I'm really scratching my head trying to understand tides..
According to theory, the tides follow the moon and sun's pull of gravity, but should be lagging somewhat behind due to the mass of water that needs to catch up. To take the simplistic view, when a full moon is at the zenith, there should be a high tide. However, if you look at the tide tables, there doesn't seem to be such a correlation.. In fact, for Singapore tomorrow, the high tide is 11.53pm, a full hour BEFORE the full moon reaches the highest point in the sky.. http://www.mobilegeographics.com:81/cal ... /5729.html
Anyone can help me out and explain how the tides work?
Cheers,
Chris
Understanding tides
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weixing
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Hi,
Hmm... This is what I think... may not be correct
:
When the Moon is in the open ocean, the water fom all direction will move toward where the Moon is. So when the Moon is moving toward Singapore from the east, the seawater on the west of Singapore will move towards the East direction... towards and move around Singapore land, so it'll cause a high tides even before the Moon reach Zenith.
Have a nice day.
Hmm... This is what I think... may not be correct
: When the Moon is in the open ocean, the water fom all direction will move toward where the Moon is. So when the Moon is moving toward Singapore from the east, the seawater on the west of Singapore will move towards the East direction... towards and move around Singapore land, so it'll cause a high tides even before the Moon reach Zenith.
Have a nice day.
Yang Weixing
"The universe is composed mainly of hydrogen and ignorance."
"The universe is composed mainly of hydrogen and ignorance." -
alvinsclee
- Posts: 299
- Joined: Mon Feb 20, 2006 9:21 pm
WeiXing has a point there 
"For most purposes it is useful to regard the observed sea level as the combined result of three main factors:
Observed sea level = tidal level + surge level + mean sea level"
Source of info: http://www.cambridge.org/catalogue/cata ... 183&ss=exc
"For most purposes it is useful to regard the observed sea level as the combined result of three main factors:
Observed sea level = tidal level + surge level + mean sea level"
Source of info: http://www.cambridge.org/catalogue/cata ... 183&ss=exc
Hi guys,When the Moon is in the open ocean, the water fom all direction will move toward where the Moon is. So when the Moon is moving toward Singapore from the east, the seawater on the west of Singapore will move towards the East direction... towards and move around Singapore land, so it'll cause a high tides even before the Moon reach Zenith.
Thanks for your response! However, it still doesn't make sense to me..
Does anyone else know if this is correct?
Chris
Re: Understanding tides
Tides are really one big component that is the resultant of numerous variations and factors put together in the final equation. These can range widely from influences of astronomic, hydraulic, hydrodynamic, hydrographic and topographic origins, including modifications by meteorological conditions. Although each of these factors takes a different slant or angle of how tides form and behave, all of them seem to converge on two common principles which form the basis of tidal mechanics.
The two most crucial basic forces contributing to tides on the Earth's surface are:
(1) the force of gravitation exerted by the moon (and sun) upon the earth; and
(2) centrifugal forces produced by the revolutions of the earth and moon (and earth and sun) around their common center-of-gravity (mass) or barycenter.
Understanding just these two key principles will help largely in building up on the more complex concepts later on.
Now adding on to the basic forces, more complicated contributing variations and factors (not all are listed) start to surface, which are:
Astronomical Factors:
- Differential Gravitational Forces (or simply 'differential tide-producing force')
- Tractive Force (draws the waters of the earth horizontally over its surface toward the sublunar and antipodal points)
- Tidal Force Envelope
- Frictional Drag
- Angular Velocity of Moon and Earth
Non-Astronomical Factors:
- Pattern of Tidal Flow
- Shape of the Earth's continents: Prevents the tidal bulges from simply following the moon (except around Antarctica).
- Topography of ocean basins
- Storm surges
Since this is an astronomy forum and for simplicity sake, I will only elaborate on the most relevant points in bold, since they have a more direct impact on the events raised earlier. For a more scientifically complete explanation covering most aspects of tides though, you can refer to the links provided right at the bottom, otherwise we might end up with having a continuous 3-hour long lecture instead (just kidding).
1) Frictional Drag
The cause for the supposed tidal displacement is due to a force called 'Frictional Drag'. The gravitational forces of the Moon causes the surface water on Earth to bulge on two sides (one point being nearest the Moon, the other on the opposite side of Earth). As Earth tries to drag the tidal lumps around with it in its daily rotation, the Moon tries to retard it. The net result of this is that the tidal bulge 'leads' the Moon, and the term describing this is called 'tidal priming' which therefore brings us to a second explanation.
Tidal priming is the main cause behind the acceleration in tidal arrival times which in this instance, an hour earlier. How do all these work out?
"An astronomical factor influencing the time of arrival of tides of a given phase at any location results from the interaction between the tidal force envelopes of the moon and sun. Between new moon and first-quarter phase, and between full moon and third-quarter phase, this phenomenon can cause a displacement of force components and an acceleration in tidal arrival times (known as priming the tides) resulting in the occurrence of high tides before the moon itself reaches the local meridian of the place (Greater 'lead'). Between first-quarter phase and full moon, and between third-quarter phase and new moon, an opposite displacement of force components and a delaying action (known as lagging of the tides) can occur, as the result of which the arrival of high tides may take place several hours after the moon has reached the meridian (Reduced 'lead')."
2) Angular Velocity of Moon and Earth
While orbiting Earth, the Moon rotates with an angular velocity of ~12.2o per day. Likewise, Earth rotates on its axis with an angular velocity of 360o per day. Due to an additional difference of the ~12.2o angular velocity of the Moon that a point on Earth has to catch up with, a total average lag time of ~50 minutes is incurred and this causes the time between two successive upper transits of the moon across the local meridian of the place (a period known as the lunar or "tidal" day) to exceed the 24 hours of the earth's rotation period - the mean solar day. In consequence, the recurrence of a tide of the same phase and similar rise would take place at an interval of 24 hours 50 minutes after the preceding occurrence, if this single astronomical factor known as lunar retardation were considered. If we were to backtrack to find out the chain of events leading to the irregularities in the maximum high tide versus the observed mean solar day, we will find that first, the length of the tidal day whose mean is 24h 50min can vary, which is due largely to the varying period between the Moon’s meridian passage (i.e. culmination, transit), and therefore, it can also be derived that due to the Moon's inconsistent meridian passage period (possibly also attributed to the ellipticity and obliquity of the Moon's orbit), the probability of forming a correlation of a maximum high tide to a specific time or period of a solar day (E.g. 10pm +/- 1 hr) is virtually close to zero.
3) Alignment of the Three Gravitational Bodies
Mentioned earlier in the events above, the high tides we are experiencing now are called ‘spring tides’, from the German word ‘springen’, to spring up. These types of tides cause the highest water levels at the beach. Spring tides occur when the Sun, Earth and Moon are at syzygy, whereby the gravitational pull of the Sun and Moon are lined up during new moon and full moon periods. Using this analogy of a gravitational alignment, I wish to propose a theoretical model of a different perspective that could possibly explain why the high tide does not have to coincide with the full moon at zenith.
Based on the three-body alignment during new moon or full moon:
(1) As long as the Earth, Moon and Sun are in line, we will experience Spring tides and therefore,
(2) Regardless of the Moon’s hour angle or position over the local meridian, if a geographical point on Earth is in line with the Moon and Sun, we will also experience Spring tides.
Since the Moon's elliptical orbit is at an angle to the Earth's equator, the period between its meridian passage (i.e culmination) varies, thus varying the length of the tidal day whose mean is 24h 50min, and therefore varying the period between two high tides.
To visualize this:
Scenario #1:
Imagine aligning three plastic balls placed on a horizontal plane. They represent Moon, Earth and Sun. This alignment shall denote the formation of spring tides on a point on Earth.
Scenario #2:
Imagine inclining the horizontal plane (with the three plastic balls on it) at an angle. The inclined (slanted) plane with the three plastic balls now represents Moon, Earth and Sun revolving on their tilted axes. Since there is still alignment of Moon, Earth and Sun here, therefore there is formation of spring tides on a point on Earth.
Scenario #3:
Imagine further inclining the horizontal plane at any angle and direction you like. The three plastic balls are still found aligned in a straight line, and based on the previous two scenarios, spring tides will still form on a point on Earth.
Therefore, from the observations above, it can be deduced that even though the Moon may not be at the zenith over a local meridian of a place, as long as it is at an angle or position which is aligned with the Earth and Sun, spring tides can still form on a point on Earth and there will be high tides. Hence, in the event of the Moon’s obliquity affecting the period between its meridian passage, I reckon the difference lies in the distance the Moon has to cover before reaching zenith, in which case the length of a tidal day is altered, and thereby resulting in the spring tides occurring even before the moon reaches its peak.
P.S. If, at any point in the proposed model you find that there are some loopholes in deducing the phenomena, of if there are major flaws in my train of thoughts leading to an inaccurate hypothesis, I will be glad to hear of your views.
Cheers,
- ALPiNe
.- .-.. .--. .. -. .
_____________________________________________________________
Resources:
http://tidesandcurrents.noaa.gov/restles1.html
http://www.astunit.com/tutorials/tides.htm
http://www.bartleby.com/30/16.html
The two most crucial basic forces contributing to tides on the Earth's surface are:
(1) the force of gravitation exerted by the moon (and sun) upon the earth; and
(2) centrifugal forces produced by the revolutions of the earth and moon (and earth and sun) around their common center-of-gravity (mass) or barycenter.
Understanding just these two key principles will help largely in building up on the more complex concepts later on.
Now adding on to the basic forces, more complicated contributing variations and factors (not all are listed) start to surface, which are:
Astronomical Factors:
- Differential Gravitational Forces (or simply 'differential tide-producing force')
- Tractive Force (draws the waters of the earth horizontally over its surface toward the sublunar and antipodal points)
- Tidal Force Envelope
- Frictional Drag
- Angular Velocity of Moon and Earth
Non-Astronomical Factors:
- Pattern of Tidal Flow
- Shape of the Earth's continents: Prevents the tidal bulges from simply following the moon (except around Antarctica).
- Topography of ocean basins
- Storm surges
Since this is an astronomy forum and for simplicity sake, I will only elaborate on the most relevant points in bold, since they have a more direct impact on the events raised earlier. For a more scientifically complete explanation covering most aspects of tides though, you can refer to the links provided right at the bottom, otherwise we might end up with having a continuous 3-hour long lecture instead (just kidding).
There are 3 interlinked reasons which explain the above phenomenon.chrisyeo wrote:I'm really scratching my head trying to understand tides..
According to theory, the tides follow the moon and sun's pull of gravity, but should be lagging somewhat behind due to the mass of water that needs to catch up. To take the simplistic view, when a full moon is at the zenith, there should be a high tide. However, if you look at the tide tables, there doesn't seem to be such a correlation.. In fact, for Singapore tomorrow, the high tide is 11.53pm, a full hour BEFORE the full moon reaches the highest point in the sky.. http://www.mobilegeographics.com:81/cal ... /5729.html
Anyone can help me out and explain how the tides work?
1) Frictional Drag
The cause for the supposed tidal displacement is due to a force called 'Frictional Drag'. The gravitational forces of the Moon causes the surface water on Earth to bulge on two sides (one point being nearest the Moon, the other on the opposite side of Earth). As Earth tries to drag the tidal lumps around with it in its daily rotation, the Moon tries to retard it. The net result of this is that the tidal bulge 'leads' the Moon, and the term describing this is called 'tidal priming' which therefore brings us to a second explanation.
Tidal priming is the main cause behind the acceleration in tidal arrival times which in this instance, an hour earlier. How do all these work out?
"An astronomical factor influencing the time of arrival of tides of a given phase at any location results from the interaction between the tidal force envelopes of the moon and sun. Between new moon and first-quarter phase, and between full moon and third-quarter phase, this phenomenon can cause a displacement of force components and an acceleration in tidal arrival times (known as priming the tides) resulting in the occurrence of high tides before the moon itself reaches the local meridian of the place (Greater 'lead'). Between first-quarter phase and full moon, and between third-quarter phase and new moon, an opposite displacement of force components and a delaying action (known as lagging of the tides) can occur, as the result of which the arrival of high tides may take place several hours after the moon has reached the meridian (Reduced 'lead')."
2) Angular Velocity of Moon and Earth
While orbiting Earth, the Moon rotates with an angular velocity of ~12.2o per day. Likewise, Earth rotates on its axis with an angular velocity of 360o per day. Due to an additional difference of the ~12.2o angular velocity of the Moon that a point on Earth has to catch up with, a total average lag time of ~50 minutes is incurred and this causes the time between two successive upper transits of the moon across the local meridian of the place (a period known as the lunar or "tidal" day) to exceed the 24 hours of the earth's rotation period - the mean solar day. In consequence, the recurrence of a tide of the same phase and similar rise would take place at an interval of 24 hours 50 minutes after the preceding occurrence, if this single astronomical factor known as lunar retardation were considered. If we were to backtrack to find out the chain of events leading to the irregularities in the maximum high tide versus the observed mean solar day, we will find that first, the length of the tidal day whose mean is 24h 50min can vary, which is due largely to the varying period between the Moon’s meridian passage (i.e. culmination, transit), and therefore, it can also be derived that due to the Moon's inconsistent meridian passage period (possibly also attributed to the ellipticity and obliquity of the Moon's orbit), the probability of forming a correlation of a maximum high tide to a specific time or period of a solar day (E.g. 10pm +/- 1 hr) is virtually close to zero.
3) Alignment of the Three Gravitational Bodies
Mentioned earlier in the events above, the high tides we are experiencing now are called ‘spring tides’, from the German word ‘springen’, to spring up. These types of tides cause the highest water levels at the beach. Spring tides occur when the Sun, Earth and Moon are at syzygy, whereby the gravitational pull of the Sun and Moon are lined up during new moon and full moon periods. Using this analogy of a gravitational alignment, I wish to propose a theoretical model of a different perspective that could possibly explain why the high tide does not have to coincide with the full moon at zenith.
Based on the three-body alignment during new moon or full moon:
(1) As long as the Earth, Moon and Sun are in line, we will experience Spring tides and therefore,
(2) Regardless of the Moon’s hour angle or position over the local meridian, if a geographical point on Earth is in line with the Moon and Sun, we will also experience Spring tides.
Since the Moon's elliptical orbit is at an angle to the Earth's equator, the period between its meridian passage (i.e culmination) varies, thus varying the length of the tidal day whose mean is 24h 50min, and therefore varying the period between two high tides.
To visualize this:
Scenario #1:
Imagine aligning three plastic balls placed on a horizontal plane. They represent Moon, Earth and Sun. This alignment shall denote the formation of spring tides on a point on Earth.
Scenario #2:
Imagine inclining the horizontal plane (with the three plastic balls on it) at an angle. The inclined (slanted) plane with the three plastic balls now represents Moon, Earth and Sun revolving on their tilted axes. Since there is still alignment of Moon, Earth and Sun here, therefore there is formation of spring tides on a point on Earth.
Scenario #3:
Imagine further inclining the horizontal plane at any angle and direction you like. The three plastic balls are still found aligned in a straight line, and based on the previous two scenarios, spring tides will still form on a point on Earth.
Therefore, from the observations above, it can be deduced that even though the Moon may not be at the zenith over a local meridian of a place, as long as it is at an angle or position which is aligned with the Earth and Sun, spring tides can still form on a point on Earth and there will be high tides. Hence, in the event of the Moon’s obliquity affecting the period between its meridian passage, I reckon the difference lies in the distance the Moon has to cover before reaching zenith, in which case the length of a tidal day is altered, and thereby resulting in the spring tides occurring even before the moon reaches its peak.
P.S. If, at any point in the proposed model you find that there are some loopholes in deducing the phenomena, of if there are major flaws in my train of thoughts leading to an inaccurate hypothesis, I will be glad to hear of your views.
Cheers,
- ALPiNe
.- .-.. .--. .. -. .
_____________________________________________________________
Resources:
http://tidesandcurrents.noaa.gov/restles1.html
http://www.astunit.com/tutorials/tides.htm
http://www.bartleby.com/30/16.html
As the Moon travels across the local meridian and reaches the zenith, tide-producing forces known as ‘tractive’ forces acts with the Earth’s gravitational force to draw ocean waters to positions on the Earth's surface known as ‘sublunar’ and ‘antipodal’ points. (Refer to the first link in the above post for more details.) This happens even for our local seawaters, as they are drawn towards the sublunar and antipodal points on Earth’s surface, rather than towards the eastern or western side of Singapore as mentioned. Even from a topographic point of view, Singapore’s land mass is just too small to have a profound effect on the tidal flow. Though, if the Moon’s gravity were to specifically impact the Singapore waters, the most likely consequence would be a slight variation in tide formation. At present, it seems to me that there are only semidiurnal tides in Singapore. Maybe I’m missing out something? The slight variation might result in a difference between the heights of two daily tides of the same phase, a phenomenon known as ‘Diurnal Inequality’. This is attributed to the changes in the declination angle of the moon, including its gravitational forces.weixing wrote:Hi,
Hmm... This is what I think... may not be correct
When the Moon is in the open ocean, the water fom all direction will move toward where the Moon is. So when the Moon is moving toward Singapore from the east, the seawater on the west of Singapore will move towards the East direction... towards and move around Singapore land, so it'll cause a high tides even before the Moon reach Zenith.
The only time possible to have seawater moving towards the East or West of Singapore is when there are strong east winds or west winds. However, the downside of it is these will result in ‘wind waves’ rather than true tides. The East/West winds will drag the surface water and cause it to heap up on the side towards which the wind is blowing. The opposing return current in the sea will then try to flow back to the displaced end to maintain hydrostatic equilibrium. Therefore, even though the Moon is not involved in such wave formations, it is still hydrodynamically possible to have Eastbound/Westbound wind waves.
Cheers,
- ALPiNe
.- .-.. .--. .. -. .
Hi Chris,
No problem. =) Glad you found the information useful.
How about this: Try typing the title of this thread in Yahoo search engine and see what you get. (Remember to click on the first link.)
Hope you are satisfied with the answer.
Cheers,
- ALPiNe
.- .-.. .--. .. -. .
No problem. =) Glad you found the information useful.
You can say that again.chrisyeo wrote:Yes. What I meant was, did you research all of this yourself? Wow!
How about this: Try typing the title of this thread in Yahoo search engine and see what you get. (Remember to click on the first link.)
Hope you are satisfied with the answer.
Cheers,
- ALPiNe
.- .-.. .--. .. -. .