Eclipse of the moon

[BobE]

Daily Telegraph says that there is a total eclipse of the moon tomorrow at around 0300 GMT..
Now that means that the three bodies must be dead in line at that time...
So here's my query..
Christchurch Entry
Belfield give a low tide of 0.5 m at 03:59
and a lower one of 0.2 m at 16:17
Now I know there are time delays due to friction land shapes etc but I would have thought that the tide nearest the eclipse time would have the greater range?
Furthermore Autotide gives ...

Location: Christchurch - Entrance 50°43'N 1°45'W
Time Zone: 00:00
Start From: 20 Feb 2008 Time: 11:04. Duration: 30 days
Listing of: Lowest 5
1 LO 16:40 8 Mar 2008 Sat 0.17m
2 LO 16:04 7 Mar 2008 Fri 0.19m
3 LO 17:22 9 Mar 2008 Sun 0.21m
4 LO 16:28 21 Feb 2008 Thu 0.27m
5 LO 15:28 6 Mar 2008 Thu 0.28m
and
Listing of: Highest 5
1 HI 09:03 8 Mar 2008 Sat 1.78m
2 HI 09:45 9 Mar 2008 Sun 1.77m
3 HI 08:27 7 Mar 2008 Fri 1.76m
4 HI 08:57 21 Feb 2008 Thu 1.73m
5 HI 09:27 22 Feb 2008 Fri 1.72m

Which show the March 6th tides to be the highest/lowest ones.. in fact for the year.
Again I find this odd even though the vernal equinox is probably involved surely the eclipse tides should be the highst./lowest ones??

Anyway I'll be at Mudeford quay on Sat 8th March at about 16:15 onwards to see what changes have taken place in "The Run"

Cheers Bob E...

 

[dt4134]

My understanding is that the movement of the Sun and Moon generate the tides out in the Atlantic, When those tides reach shallower water they are slowed down and the tides we experience around the coast lag about two or three days behind the astronomical cycle.

Usually one high tide per day is higher than the other because the pull of the moon and sun are rarely in the plane of the equator.

The area between Christchurch and Portland is unusual in that the tidal range is very small compared to the surrounding areas. I believe this is because two much larger tidal waveforms pretty nearly cancel each other out. Small variations in the phase differences between these two 'waves' would have a significant effect that is likely to be greater than the effect of the alignment of the Sun & Moon in the eclipse.

I used to be a physicist but I have to admit I've only looked at the science behind the tides quite casually I'd certainly be interested if there's a Hydrographer out there who could give a proper answer.

 

[BobE]

I just did the same for St.Marys in the Scillies
Location: St.Mary's 49°55'N 6°19'W
Time Zone: 00:00
Start From: 20 Feb 2008 Time: 11:52. Duration: 30 days
Listing of: Lowest 5
1 LO 12:28 9 Mar 2008 Sun 0.38m
2 LO 00:46 10 Mar 2008 Mon 0.41m
3 LO 00:04 9 Mar 2008 Sun 0.43m
4 LO 11:46 8 Mar 2008 Sat 0.46m
5 LO 13:04 10 Mar 2008 Mon 0.47m
Listing of: Highest 5
1 HI 05:57 9 Mar 2008 Sun 6.13m
2 HI 06:33 10 Mar 2008 Mon 6.08m
3 HI 18:15 9 Mar 2008 Sun 6.00m
4 HI 05:15 8 Mar 2008 Sat 6.00m
5 HI 17:39 8 Mar 2008 Sat 5.94m

and all the March tides are higher/lower than the Febs????

Where else should we check... The program should cover the world!!!
Dunno how much of the earth's surface is subject to the eclipse tho'
Cheers Bob E..

 

[Daipal]

Tides are caused by the gravitational pull of the Moon and the Sun. The Moon has a much greater effect than the Sun as it is much closer to Earth. When there is alignment between the Sun, Earth and Moon there will be Spring tides. This happens twice every 28 days. The shadow causing the eclipse is relatively small and the difference between the alignment for a lunar eclipse and a normal new moon is very small and has a negligible effect on the tides.
However the orbits of the Moon and Earth are elliptical. The Moon varies from about 216,000 miles to about 247,000 miles from the Earth. The closer it is the higher the Spring tide. The distance to the Sun also has a similar, but smaller effect. In fact all of the Solar system has a minute effect and this is used to calculate HAT (Highest Astronomical Tide) and LAT values for charts.
Daipal

 

[saltwater_gypsy]

A key factor that is often ignored when discussing the origin of tides is that the Earth- Moon system has a centre of gravity that is inside the earth(about 1/3 of the way to the centre). The Earth-moon system spins about an axis through this centre of gravity and this causes a bulge of water on the side of the Earth furthest from the Sun.The bulge on the side nearest the Sun is mainly gravitational and is largely due to the Moon. Both bulges are almost equal as the Earth is in a stable spin. The Alignment of the Earth-Moon system fine tunes the whole effect to give Spring and Neap tides.
What was the question again?
Oh yes, when you get a Lunar eclipse the gravitational effects from the Sun and The Moon are in opposition so you would expect the high water level to be reduced.
I await a barrage of corrections!!!!!!

 

[dt4134]

The spring tides are always highest approximately around Easter and aroundabout September.

The tides are not actually caused by the gravitation effect of the moon and the sun but by the differential of the gravity. The sun has a much greater gravitational pull on the Earth, which is why the Earth goes around the sun, but the difference between the gravitational force at a point on the surface facing the sun and the opposite surface is comparatively much less than that for the moon which is why the moon has a larger effect on the tides.

The moon's orbit is an eclipse and, as has been pointed out, the moon has more impact when it is closer because the difference in gravitational pull between the point facing the moon and the opposite point is greater when the moon is closer. The force due to gravity falls off as 1/r^2.

I think the moons orbit is roughly 29 days, so the effect it has on the tides will diminish and grow over this cycle as it moves further away and comes closer. I know the sun is closest roughly around the beginning of January but don't know when the moon is closest. However, I suppose that the larger spring tides in spring & autumn occur because the time when the moon is closest and having the greatest effect coincides with when the sun and moon are aligned. I vaguely remember there being something about the alignment being better in spring & autumn too.

Although for the eclipse the sun & moon are obviously well aligned, probably the moon is nowhere near its closest point in its orbit so the effect is diminished.

Bowditch also has some useful info on diurnal and semi-diurnal tides in case you're looking further afield than the British Isles (it's distinctly semi-diurnal here). Bowditch

 

[macd]

"The spring tides are always highest approximately around Easter and aroundabout September"

I invite you to Google they way the dates of Easter are determined, a mix of gobbledegook and nonsense involving, amongst other crap, the 'golden number' -- for a date that's supposedly an anniversary (spot the bloke who thinks anyone who has a god is delusional). Spring and autumn equinox might be a better way of expressing it.

Incidentally, I've read that tides are less caused by the 'lifting' and 'lowering' of sea water than the gravity-induced deformatiom of the sea floor. Anyone know any more about this?

 

[alan_d]

[ QUOTE ]
Spring and autumn equinox might be a better way of expressing it.


[/ QUOTE ]

The date of easter is linked to the Vernal Equinox. In the Western churches it is the first Sunday after the first full moon after the 21st March. The Orthodox churches use a different method based on the Julian calendar, but still linked to the equinox. Although there is a clear symbolic link between the Resurrection and Spring equinoctal celebrations it is also (roughly) the correct date for the anniversary, as the Crucifixion took place on the eve of the Passover, a Jewish festival linked to the equinox.

More information here.

 

[peterb]

[ QUOTE ]
My understanding is that the movement of the Sun and Moon generate the tides out in the Atlantic, When those tides reach shallower water they are slowed down and the tides we experience around the coast lag about two or three days behind the astronomical cycle.

Usually one high tide per day is higher than the other because the pull of the moon and sun are rarely in the plane of the equator.

The area between Christchurch and Portland is unusual in that the tidal range is very small compared to the surrounding areas. I believe this is because two much larger tidal waveforms pretty nearly cancel each other out. Small variations in the phase differences between these two 'waves' would have a significant effect that is likely to be greater than the effect of the alignment of the Sun & Moon in the eclipse.

I used to be a physicist but I have to admit I've only looked at the science behind the tides quite casually I'd certainly be interested if there's a Hydrographer out there who could give a proper answer.

[/ QUOTE ]

You can think of the tide as a wave; slow (two waves a day) but actually quite fast moving. In deep water the speed depends on the wavelength, but as the water shallows to depths comparable or less than the wavelength, the speed depends more on the depth of the water.

The tide coming up the channel is in water much shallower than the wavelength, so its speed depends almost entirely on the depth. The wave comes up the channel until it reaches the Dover Straits, which act like a wall and reflect the wave. At some point the crest of the east-going wave will meet the valley of the reflected (now west-going) wave and the two will tend to cancel out. If you do the sums, you find that this happens somewhere between Hurst and Poole, hence the small range at Poole.

The wave moves faster in deep water than in shallow, and hence the crest of the wave (deep water) moves slower than the valley (shallow water), and the crest catches up the valley. You can see it happening on the beach, with the crest overtaking the valley until finally the wave breaks. Look at the tidal curve for Dover; the front of the wave rises in five hours, but then takes seven and a half hours to fall.

This difference causes the shape of the wave to be distorted; instead of the nice smooth sine wave that you get in the open ocean, yuou get the distoted wave, almost a saw-tooth, at Dover. Mathematically (or musically) this can be expressed as the generation of harmonics. The harmonics don't move at the same speed as the main wave, so the point of cancellation is different. Hence near Poole and the Solent the ratio of harmonics to the fundamental sine wave is increased, and that's why the Solent sees double high waters.

On the original question, look at any of the tidal curves in the almanac, and you'll find a statement like "Springs occur 2 days after new and full moon". I've always believe that it's a resonance effect, rather like moving your hand in a bath full of water. It takes time for the resonance to build up or die away. The driving force of sun and moon in line may have reached or passed its maximum, but the water swing still increase for a couple of days.

 

[cnh]

Well, yes and no.

The 'wave' stays pointing towards the moon. The earth rotates underneath. The 'wave' is actually a bulge in the liquid surface of the ocean caused by [1] the difference in the moon's gravity from one side of the earth to the other and [2] the fact the earth and moon rotate round a common centre of gravity.

Similar arguments apply to the sun and earth, though the effect is much less.

Nicholas Hill

 

[VicS]

[ QUOTE ]
it is the first Sunday after the first full moon after the 21st March

[/ QUOTE ] Nearly but not quite right

It is the first Sunday after the first full moon <u>on or</u> after the 21st March

 

[damo]

Another resonance effect is seen most remarkably in the Bristol Channel, Ungava Bay and the Bay of Fundy. The time it takes a large wave to go from the mouth of the inlet to the opposite end and back, is the same as the time from one high tide to the next. In other words, the 12.4 (approx) hour period of the lunar tides is close to the natural sloshing period of the bay, so the moon sloshes it as if it were a bathtub.

So where somewhere like Poole receives the cancelling effect, Avonmouth gets the opposite, with 14.5 -15m tides occurring regularly (and a range of nearly 18m on the last big springs). Ungava and Fundy have a statistical tie for the record: 16.8 - 17m

Good explanation of tides on wikipaedia, with a nice animation showing the earth-moon centre of rotation.

Here endeth the lesson! /forums/images/graemlins/grin.gif

 

[earlybird]

Quote, slightly abridged, from "Cruising under Sail", ( Eric Hiscock ).
"Part of the main tidal wave sweeping round the Southern Ocean strikes the east coast of Africa and is deflected towards the Cape. From there it travels northward up the length of the Atlantic. About 2 days after leaving the Cape, this wave reaches the British Isles where it combines with the true tides, so the highest tides in the British Isles are usually 2 days after full and change of the moon."