reporting this as a contributing factor to the opposite bulge. Not so! In any case, again as sailors we don’t necessarily care too much. We just need to know and realize that the double bulge occurs to explain two tides in a day rather than matching the periodicity of one moon rotation per day.

The exact same concept is valid for the sun. A solar high tide exists on the same side of earth as the sun and simultaneously on the opposite side of the sun.

Theoretical Tide Heights:

Now let’s look at a few theoretical mathematical numbers for comparison. If the earth’s oceans were uniform in depth and no landmasses existed and just considering the moon and if the moon was rotating around the earth every 24 hours, plus a few other ands and ifs, then the theoretical rise of the water at high tide would be 54 cm (21 inches). Now consider the suns effect in the same way. This would lead to a theoretical tide rise of 25 cm (9.8 inches). If the moon and the sun aligned or were directly opposite (from the bulge theory above) then we could add these heights together and get 79 cm (31 inches). If the moon and sun were acting against each other by being 90 degrees apart, then we would subtract the numbers and be left with a tidal height of 29 cm (11 inches).

The phenomenon then of a spring tide has thus been explained. Spring tide is when the sun and moon align or are diametrically opposite. The word spring does not come from a relationship to the season but from the concept of 'force'. Meaning there is more force during this period of alignment, which occurs about every 14 days – full moon and new moon.

Consequently, a neap tide is the minimal tide effect from the action of the sun and moon opposing each other. When they are at 90 degrees from each other – not diametrically opposite each other due to the double bulge effect. Again, this occurs at a period of every 14 days.

In addition to the above, the sun and moon operate in elliptical orbits relative to the earth. You can then imagine as the sun and moon become at their closest points to the earth that a greater effect would take place. The effect is increased 18% for the moon and 5% for the sun. Since the earth rotates about the sun once per year, at a certain times of the year then you would expect all these phenomena to exist simultaneously and create a theoretical tidal height of 93 cm (37 inches).

You can also imagine a common high high tide condition existing when the moon is closest to the earth with the moon and sun aligned (or diametrically opposite). This is called a perigee spring tide and occurs every 7.5 moon cycles.

This can be further expanded to include the sun being at its closest point along with the moon and in a spring tide situation. This occurs every 18.6 years. For this reason tidal high high and low low measurements are recorded and reported over periods of 19 years by statistical governmental recording agencies.

There is one last effect to discuss and that is that the sun and moons orbits are not directly aligned with the pole of the earth. Both the sun and moon’s orbits are not in unison and change day by day. For this reason the height of each high tide will vary day by day.

And just in case you’re wondering, Venus provides the next greatest tidal effect on the earth. But it is less than 0.001% of the magnitude of the Sun’s effect.

Here is a rudimentary animation of the Moon’s and the Sun’s tidal forces on the Earth:'>

by Grant Headifen
- 10:15 PM Fri 17 Sep 2010 GMT

Click here for printer friendly version
Click here to send us feedback or comments about this story.
If you want to send this story to a friend, please use link below

Based on your current download speed Sail-World has delivered large images (660 pixels long side.)  If you would like to see only small images (300 pixels long side) click here .  For medium sized images (500 pixels long side) click here .  If you would prefer to see text only (without pictures) click here

Click for further information on

Related News Stories:
05 Apr 2013  World's tiniest PLB now certified for use
27 Mar 2011  Low DSC connect rate:Sailor irresponsibility or technological failure?
05 May 2007  AIS - a major navigation advance
16 Jun 2006  The Necessary Sextant and How to Choose It
25 May 2006  Man Overboard! - See the Video
18 Apr 2006  Yachtmaster stable after flare explosion
23 Mar 2006  Seabrakes - a must have for offshore sailors?
12 Jan 2006  Retrieving your Anchor
01 Jan 2006  New Products 21DEC05
29 Dec 2005  Dinghies

Switch Default Region to:

Social Media





New Zealand

United Kingdom

United States

Cruising Northern

Cruising Southern

MarineBusiness World

PowerBoat World

FishingBoating World






Contact Us

Advertisers Information

Submit news/events

Search Stories/Text


Advertisers Directory

Newsletter Archive

Photo Gallery


Banner Advertising Details

Newsletter Subscribe

Video Gallery





Privacy Policy



Cookie Policy



This site and its contents are © Copyright TetraMedia and/or the original author, photographer etc. All Rights Reserved.  Photographs are copyright by law.  If you wish to use or buy a photograph contact the photographer directly.
LocalAds   DE  ES  FR  IT