[StarainBoy]

In High Tide but No Green Grass, I wonder how spring tides end up in spring and not in January.

[captaincarot]

in january the sun lies below the equator so the suns gravitation vector is pulling slightly south at the equinox it's on the equator so the gravitational forces are pulling in the same direction, straight out from the equator, well that's my guess anyway

[Keith Dickinson]

ripped from wiki, containing my favourite scrabble word syzygy, it does explain things

Around new and full moon when the Sun, Moon and Earth form a line (a condition known as syzygy[8] ), the tidal force due to the Sun reinforces that due to the Moon. The tide's range is then at its maximum: this is called the spring tide, or just springs. It is not named after the season but, like that word, derives from an earlier meaning of "jump, burst forth, rise" as in a natural spring. When the Moon is at first quarter or third quarter, the Sun and Moon are separated by 90° when viewed from the Earth, and the solar gravitational force partially cancels the Moon's. At these points in the lunar cycle, the tide's range is at its minimum: this is called the neap tide, or neaps (a word of uncertain origin). Spring tides result in high waters that are higher than average, low waters that are lower than average, slack water time that is shorter than average and stronger tidal currents than average. Neaps result in less extreme tidal conditions. There is about a seven day interval between springs and neaps.

[captaincarot]

but it doesn't answer the question that he asked though, i don't think.

that just tells us why we get tides, rather than why we get the biggest tide when the sun isn't at it's closest point to the earth.

[Ficedula]

Originally Posted by captaincarot

in january the sun lies below the equator so the suns gravitation vector is pulling slightly south at the equinox it's on the equator so the gravitational forces are pulling in the same direction, straight out from the equator, well that's my guess anyway

I am always confused by tides but i think the above is close. The moon orbits the earth in the plane of the ecliptic so when the sun is in the southern hemisphere the moon is in the northern so exert their greatest force on the respective hemispheres, dragging water away from the equator and evening out the tides globally. At the equinoxes the equatorial bulge caused by the spinning of the planet is added to by the gravitational pull of the sun and moon aligned over the equator.

Yesterdays high tide happened at a full moon, ie with the moon on the opposite side of the earth than the sun, thus pulling in the opposite direction. I think this is explained because the the moon does not simply orbit the earth but both the moon and earth are orbiting a common point which lies about 4,700 kilometres from Earth's centre, about three quarters of the Earth's radius.

[markgrubb]

Originally Posted by captaincarot

but it doesn't answer the question that he asked though, i don't think.

that just tells us why we get tides, rather than why we get the biggest tide when the sun isn't at it's closest point to the earth.

Because the moon contributes more than twice as much force to tides relative to the sun

[StarainBoy]

Originally Posted by Ficedula

The moon orbits the sun in the plane of the ecliptic so when the sun is in the southern hemisphere the moon is in the northern so exert their greatest force on the respective hemispheres, dragging water away from the equator and evening out the tides globally. At the equinoxes the equatorial bulge caused by the spinning of the planet is added to by the gravitational pull of the sun and moon aligned over the equator.

This is getting closer and it makes sense if you say, "The moon orbits the Earth", which is what I think you meant. In fact, the Moon has a slight tilt too of 5 degrees, which might explain why it's not always the full or new moon nearest the equinox that does the most pulling.

Thanks to everyone for their input.

[bartolli]

I think the biggest tides occur when both the moon & sun are pulling in roughly the same direction rather than when the sun is at its closest.

So if the sun is below the equator in january then it is pulling away from the moon, in the northern hemisphere?

When the sun is 5 million kilometers closer then this is only about 3% of the total distance & therefore this probably "doesnt amount to a hill of beans" (forgive the misquote).

[StarainBoy]

There's no guarantee that the Moon is in the northern hemisphere; it can equally be in the south. In fact, in our winter, full moon is in the north, new moon the south. It's the reverse in our summer.

I've just checked this with an orange and a tomato - those well known astronomical instruments. But that's also shown me that, ignoring the Moon's 5 degree wandering above and below the plane of our orbit, new moon is pretty much always augmenting the Sun's pull. A strange way of putting it since the Moon is pulling more than the Sun but it's easier to work out what's going on.

In that case, logic dictates that the northern highest tides should be at the summer solstice when a new moon and the Sun are combining to pull the tide along the Tropic of Cancer. At either of the equinoxes the highest tides should be at the equator.

This tends to suggest that the equatorial bulge in the oceans due to the Earth's spin actually has the greatest effect on the height of the tide. If the Moon and Sun are reinforcing that, then the whole world gets a good high tide.

I've tried finding confirmation of this on the Web but the best I get is a site saying that it’s complicated. Yeah, we know that, thanks.

[Ficedula]

the more one tries to get ones head around this the more slippery it gets. A little more reading and thinking and i believe that the equatorial bulge is a red herring, should be constant affecting all tides equally. I think the whole idea of moon being in northern hemisphere is also wrong because motion of earth around centre of mass creates an equal tidal bulge on the far side of the earth, ie in southern hemisphere.

i think the clue to the highest tides being equinoctial is here

The changing distance separating the Moon and Earth also affects tide heights. When the Moon is at perigee the range increases, and when it is at apogee the range shrinks. Every 7½ lunations (the full cycle from full moon to new to full), perigee coincides with either a new or full moon causing perigean spring tides with the largest tidal range.

masses of general info here but does not seem to specifically address the op question.

http://www.uwgb.edu/dutchs/EarthSC202Notes/TIDES.HTM

also see here

http://co-ops.nos.noaa.gov/restles4.html

[captaincarot]

i think i'll stick to qquantum mechanics it makes more sense

[StarainBoy]

Yes, the bulge at the equator must be irrelevant because it’s always there and always acts as the baseline from which to measure tidal variation.

I now like the idea of the Moon’s distance having the biggest effect. This varies from 407,000 kilometres down to 357,000, but not in the regular yearly way that the Earth’s orbit varies round the Sun. Why not? Because the Sun, as well as the Earth, is shunting the Moon around; it’s a complex three-body problem.

Newton’s calculation for gravity has it grow proportional to the square of the distance between two bodies. So, plugging 407 and 357 into my calculator, I get a 30% increase in the Moon’s pull on the oceans from furthest to closest approaches. Incidentally, I get an 8% increase in the Sun’s pull from July to January.

OK, so why does this happen close to the vernal equinox? It may just be coincidence and may be why such high tides got their name. Give it another few thousand years and spring tides may be happening in September. I’m sure it’s all to do with the complex dance that the Sun is choreographing between Earth and Moon.

[Ficedula]

although the explanation is far from clear i think it can be found in the NOAA website.

Spring tides do not happen around the solstices because the Lunar Declination means that when, at this time, sun earth and moon are aligned at syzygy the moon is 5 degrees off the ecliptic plane thus not pulling exactly in line with the sun. Additionally in the summer months we are further from the sun (Aphelion 2 July) so spring tides tend to be on the winter side of an equinox when we are closer to the sun (Perihelion 2 Jan.)

However, the earth's axial tilt is means that only at the equinoxes is the sun directly over the equator. Within a 28 day period around each equinox the moon will also cross the equator thus syzygy can coincide with the moon pulling directly along the ecliptic plane so you have your spring tides.

I think this perfect syzygy (i.e. sun, moon and earth not only in line but also with moon on the plane of the ecliptic) does not have to occur over the equator. The 2 March spring tide was well before the equinox so presumably the moon passed behind the earth very close to the time it crossed the plane of the ecliptic. Can't have been exact though as this would have caused an eclipse of the moon.

the attached sketch might help. [note i have not even attempted to include the affects of lunar apogee/perigee]

[markgrubb]

Originally Posted by Ficedula

although the explanation is far from clear i think it can be found in the NOAA website.

Spring tides do not happen around the solstices because the Lunar Declination means that when, at this time, sun earth and moon are aligned at syzygy the moon is 5 degrees off the ecliptic plane thus not pulling exactly in line with the sun. Additionally in the summer months we are further from the sun (Aphelion 2 July) so spring tides tend to be on the winter side of an equinox when we are closer to the sun (Perihelion 2 Jan.)

However, the earth's axial tilt is means that only at the equinoxes is the sun directly over the equator. Within a 28 day period around each equinox the moon will also cross the equator thus syzygy can coincide with the moon pulling directly along the ecliptic plane so you have your spring tides.

I think this perfect syzygy (i.e. sun, moon and earth not only in line but also with moon on the plane of the ecliptic) does not have to occur over the equator. The 2 March spring tide was well before the equinox so presumably the moon passed behind the earth very close to the time it crossed the plane of the ecliptic. Can't have been exact though as this would have caused an eclipse of the moon.

the attached sketch might help. [note i have not even attempted to include the affects of lunar apogee/perigee]

This is more or less correct. As I stated befor the moons effect on tides is roughly twice that of the sun. This is by far the biggest effect so when in line with the sun and at its closest(in distance and in plane) you will get the biggest tides.-see http://www.pol.ac.uk/ntslf/tidalp.html for dates. The equinox is a bit of a red herring and is a smaller factor-the biggest tide for many years will occur a month before the equinox in 2015-click on XHL and your local area for highest tides

[StarainBoy]

The diagram is a great idea and it does show that, at the moment, the Moon’s orbit does cause spring tides in the spring. But you could swap the orbits between the two diagrams to show spring tides in the summer. I guess the question here is: does the axis of the Moon’s orbit remain fixed forever? Or does it precess over the centuries? This question just got bigger than the one I originally asked!

[StarainBoy]

I've done some more digging and the precession of the Moon’s orbit means it works its way round the Earth in 18.6 years. Right now it crosses the ecliptic in January and July. In a couple of years it'll be November and May. But the spring tides will keep happening somewhere near March.

Dang! I thought we’d cracked it then!

[markgrubb]

Originally Posted by StarainBoy

I've done some more digging and the precession of the Moon’s orbit means it works its way round the Earth in 18.6 years. Right now it crosses the ecliptic in January and July. In a couple of years it'll be November and May. But the spring tides will keep happening somewhere near March.

Dang! I thought we’d cracked it then!

The perihelion (when the sun is nearest the earth) is always in January. The perigee(when the moon is nearest the earth) is at a variety of times. Presumably the very highest tides occur when both the sun and moon are both relatively close to the earth and in line. And because the sun is always nearest the earth early in the year then the highest spring tides occur early in the year. Or is that too simplistic?

[Ficedula]

Originally Posted by StarainBoy

Right now it crosses the ecliptic in January and July.

are you sure? the moon orbits the earth once every 28 days so it should cross the ecliptic twice in this period. What is critical for the highest tides is that a crossing of the ecliptic coincides with syzygy and perigee while still not too far from perihelion. Simples

[halftwo]

Put simply (as far as I understand it) highest tides happen when the sun & moon are opposite each other - the moon has the bigger effect.
The effect of this is that the seas 'stretch' towards moon & sun. When the three bodies are aligned the effect is greatest - and the seas bulge in the direction of sun & moon.

This link explains:

www.redfishpro.org/images/SpringTides.gif&imgrefurl=http://www.redfishpro.org/understanding-how-tide-cycles-follow-moon.html&h=262&w=402&sz=18&tbnid=nzCVWfWAsbxPVM:& tbnh=81&tbnw=124&prev=/images%3Fq%3Dspring%2Btides&usg=__OaJmiEz7XmCK6Ch_ 86nw8XsuGTA=&ei=CcmSS7mBIaay0gT1r5zsDA&sa=X&oi=ima ge_result&resnum=7&ct=image&ved=0CBwQ9QEwBg


However the highest tidal ranges are because of geography - google Bay of Fundy eg.