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Christine112
2002-Jan-15, 05:22 PM
I understand how there are really high/low tides during a New Moon (because the sun and moon are both pulling on the Earth in the same direction), but why are there really high tides during a Full Moon? The sun and moon would be on different sides of the Earth, so I don't understand why there would be high tides. I would think there would be less drastic tides when the moon is full...

Azpod
2002-Jan-15, 05:59 PM
On 2002-01-15 12:22, Christine112 wrote:
I understand how there are really high/low tides during a New Moon (because the sun and moon are both pulling on the Earth in the same direction), but why are there really high tides during a Full Moon? The sun and moon would be on different sides of the Earth, so I don't understand why there would be high tides. I would think there would be less drastic tides when the moon is full...


That's because when the moon is new, the oceans are pulled toward both the Sun and the Moon, but of the opposite side of the planet, the total pull of gravity is less than it is on the closer side, so the oceans actually bulge there too! That is because there is less gravitational force holding the ocean on the Earth.

However, when the moon is full, both sides get pulled of the Earth are pulled, one by the Sun and the other by the Moon. It may sound strange, but the total tidal effect is the same as if the Sun and Moon were on the same side of the Earth.

You actually get the smallest tides when the Sun and the Moon are at 90 degrees with respect to each other from the viewpoint of the Earth.

That's my best shot at explaining it; I'm hoping that someone who can explain it better than I can fills in the holes that I'm certain that I left. /phpBB/images/smiles/icon_smile.gif

Valiant Dancer
2002-Jan-15, 06:12 PM
On 2002-01-15 12:22, Christine112 wrote:
I understand how there are really high/low tides during a New Moon (because the sun and moon are both pulling on the Earth in the same direction), but why are there really high tides during a Full Moon? The sun and moon would be on different sides of the Earth, so I don't understand why there would be high tides. I would think there would be less drastic tides when the moon is full...


During a full moon, the tides are high because the moon cannot offset the pull of the sun as it could during the first or last quarter. During the new moon produces the highest of the high tides (both are pulling on the same side). During the full moon, there is nothing to keep the tides somewhat balanced to the side of the sun, so the high tides occur, but in a much lesser degree. During the first and last quarter, the tides are kept relatively in balance with pulling to the "side" of the sun. (Neap tide lowest of the high tide situations.)

Link to information:

http://www.whoi.edu/seagrant/education/bulletins/tides.html

A sixth grader in Centreville, MD had this to say

"What is a tide?

A tide is the daily rise and fall of the oceans. The lowest level of water is defined, as the "low tide water mark" while the highest water level is the "high tide water mark". The usual frequency of tides is two high tides and two low tides daily. Sometimes there is only one of either. This is because of the specific shape of the land, and the water gets trapped within an enclosed area of land.

What causes tides?

Tides are caused by the gravitational pull of the sun and moon and by the rotation of the earth on its axis. As the earth spins, each part of the water is under the moon once about every 24 hours. The water bulging towards the moon makes the high tide. The centrifugal force causes another high tide on the opposite side of the earth. The centrifugal force is the force that pulls a thing outward from the center when it is spinning. This force pulls the water away from the earth to form a bulge while it is spinning.

What affects tides?

Tides are affected by the sunís gravitational pull. The moonís gravity also affects them. Although the sun is 27 million times larger than the moon, it is also 400 times further away. So the tidal force caused by the Sun is 50% less than the tidal force of the moon. Also moon phases affect tides. During the full and new moon, also referred to as spring tides, the tides are 20% larger and during first and third quarter moons, also referred to as neap tides, the tides are 20% smaller. "

pretty much sums it up.

SeanF
2002-Jan-15, 06:24 PM
On 2002-01-15 13:12, Valiant Dancer wrote:

A sixth grader in Centreville, MD had this to say

[Snipped]

The centrifugal force causes another high tide on the opposite side of the earth. The centrifugal force is the force that pulls a thing outward from the center when it is spinning. This force pulls the water away from the earth to form a bulge while it is spinning.



Um . . . centrifugal force from the spinning Earth causes the tidal bulge on the other side of the Earth? I don't know about that.

The Moon's gravity pulls on the Earth, but, since gravity is dependent on distance, it pulls "harder" on the near side. So if, for example, the moon's gravity is at a 10 on the side of the earth closest to it, at an 8 in the middle of the earth, and at a 6 on the far side, then you have a difference of 2 between the center and near side and a difference of 2 between the center and far side, so you've got a bulge on both sides (I'm just making up numbers here, but the concept remains).

This very site has a nice tide-related page right here (http://www.badastronomy.com/bad/misc/tides.html).


_________________
SeanF

<font size=-1>[ This Message was edited by: SeanF on 2002-01-15 13:25 ]</font>

Christine112
2002-Jan-15, 06:33 PM
But then wouldn't that be the case for all tides? Not just full ones?

Valiant Dancer
2002-Jan-15, 07:05 PM
On 2002-01-15 13:24, SeanF wrote:


On 2002-01-15 13:12, Valiant Dancer wrote:

A sixth grader in Centreville, MD had this to say

[Snipped]

The centrifugal force causes another high tide on the opposite side of the earth. The centrifugal force is the force that pulls a thing outward from the center when it is spinning. This force pulls the water away from the earth to form a bulge while it is spinning.



Um . . . centrifugal force from the spinning Earth causes the tidal bulge on the other side of the Earth? I don't know about that.

The Moon's gravity pulls on the Earth, but, since gravity is dependent on distance, it pulls "harder" on the near side. So if, for example, the moon's gravity is at a 10 on the side of the earth closest to it, at an 8 in the middle of the earth, and at a 6 on the far side, then you have a difference of 2 between the center and near side and a difference of 2 between the center and far side, so you've got a bulge on both sides (I'm just making up numbers here, but the concept remains).

This very site has a nice tide-related page right here (http://www.badastronomy.com/bad/misc/tides.html).


_________________
SeanF

<font size=-1>[ This Message was edited by: SeanF on 2002-01-15 13:25 ]</font>


And for the sun it would be 5 for the near side, 4 for the middle, and 3 on the far side.

This would equate to:

new moon: 15 closest to moon, 12 middle, and 9 far

1st and last quarters: 14 closest to moon, 13 sunward, 11 anti-sunward, and 10 far

full moon: 13 closest to moon, 12 middle, and 11 far.

Doesn't quite equate to gravitation alone. Centrifugal force could expain it.

SeanF
2002-Jan-15, 07:35 PM
On 2002-01-15 14:05, Valiant Dancer wrote:

full moon: 13 closest to moon, 12 middle, and 11 far.



Well, no, actually at full moon the numbers would be 7, 4, and 1 (the Moon and Sun are pulling against each other so you've got to subtract the numbers rather than add). At any rate, these were just numbers that I pulled out of my . . . hat, so they shouldn't necessarily add up. (On the other hand, New Moon gives 15-12-9 for a difference of 3 each way. Full Moon gives 7-4-1 for a difference of 3 each way! How about that?)

Centrifugal force from the spinning Earth would not create tidal variations -- that force would be the same all the way around the globe, all the time -- it would just make the Earth a slightly larger ball for the Moon and Sun to pull at different strengths against.


_________________
SeanF

<font size=-1>[ This Message was edited by: SeanF on 2002-01-15 14:37 ]</font>

<font size=-1>[ This Message was edited by: SeanF on 2002-01-15 14:48 ]</font>

SeanF
2002-Jan-15, 07:44 PM
On 2002-01-15 13:33, Christine112 wrote:
But then wouldn't that be the case for all tides? Not just full ones?


I'm not sure what you're asking about here . . .

On any given day, a location will have two high tides (when the Moon is directly overhead, and when the Moon is directly beneath your feet) and two low tides (when the Moon is directly to either side of you). The overhead high tide should be slightly higher than the underfoot high tide, and the two low tides should be pretty much the same.

During the Full Moon and New Moon times of the month, the high tide will be higher and the low tide lower, because the Moon and Sun will be pulling along the same axis. New Moon tides will be higher (and lower) than Full Moon tides.

During the Quarter Moon times of the month, the high tide will be lower and the low tide will be higher, because the Moon and Sun will be pulling at cross-axes and "evening things out."

Actually, that's the way it should probably be stated: During Quarter Moons, the tides will be at their most "even", relatively little difference between low tide and high tide. At Full Moon, there is a greater difference between high and low tides, and at New Moon an even greater difference.

Valiant Dancer
2002-Jan-15, 08:14 PM
On 2002-01-15 14:35, SeanF wrote:


On 2002-01-15 14:05, Valiant Dancer wrote:

full moon: 13 closest to moon, 12 middle, and 11 far.



Well, no, actually at full moon the numbers would be 7, 4, and 1 (the Moon and Sun are pulling against each other so you've got to subtract the numbers rather than add). At any rate, these were just numbers that I pulled out of my . . . hat, so they shouldn't necessarily add up. (On the other hand, New Moon gives 15-12-9 for a difference of 3 each way. Full Moon gives 7-4-1 for a difference of 3 each way! How about that?)

Centrifugal force from the spinning Earth would not create tidal variations -- that force would be the same all the way around the globe, all the time -- it would just make the Earth a slightly larger ball for the Moon and Sun to pull at different strengths against.


_________________
SeanF

<font size=-1>[ This Message was edited by: SeanF on 2002-01-15 14:37 ]</font>

<font size=-1>[ This Message was edited by: SeanF on 2002-01-15 14:48 ]</font>


You're right (darn you /phpBB/images/smiles/icon_smile.gif ). I hadn't thought of subtracting the numbers due to opposite effect. (dang, dang, dang) Obvious mistake on my part. (not the first time, won't be the last.)

Thanks for the clarification.