Tidal Cycle? Help me out here!

[Mike1956]

Having noticed a statement in a training booklet "Moon orbits the Earth over a 28 day cycle creating Spring & Neap tides" I emailed the publisher advising that a Tidal Cycle in fact averages 29 days, 12 hours, 44 mins & 3 seconds approximately. It is known as a Lunar month and does not vary much from this average and is never as little as 28 days. A google determined between the years 1760 and 2200 the shortest Lunar month is 29d 6h 34m and the longest 29d 19h 58m. Casual analysis of my tide table seems to confirm the above.

I was surprised when their reply strongly disagreed with my understanding and they went on to explain why....

The Tidal Cycle is due to an average of 3 cycles of the Moon.

Synodic Month 29.5 days
Tropical Month 27.5 days
Anomalistic Month 27.3 days

Average 28.1 days

I still feel my understanding is correct and have no comprehension of how the Tropical & Anomalistic would affect the Tidal Cycle.

I am sure someone on here will have a thorough knowledge of this subject and I look forward to reading replies.

Thanks

Mike

[RainDog]

I am with you. Tidal Cycle should follow the Synodic month.

If this is not the case, Spring or Neap tide will generally not occur during the Full or New moon.

[hellosailor]

From Wikipedia:
https://en.wikipedia.org/wiki/Lunar_month


"Sidereal month[edit]

The period of the Moon's orbit as defined with respect to the celestial sphere (of the fixed stars, nowadays the International Celestial Reference Frame (ICRF)) is known as a sidereal month because it is the time it takes the Moon to return to a similar position among the stars (Latin: sidera): 27.321661 days (27 d 7 h 43 min 11.5 s).[1] This type of month has been observed among cultures in the Middle East, India, and China in the following way: they divided the sky into 27 or 28 lunar mansions, one for each day of the month, identified by the prominent star(s) in them.
Synodic month[edit]

This is the average period of the Moon's revolution with respect to the line joining the Sun and Earth. The synodic month is the period of the Moon's phases, because the Moon's appearance depends on the position of the Moon with respect to the Sun as seen from the Earth.
While the Moon is orbiting the Earth, the Earth is progressing in its orbit around the Sun. After completing a sidereal month the Moon must move a little further to reach the new position having the same angular distance from the Sun. This longer period is called the synodic month (Greek: συνοδικός, sunodikos, meaning "pertaining to a synod, i.e., a meeting" [in this case of the Sun and the Moon]).
Since the Earth's orbit around the Sun is elliptical and not circular, the angular rate of Earth's progression around the Sun varies during the year. The angular rate is faster nearer periapsis and slower near apoapsis. The same is so for the Moon's orbit around the Earth. Because of these variations in angular rate, the actual time between lunations may range from about 29.18 to about 29.93 days"


So apparently the term "lunar month" is about as precise as "How long is a piece of string?"


It isn't bad enough having three different definitions of "sunrise" and "sunset"...

[a64pilot]

Lord, you guys are way over my head, all I know about the cycle is it goes out, where does it go? I don't know, seems to always come back though

[jackdale]

If it helps - from Wikipedia.

The sidereal month is defined as the Moon's orbital period in a non-rotating frame of reference (which on average is equal to its rotation period in the same frame). It is about 27.32166 days (27 days, 7 hours, 43 minutes, 11.6 seconds). The exact duration of the orbital period cannot be easily determined, because the 'non-rotating frame of reference' cannot be observed directly. However, it is approximately equal to the time it takes the Moon to pass twice a "fixed" star (different stars give different results because all have proper motions and are not really fixed in position).

A synodic month is the most familiar lunar cycle, defined as the time interval between two consecutive occurrences of a particular phase (such as new moon or full moon) as seen by an observer on Earth. The mean length of the synodic month is 29.53059 days (29 days, 12 hours, 44 minutes, 2.8 seconds). Due to the eccentricity of the lunar orbit around Earth (and to a lesser degree, the Earth’s elliptical orbit around the Sun), the length of a synodic month can vary by up to seven hours.

The tropical month is the average time for the Moon to pass twice through the same equinox point of the sky. It is 27.32158 days, very slightly shorter than the sidereal month (27.32166) days, because of precession of the equinoxes. Unlike the sidereal month, it can be measured precisely.

An anomalistic month is the average time the Moon takes to go from perigee to perigee - the point in the Moon's orbit when it is closest to Earth. An anomalistic month is about 27.55455 days on average.

The draconic month or nodal month is the period in which the Moon returns to the same node of its orbit; the nodes are the two points where the Moon's orbit crosses the plane of the Earth's orbit. Its duration is about 27.21222 days on average.

The time frame of the phases of the moon will not be identical to the orbit moon. because the moon is also orbiting the sun with the earth.

The anomalistic month will affect the height of tides, but not the timing. At perigee the gravitional force a will add to the height of the tide.

The draconic month or nodal month will also affect the height of the tide; when the moon is north of the equator, it will add to the height of tide in the northern hemisphere.

That is my understanding.

Canadian tide tables include the phase of the moon, apogee and perigee, and hemisphere on a calendar at the back of the tables. I just bought my set on Saturday.

The moon was full on the 23, farthest north on the 21, on the equator on the 25, and in apogee on the 30th.

[RainDog]

Originally Posted by Mike1956

"Moon orbits the Earth over a 28 day cycle creating Spring & Neap tides"

<snip>

The Tidal Cycle is due to an average of 3 cycles of the Moon.

Synodic Month 29.5 days
Tropical Month 27.5 days
Anomalistic Month 27.3 days

Average 28.1 days

Seems like this is the statement in question. If this statement is true, that means that Spring Tides would vary on a ~28.1 day cycle. Since the full/new moon vary on a ~29.5 day cycle, then we would see spring tides during any phase of the moon. I do not believe this is the case. Thus the cycle of spring/neap tides average must be the same as the cycle of full/new moon average, which is the Synodic month.

[Reefmagnet]

Think of it this way, Earth's rotation is 23:56 hours. The additional 4 minutes of our day comes from the approx 1 degree the Earth has progressed along it's orbit. After the month long orbit of the moon, earth is now offset relative to the sun by about 25 degrees or so which means the moon needs to rotate further to align to a same relative angle to the sun as seen from the earth then it did on the previous cycle.

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[RainDog]

Originally Posted by Reefmagnet

Think of it this way, Earth's rotation is 23:56 hours. The additional 4 minutes of our day comes from the approx 1 degree the Earth has progressed along it's orbit. After the month long orbit of the moon, earth is now offset relative to the sun by about 25 degrees or so which means the moon needs to rotate further to align to a same relative angle to the sun as seen from the earth then it did on the previous cycle.

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You are just explaining the difference between a sidereal and synodic month. You are saying nothing about tides.

[Reefmagnet]

I just assumed everyone knew the moon influenced tides. Sorry about that.

The moon is the primary driver of tides for those that aren't aware.

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[RainDog]

Originally Posted by Reefmagnet

I just assumed everyone knew the moon influenced tides. Sorry about that.

The moon is the primary driver of tides for those that aren't aware.

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The whole premise of this thread is if the text book authors understand that. Sounds like you agree with the OP, and think the texts book authors are wrong.

[Reefmagnet]

Originally Posted by RainDog

The whole premise of this thread is if the text book authors understand that. Sounds like you agree with the OP, and think the texts book authors are wrong.

Agreed. The high tide points, more or less, directly at the moon therefore the technical answer is the synodic period.

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[deblen]

Originally Posted by Reefmagnet

I just assumed everyone knew the moon influenced tides. Sorry about that.

The moon is the primary driver of tides for those that aren't aware.

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The sun gets involved also especially during spring & neap. If the sun's pull is aligned with the moons pull,you get a spring.Conversely.if sun's pull is opposite the moon's,you get a neap. This happens twice per month roughly.
At least twice a year (Spring & Fall) we get extra high spring & extra low neap. This is really only noticeable to the eye when approx 40deg north or south of equator where tides are more extreme & very noticeable in places like the Bay of Fundy.

Then,to complicate matters more,there is the slosh effect.This occurs in narrow channels,bays & river estuaries. Slosh is the same effect you would get by pushing water along the length of a bathtub with your hand. The pushed water sloshes up the end of the tub well above avg. water level. If you remove your hand,water at high end rushes back down by gravity & ,instead of just leveling out in tub,it will slosh up the other end a bit. Then it repeats & sloshes up the first end. Each repetition is smaller in height but is still happening.

Expand the tub to several hundred(thousand) sq. Km of water surface with several narrow channels,bays & estuaries sloshing at different rates & frequencies & tidal height & even time (to the minute) is extremely complex.

I am not a tidal expert. I have just observed the Bay of Fundy all my life.

I have several copies of the tide book plus online sources & rely on them. But I also make allowance for strong winds which can affect tide time & height substantially also.

Cheers/ Len

[Reefmagnet]

Originally Posted by deblen

The sun gets involved also especially during spring & neap. If the sun's pull is aligned with the moons pull,you get a spring.Conversely.if sun's pull is opposite the moon's,you get a neap. This happens twice per month roughly.
At least twice a year (Spring & Fall) we get extra high spring & extra low neap. This is really only noticeable to the eye when approx 40deg north or south of equator where tides are more extreme & very noticeable in places like the Bay of Fundy.

Then,to complicate matters more,there is the slosh effect.This occurs in narrow channels,bays & river estuaries. Slosh is the same effect you would get by pushing water along the length of a bathtub with your hand. The pushed water sloshes up the end of the tub well above avg. water level. If you remove your hand,water at high end rushes back down by gravity & ,instead of just leveling out in tub,it will slosh up the other end a bit. Then it repeats & sloshes up the first end. Each repetition is smaller in height but is still happening.

Expand the tub to several hundred(thousand) sq. Km of water surface with several narrow channels,bays & estuaries sloshing at different rates & frequencies & tidal height & even time (to the minute) is extremely complex.

I am not a tidal expert. I have just observed the Bay of Fundy all my life.

I have several copies of the tide book plus online sources & rely on them. But I also make allowance for strong winds which can affect tide time & height substantially also.

Cheers/ Len

I did say "primary", but you are indeed correct. The Bay of Fundy extreme tides are mostly related to geography.

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[jackdale]

BTW - while searching around I came across this presentation from Canadian Hydrographic Service. Tide geeks might enjoy it.

https://www.acls-aatc.ca/files/abstract/MacAulay.pdf

[SimonV]

Moon shmoon. It's simple if the tide is out, it will be back in about six hours. Simple.

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