Why do the lowest tides follow the full moon by a few days?

[sea slug]

Hi,
I'm an old biologist/naturalist who loves to go tidepooling, so this is not a homework question. Why do the lowest tides follow the full moon by a few days? For example, the "super" full moon of yesterday March 19, will pull the lowest tides (here in Maine) on
Monday and Tuesday March 21 & 22.

Saturday March 19th 4:46 am -1.3 5:11 pm -1.5
Sunday March 20th 5:37 am -1.9 5:59 pm -1.6 (talk about spring tides! ha ha)
Monday March 21st 6:28 am -2.1 6:48 pm -1.5
Tuesday March 22nd 7:19 am -2.0 7:38 pm -1.0
Wednesday March 23 8:13 am -1.6

Does this delay have anything to do with tidal bulges and despinning of the moon?

Thanks!
Lynn

 

[DaveC426913]

1] The point directlon the Earth directly under the Moon is moving at approx. 1000mph. The movement of water takes more time than the time it takes for Earth to spin under the Moon.

2] A landless Earth (sans continents) would have a much smoother correlation between Moon and tides.

I am not sure if these are the only two factors to consider.

 

[sea slug]

Thanks Dave,

Would the delay of water movement be considered inertia? Since the water is already moving at 1,000mph due to rotation?

Sidereal time doesn't factor in does it?
Thanks,
Sea Slug

 

[AlephZero]

There are two major causes of the ocean tides, the gravitational pull of the sun and the moon. At new and full moon the two forces add up. At the first and third quarters they partially cancel out.

You can think of the tides as a very large wave whose "length" extends half way around the Earth (because there are 2 tides per day, not one), and travels once round the Earth in about 24 hours 50 minutes. Only a small fraction of the amount of energy in the wave is lost each day by "friction", mainly along coastlines rather than in the open oceans, and that fraction is replaced by the pull of the sun and moon.

If the sun and moon stayed permanently "in line" with each other, the tides would increase over several days or weeks to a level higher than the current maximum tides, in the same way that the motion of a child's swing builds up slowly from a constant "push" every cycle. The amount of energy lost in each cycle increases as the height of the tide increases, until the energy being added equals the energy being lost.

The build-up in height of the tides is still in progress when the force reaches its maximum at new or full moon. Even though the force is a bit less for the next one or two days, it is still big enough to keep increasing the height. As the force decreases further it is not enough to maintain the height of the tide and it starts to drop.

The same thing happens "in reverse" after the minumum force, at the moon's first and third quarters. It takes 2 or 3 days for the force to increase enough to start to increase the height of the tides again.

Note, the details of the tide at any point on a coastline are strongly affected by the local geography. The "tidal wave" is deflected from traveling around the Earth to traveling along the coast. The times of high tides relative to the rising or setting time of the moon can vary by several hours at points only a few hundred miles apart along a coastline.

 

[DaveC426913]

Thanks Dave,

Would the delay of water movement be considered inertia? Since the water is already moving at 1,000mph due to rotation?

The water is not moving at 1000mph, the point directly under the Moon is moving at 1000 mph across the ocean.

 

[sea slug]

Thanks AlephZero and Dave!
Sea Slug

 

[granpa]

http://volkov.oce.orst.edu/tides/

[URL]http://volkov.oce.orst.edu/tides/pic/tpxo7.2.gif[/URL]

 

[D H]

Out of order response,

Does this delay have anything to do with tidal bulges and despinning of the moon?

I'm dealing with this first (I'll get to the delay later) because this question represents a bit of an oversimplification on the part of the educational system. In a sense, there is no such thing as a tidal bulge, at least not in the sense taught in schools. There would be a tidal bulge if the Earth wasn't rotating and if Earth had no land and was instead covered by a very deep ocean. There are several reasons why this simple model is just too simple:

• Eurasia+Africa+Australia and North+South America, that block the flow of this tidal bulge.
• The oceans aren't deep enough to sustain a surface wave moving at 460 km/hr. Such a wave would require an ocean depth of 22 km or more.
• The sub-lunar point moves at 460 km/hr (at the equator) because the Earth is rotating. This means the Coriolis effect comes into play. Even if the continents were gone and the ocean was 22 km deep, there still wouldn't be a tidal bulge of the simple sort depicted in schools and across the 'net.

The animated gif provided by granpa (previous post) shows a better model of the tides based on modern remote sensing data and the not so modern Laplace's Tidal Equations. Can you see the tidal bulge in that image? What you see is a set of gyres rotating around various amphidromic points. The image below depicts the M₂ tidal (the principal lunar semidiurnal component of the tides) amphidromic systems. Click on the image for an enlarged version.

http://www.aviso.oceanobs.com/uploads/pics/200010_m2_amp_pha_fes99_sm.gif

So what about the tidal bulges? They appear as residual components if you look at things from the perspective of a frame in which the Earth and Moon are fixed and average out ocean heights over time and over latitude -- and they are small, about 3 or 4 centimeters high.

Now to answer your question in full.

Hi,
I'm an old biologist/naturalist who loves to go tidepooling, so this is not a homework question. Why do the lowest tides follow the full moon by a few days? For example, the "super" full moon of yesterday March 19, will pull the lowest tides (here in Maine) on
Monday and Tuesday March 21 & 22.

Saturday March 19th 4:46 am -1.3 5:11 pm -1.5
Sunday March 20th 5:37 am -1.9 5:59 pm -1.6 (talk about spring tides! ha ha)
Monday March 21st 6:28 am -2.1 6:48 pm -1.5
Tuesday March 22nd 7:19 am -2.0 7:38 pm -1.0
Wednesday March 23 8:13 am -1.6

Does this delay have anything to do with tidal bulges and despinning of the moon?

Thanks!
Lynn

This has nothing to do with the tidal bulges and the despinning of the moon. There are multiple frequency components to the tides due to the tides being caused by the Moon and the Sun and by interactions between the tides, the depths of the oceans, and the geography of the coasts. The best way to describe the tides for a given location is to perform a Fourier analysis of the historical observations the tides at that location. The result of this analysis will attribute the tides at the location as amplitudes and phases the various components of the tide (principal lunar semidiurnal (12.42 hour period), lunisolar diurnal (22.93 hours), principal solar semidiurnal (12 hours), principal lunar diurnal (25.82 hours), etc.). This attribution is strictly empirical and applies only to the given location. It is however extremely accurate in terms of predictability.

Those amphidromic systems can get quite complex. For example, it is always high tide somewhere in the North Sea due to the presence of three amphidromic points in the North Sea / English Channel.

 

[sea slug]

Oh granpa,

Thank you, but these tidal facts are way over my head! Remember, I'm just a biologist!

So is it correct to say that the highest/lowest tides follow full/new moon by almost a day because the net effect of solar and lunar "pull" is max/min tides every 12/24 hours?

In deep water with a pair of gees,
Sea Slug

 

[D H]

So is it correct to say that the highest/lowest tides follow full/new moon by almost a day because the net effect of solar and lunar "pull" is max/min tides every 12/24 hours?

No. You are extrapolating from a sample size of one (the conditions in your local part of Maine) to the world as a whole. As a biologist, you should know that extrapolating from a sample size of one is almost invariably a very bad thing to do.