It's a really harmonic probelm with the moon being just one factor. As a
result it's not even a constant delay at a fixed spot.

Cheers

Wally wrote:

N1EE wrote:

1 pt to you Wally.

High Tide will be somewhat close to the
mid point between moonrise and moonset.

I have not researched many lcoations but I'd
estimate +/- an hour for more locations.

In my location High tide occurs about an hour
before that mid point.

Geographical factors will have a big influence.
A body of water might be north-south like the
Bay of Fundy or Gulf or California versus
east-west like Long Island Sound.


Yup, guess the geography is the single biggest factor affecting the
interval between lunar noon and high tide. At least, I'd imagine that a
smooth planet completely covered in water would have a constant interval for
all locations.



I find it handy to be able to judge high tide
by looking at the moon. I can add or subtract
to estimate when the next high or low tide will
be.


That's a habit I might try to develop. That said, I only daysail (club
racing), so I tend to use published tide info and the little program in my
PDA (Tide Tool).



As you can see we have at times nearly a 9
foot swing, so tidal currents coupled with river
current can cause problems with launching if you
are like me, and using a chain hoist. At times
I cannot drop the boat all the way into the river.
I need longer lift straps.


My boat isn't in the water, but the one I crew on has a marina berth, which
is very convenient. We have a similar situation with current - we're at a
narrow on a tidal river a couple of miles across which immediately opens out
to an estuary to the east. On the ebb, the current can get up to 3 knots in
places. There are islands and bridge piers dotted around, a deep channel on
the north side, and shoals to the south, all of which seem to make for a
great variety of water to contend with. Tidal range gets close to 6m at
springs (about 20 feet).




2004-05-04 11:42 PM EDT 8.21 feet High Tide

2004-05-05 5:44 AM EDT Sunrise
2004-05-05 6:06 AM EDT Moonset

You will note that the time of high tide slips
about 50 minutes every day and the interval
between high tides is about 12.5 hours.


A quick scan at my local data suggests slightly less slippage (30-40
minutes), but I'm not sure how reliable the info is. Did you use software to
derive your numbers?



A good sailor will know what these factors are
for his or her location to figure the next
high tide.


Aye, still getting there. :-)

Navigator wrote:

No, I'm trying to get people to think about the relationship between
altitude and period between rise and set for a fixed lunar orbital
period.

Not so much a side-step, as a stumble.

C'mon you can try better than that.

Given that the moon doesn't spend each day jumping up and down like a
bouncing ball, it wouldn't be too far off the mark to say that its altitude
is a function of the the azimuths of its rise and set, and the observer's
latitude. In other words, its apparent path across the sky is largely due to
the rotation of the earth - like that of the sun.

They key thing about the sun is that its altitude for a given azimuth
changes from day to day due to the obliquity of the ecliptic - the plane of
the equator is different from the plane of the earth's orbit around the sun.
This, combined with the earth spinning on its axis, results in the sun's
rise and set azimuths changing daily and producing summer and winter
solstices, and spring and vernal equinoxes. At the equinoxes, the sun is
passing over the equator (the intersection of the equatorial and solar
orbital planes is in the direction of the sun) and, to the observer on
earth, it appears as though the obliquity of the ecliptic is zero, resulting
in the sun's rise/set being exactly due east/west. Anyone who cares to sit
on a hill for a long time will notice that this cycle is yearly.

A notable aspect of the moon's orbit is that it, too, isn't parallel to the
plane of the equator. For the observer, this is manifest as a series of
sun-like solstices and equinoxes, but with a much shorter period - monthly.
Each month, the moon has a northern and southern solstice (or standstill) ,
and twice passes over the equator midway between these, once when going from
the northern to the southern standstill, and vice versa.

The relationship between rise and set azimuths and altitude (at zenith) is
important insofar as the earth doesn't speed up and slow down during its
axial spin. Further, the altitude of the moon is lower when the rise/set
azimuths are closer to south (for the northern hemisphere). In other words,
when moonrise is towards the north, it takes a longer path across the sky
than it does when moonrise is towards the south. If the earth's rotation
speed is constant, then it must follow that the moon takes more time to
traverse the sky with a northerly rise point than with a southerly one. So,
over the period of one month, the time taken for the moon to traverse the
sky varies from day to day.

As I said earlier, if moon rise/set are due east/west, the time to traverse
the sky is about 12 hours. It isn't *exactly* 12 hours because the moon is
orbiting the earth, but it is *about* 12 hours, because it's orbiting the
earth *slowly*. To render some precision to the description: If I stand at
the Greenwich Observatory and note the moon's azimuth at midnight tonight,
and then wait for it to pass through that azimuth tomorrow night, it'll take
nearly an hour for the moon to 'catch up' - nearly 25 hours.

For an east/west rise/set, the nominal time taken to traverse the sky is 12
hours - half the period of the earth's axial rotation. However, since the
moon takes time to catch up, the time to traverse would be longer - I guess
about 12.5 hours (half the earth's rotation period, plus half the moon's
'catch up' time). It would seem that moonrise at the northern standstill
would entail a greater proportion of the moon's catch up time being added
because the moon is visible for a greater portion of the earth's rotation
period, and vice versa for moonrise at the southern standstill. A true 12
hour traverse of the sky by the moon would consequently entail a rise/set a
little to the south of east/west to account for the delay in the moon
catching up to reach the same azimuth.


--
Wally
www.forthsailing.com
www.wally.myby.co.uk

Excellent informative answer at last. Just to add to your post, at the
equator I think the moon takes about 12 1/2 hours from rise to set.

Cheers

Wally wrote:

Navigator wrote:


No, I'm trying to get people to think about the relationship between
altitude and period between rise and set for a fixed lunar orbital
period.


Not so much a side-step, as a stumble.


C'mon you can try better than that.


Given that the moon doesn't spend each day jumping up and down like a
bouncing ball, it wouldn't be too far off the mark to say that its altitude
is a function of the the azimuths of its rise and set, and the observer's
latitude. In other words, its apparent path across the sky is largely due to
the rotation of the earth - like that of the sun.

They key thing about the sun is that its altitude for a given azimuth
changes from day to day due to the obliquity of the ecliptic - the plane of
the equator is different from the plane of the earth's orbit around the sun.
This, combined with the earth spinning on its axis, results in the sun's
rise and set azimuths changing daily and producing summer and winter
solstices, and spring and vernal equinoxes. At the equinoxes, the sun is
passing over the equator (the intersection of the equatorial and solar
orbital planes is in the direction of the sun) and, to the observer on
earth, it appears as though the obliquity of the ecliptic is zero, resulting
in the sun's rise/set being exactly due east/west. Anyone who cares to sit
on a hill for a long time will notice that this cycle is yearly.

A notable aspect of the moon's orbit is that it, too, isn't parallel to the
plane of the equator. For the observer, this is manifest as a series of
sun-like solstices and equinoxes, but with a much shorter period - monthly.
Each month, the moon has a northern and southern solstice (or standstill) ,
and twice passes over the equator midway between these, once when going from
the northern to the southern standstill, and vice versa.

The relationship between rise and set azimuths and altitude (at zenith) is
important insofar as the earth doesn't speed up and slow down during its
axial spin. Further, the altitude of the moon is lower when the rise/set
azimuths are closer to south (for the northern hemisphere). In other words,
when moonrise is towards the north, it takes a longer path across the sky
than it does when moonrise is towards the south. If the earth's rotation
speed is constant, then it must follow that the moon takes more time to
traverse the sky with a northerly rise point than with a southerly one. So,
over the period of one month, the time taken for the moon to traverse the
sky varies from day to day.

As I said earlier, if moon rise/set are due east/west, the time to traverse
the sky is about 12 hours. It isn't *exactly* 12 hours because the moon is
orbiting the earth, but it is *about* 12 hours, because it's orbiting the
earth *slowly*. To render some precision to the description: If I stand at
the Greenwich Observatory and note the moon's azimuth at midnight tonight,
and then wait for it to pass through that azimuth tomorrow night, it'll take
nearly an hour for the moon to 'catch up' - nearly 25 hours.

For an east/west rise/set, the nominal time taken to traverse the sky is 12
hours - half the period of the earth's axial rotation. However, since the
moon takes time to catch up, the time to traverse would be longer - I guess
about 12.5 hours (half the earth's rotation period, plus half the moon's
'catch up' time). It would seem that moonrise at the northern standstill
would entail a greater proportion of the moon's catch up time being added
because the moon is visible for a greater portion of the earth's rotation
period, and vice versa for moonrise at the southern standstill. A true 12
hour traverse of the sky by the moon would consequently entail a rise/set a
little to the south of east/west to account for the delay in the moon
catching up to reach the same azimuth.

Ahh Nutation? Are you perchance an astronomer?

Cheers

Wally wrote:

Navigator wrote:


... "is the period from rise to set at a
fixed location constant?"


No. See other post.



... Is it constant at a fixed location on the same day
each year? If not why?


No, because the number of lunar months in a year is not a whole number. Even
if it were, the period would likely still vary year-on-year due to the
nutation period of the moon's orbit, which causes a variation in the
azimuths of the northern and southern standstills (and the rise/set points
in between).

Navigator wrote:

... "is the period from rise to set at a
fixed location constant?"

No. See other post.


... Is it constant at a fixed location on the same day
each year? If not why?

No, because the number of lunar months in a year is not a whole number. Even
if it were, the period would likely still vary year-on-year due to the
nutation period of the moon's orbit, which causes a variation in the
azimuths of the northern and southern standstills (and the rise/set points
in between).


--
Wally
www.forthsailing.com
www.wally.myby.co.uk

I'm not being "evasive," I just don't feel the need to prove myself to you,
especially given that you've already demonstrated a less than beginner's
understanding of this. A question was asked; I gave the correct answer, you
spoke gibberish. Now you're trying to save face by posing riddles until I
falter - sorry, not playing.

One of the links I provided yesterday shows on one screen all the
moonrise/moonsets for a location for a year. It is pretty clear from that the
time is not constant. Its also clear that a 12 hour "lunar day" is quite
possible, though not common. Its also possible for this to occur with moonrise
are 4 pm. You, on the other hand have claimed this is not possible. Come back
when you have worked it out.

-jeff

p.s. Of course its not constant for a day of the year.


"Navigator" wrote in message
...
So why not answer the question "is the period from rise to set at a
fixed location constant?" Why be so evasive? I'll give you a hint and
then add to it: Is it constant at a fixed location on the same day each
year? If not why?

Cheers


Jeff Morris wrote:

I know the answer, having worked it out (for the nth time) a few minutes
after
reading the question. But it seems like you have some difficulty with the
concepts. After all, I had the correct answer to the problem; all you've
offered is nonsense.


"Navigator" wrote in message
...

So you don't know the answer?

Cheers

Jeff Morris wrote:


After you've thought about it a bit, you can report back.


"Navigator" wrote in message
...


Jeff Morris wrote:


Are you claiming its broken?

There are a number of factors that determine the time between moonrise
and
moonset. However, there is no reason why a 12 hour duration can't occur,

and no


reason why it can't happen from 4pm to 4am.


Yes, but is the period from rise to set at a fixed location constant?
And yes I'm trying to get some thinking going.

Cheers

Calm down Jeff. I think you've got the wrong end of the proverbial stick
old chap. My question was not an assertion...

Cheers

Jeff Morris wrote:

I'm not being "evasive," I just don't feel the need to prove myself to you,
especially given that you've already demonstrated a less than beginner's
understanding of this. A question was asked; I gave the correct answer, you
spoke gibberish. Now you're trying to save face by posing riddles until I
falter - sorry, not playing.

One of the links I provided yesterday shows on one screen all the
moonrise/moonsets for a location for a year. It is pretty clear from that the
time is not constant. Its also clear that a 12 hour "lunar day" is quite
possible, though not common. Its also possible for this to occur with moonrise
are 4 pm. You, on the other hand have claimed this is not possible. Come back
when you have worked it out.

-jeff

p.s. Of course its not constant for a day of the year.


"Navigator" wrote in message
...

So why not answer the question "is the period from rise to set at a
fixed location constant?" Why be so evasive? I'll give you a hint and
then add to it: Is it constant at a fixed location on the same day each
year? If not why?

Cheers


Jeff Morris wrote:


I know the answer, having worked it out (for the nth time) a few minutes

after

reading the question. But it seems like you have some difficulty with the
concepts. After all, I had the correct answer to the problem; all you've
offered is nonsense.


"Navigator" wrote in message
...


So you don't know the answer?

Cheers

Jeff Morris wrote:



After you've thought about it a bit, you can report back.


"Navigator" wrote in message
...



Jeff Morris wrote:



Are you claiming its broken?

There are a number of factors that determine the time between moonrise

and

moonset. However, there is no reason why a 12 hour duration can't occur,

and no



reason why it can't happen from 4pm to 4am.


Yes, but is the period from rise to set at a fixed location constant?
And yes I'm trying to get some thinking going.

Cheers

Yes, you did! So this explains why the tide times move forward about a
1/2 hour each day... for those that didn't need to read your entire post
of course ;-).

Wally wrote:

Navigator wrote:


Excellent informative answer at last.


Thanks.



Just to add to your post, at the
equator I think the moon takes about 12 1/2 hours from rise to set.


I think I pretty-much covered that. :-)

Navigator wrote:

Excellent informative answer at last.

Thanks.


Just to add to your post, at the
equator I think the moon takes about 12 1/2 hours from rise to set.

I think I pretty-much covered that. :-)


--
Wally
www.forthsailing.com
www.wally.myby.co.uk

"Navigator" wrote in message
...
Excellent informative answer at last. Just to add to your post, at the
equator I think the moon takes about 12 1/2 hours from rise to set.


Excellent, indeed.

The time at the equator is not constant, though it does vary less than at high
latitudes. Of course, at very high latitudes, the moon can stay above or below
the horizon for several days at a time. Guess how long its up at the North
Pole.