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Question: Judging High Tide by the Moon
Moonrise 4 pm local time
Moonset 4 am local time Approximately when is high tide? [ 1 pt ] |
Question: Judging High Tide by the Moon
Bart Senior wrote:
Moonrise 4 pm local time Moonset 4 am local time Approximately when is high tide? [ 1 pt ] I guess it depends on the location. For my area, it's a couple of hours or so after lunar noon, so about midnight to 1am for the above example. -- Wally www.forthsailing.com www.wally.myby.co.uk |
Question: Judging High Tide by the Moon
Ok, I'll give it a try -
Halfway between moonrise and moonset is 10 pm local time, which is the time of transit of the local meridian. To this we must add the "Local Establishment" or the "Lunitidal Interval" (actually the proper terms are "high water full and change" or "mean high water lunitidal interval") to get the time of the next high tide. This was published on old charts. My 1867 Boston Harbor chart lists the "Corrected Establishment" (average of all high water intervals) for Boston Light as "XIh XIIm". This mean you add 11 hours and 12 minutes to the time of local meridian passage of the moon to get the time of the next high tide. Tides watches usually require setting the Lunitidal Interval for a particular spot. Of course, this only works for semidiurnal tides. "Bart Senior" wrote in message . net... Moonrise 4 pm local time Moonset 4 am local time Approximately when is high tide? [ 1 pt ] |
Question: Judging High Tide by the Moon
What location has moon rise and set 12 hours apart? If moon rise is at
4pm the next high tide here will be about 5:30pm -I think. Cheers Bart Senior wrote: Moonrise 4 pm local time Moonset 4 am local time Approximately when is high tide? [ 1 pt ] |
Question: Judging High Tide by the Moon
Navigator wrote:
What location has moon rise and set 12 hours apart? Virtually anywhere, I'd guess - just had a look at moon rise and set for some random locations (Glasgow, Jerusalem and Canberra), for today, and they're all a shade over 12 hours apart. If rise/set are close to due east/west, then the time for the moon to traverse the sky will be about 12 hours - think about the solar equinoxes, where the sun rises/sets due east/west. I dare say this princple won't hold at extreme latitudes. The moon passes the due east/west points twice in its mo(o)nthly cycle. -- Wally www.forthsailing.com www.wally.myby.co.uk |
Question: Judging High Tide by the Moon
Wally wrote: Navigator wrote: What location has moon rise and set 12 hours apart? Virtually anywhere, I'd guess - just had a look at moon rise and set for some random locations (Glasgow, Jerusalem and Canberra), for today, and they're all a shade over 12 hours apart. If rise/set are close to due east/west, then the time for the moon to traverse the sky will be about 12 hours - think about the solar equinoxes, where the sun rises/sets due east/west. I dare say this princple won't hold at extreme latitudes. The moon passes the due east/west points twice in its mo(o)nthly cycle. The devil _is_ in the detail and is it ever 12 hours???? Cheers |
Question: Judging High Tide by the Moon
Navigator wrote:
The devil _is_ in the detail and is it ever 12 hours???? What part of 'find lunar noon' are you ignoring in lieu of trolling for nanosecond-precise astronomical timing? -- Wally www.forthsailing.com www.wally.myby.co.uk |
Question: Judging High Tide by the Moon
"Navigator" wrote in message
... Wally wrote: Navigator wrote: What location has moon rise and set 12 hours apart? Virtually anywhere, I'd guess - just had a look at moon rise and set for some random locations (Glasgow, Jerusalem and Canberra), for today, and they're all a shade over 12 hours apart. If rise/set are close to due east/west, then the time for the moon to traverse the sky will be about 12 hours - think about the solar equinoxes, where the sun rises/sets due east/west. I dare say this princple won't hold at extreme latitudes. The moon passes the due east/west points twice in its mo(o)nthly cycle. The devil _is_ in the detail and is it ever 12 hours???? Why not? What's the problem? Or are you making the sophomoric argument that its very unlikely to be "precisely" 12 hours? It certainly isn't hard to find places and times where its within a couple of minutes. Certain latitudes are more favorable than others for this situation to exist, but there's nothing the precludes it. Have a go with: http://aa.usno.navy.mil/ select "Data Services" and then "Table of sunrise/sunset ..." |
Question: Judging High Tide by the Moon
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. 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. 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. Here is an example for my location: 2004-05-02 5:26 PM EDT Moonrise 2004-05-02 7:51 PM EDT Sunset 2004-05-02 10:09 PM EDT 7.80 feet High Tide 2004-05-03 4:33 AM EDT -0.49 feet Low Tide 2004-05-03 5:06 AM EDT Moonset 2004-05-03 5:47 AM EDT Sunrise 2004-05-03 10:38 AM EDT 7.16 feet High Tide 2004-05-03 4:45 PM EDT -0.14 feet Low Tide 2004-05-03 6:42 PM EDT Moonrise 2004-05-03 7:52 PM EDT Sunset 2004-05-03 10:55 PM EDT 8.06 feet High Tide 2004-05-04 5:21 AM EDT -0.84 feet Low Tide 2004-05-04 5:33 AM EDT Moonset 2004-05-04 5:46 AM EDT Sunrise 2004-05-04 11:27 AM EDT 7.20 feet High Tide 2004-05-04 4:35 PM EDT Full Moon 2004-05-04 5:31 PM EDT -0.26 feet Low Tide 2004-05-04 7:53 PM EDT Sunset 2004-05-04 8:01 PM EDT Moonrise 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 good sailor will know what these factors are for his or her location to figure the next high tide. Bart Senior "Wally" wrote Bart Senior wrote: Moonrise 4 pm local time Moonset 4 am local time Approximately when is high tide? [ 1 pt ] I guess it depends on the location. For my area, it's a couple of hours or so after lunar noon, so about midnight to 1am for the above example. |
Question: Judging High Tide by the Moon
2 pts. Great answer Jeff. an extra point for adding
worthwhile content. Can you name two places where tides are diurnal? What are spring and neap tides and how can you tell by observing the moon, if a tide will be spring or neap? Jeff Morris wrote Halfway between moonrise and moonset is 10 pm local time, which is the time of transit of the local meridian. To this we must add the "Local Establishment" or the "Lunitidal Interval" (actually the proper terms are "high water full and change" or "mean high water lunitidal interval") to get the time of the next high tide. This was published on old charts. My 1867 Boston Harbor chart lists the "Corrected Establishment" (average of all high water intervals) for Boston Light as "XIh XIIm". This mean you add 11 hours and 12 minutes to the time of local meridian passage of the moon to get the time of the next high tide. Tides watches usually require setting the Lunitidal Interval for a particular spot. Of course, this only works for semidiurnal tides. |
Question: Judging High Tide by the Moon
The Gulf Coast is the main diurnal tide in the USA. Southeast Asia is another
area. http://chinook.kpc.alaska.edu/~ifeeb...altidemap.html http://www.oceanservice.noaa.gov/edu...07_cycles.html "Bart Senior" wrote in message .net... 2 pts. Great answer Jeff. an extra point for adding worthwhile content. Can you name two places where tides are diurnal? What are spring and neap tides and how can you tell by observing the moon, if a tide will be spring or neap? Jeff Morris wrote Halfway between moonrise and moonset is 10 pm local time, which is the time of transit of the local meridian. To this we must add the "Local Establishment" or the "Lunitidal Interval" (actually the proper terms are "high water full and change" or "mean high water lunitidal interval") to get the time of the next high tide. This was published on old charts. My 1867 Boston Harbor chart lists the "Corrected Establishment" (average of all high water intervals) for Boston Light as "XIh XIIm". This mean you add 11 hours and 12 minutes to the time of local meridian passage of the moon to get the time of the next high tide. Tides watches usually require setting the Lunitidal Interval for a particular spot. Of course, this only works for semidiurnal tides. |
Question: Judging High Tide by the Moon
I've had trouble finding a nice tide table for the east coast, so I checked out my 1886 Bowditch.
Sure enough, it had the following (plus tables for the rest of the world): http://www.sv-loki.com/NETides.jpg http://www.sv-loki.com/SETides.jpg I'm sure some oceanographer can explain the variations, but there's nothing obvious that would help me if I landed on a different planet. -- -jeff "N1EE" wrote in message om... 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. 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. 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. Here is an example for my location: 2004-05-02 5:26 PM EDT Moonrise 2004-05-02 7:51 PM EDT Sunset 2004-05-02 10:09 PM EDT 7.80 feet High Tide 2004-05-03 4:33 AM EDT -0.49 feet Low Tide 2004-05-03 5:06 AM EDT Moonset 2004-05-03 5:47 AM EDT Sunrise 2004-05-03 10:38 AM EDT 7.16 feet High Tide 2004-05-03 4:45 PM EDT -0.14 feet Low Tide 2004-05-03 6:42 PM EDT Moonrise 2004-05-03 7:52 PM EDT Sunset 2004-05-03 10:55 PM EDT 8.06 feet High Tide 2004-05-04 5:21 AM EDT -0.84 feet Low Tide 2004-05-04 5:33 AM EDT Moonset 2004-05-04 5:46 AM EDT Sunrise 2004-05-04 11:27 AM EDT 7.20 feet High Tide 2004-05-04 4:35 PM EDT Full Moon 2004-05-04 5:31 PM EDT -0.26 feet Low Tide 2004-05-04 7:53 PM EDT Sunset 2004-05-04 8:01 PM EDT Moonrise 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 good sailor will know what these factors are for his or her location to figure the next high tide. Bart Senior "Wally" wrote Bart Senior wrote: Moonrise 4 pm local time Moonset 4 am local time Approximately when is high tide? [ 1 pt ] I guess it depends on the location. For my area, it's a couple of hours or so after lunar noon, so about midnight to 1am for the above example. |
Question: Judging High Tide by the Moon
Bart Senior wrote:
Can you name two places where tides are diurnal? Always diurnal, i.e. only one tide per day, every day? - No. I don't think there is one. It'd be a very rare combination of geography and geography. Diurnal most of the time, i.e. only one tide per day most days, with other days showing a smaller second tide? - Lots of places. The Aleutians and the Gulf of Mexico are two. What are spring and neap tides and how can you tell by observing the moon, if a tide will be spring or neap? Spring tides are bigger than neap tides. Spring tides occur at the full moon and at the new moon. Neap tides occur at the first and last quarter. BTW, I sail on a small inland lake, so this stuff is just trivia for me. Tides schmides - the real question is which direction is the next 90 degree wind shift coming from. -- //-Walt // // Sigs suck. Oh, the irony. |
Question: Judging High Tide by the Moon
Bart Senior wrote:
Moonrise 4 pm local time Moonset 4 am local time Approximately when is high tide? [ 1 pt ] Are we right on the ocean, or somewhere up a coastal bay? On the Ocean the high tide will occur around lunar noon (when the moon is at it's highest point in the sky) and again about twelve hours and 25 minutes later. So let's say about 10pm and then about 10:30am the next day. On a basin like, say, Rehoboth Bay tides can lag by several hours. Check the local tide charts. -- //-Walt // // Sigs suck. Oh, the irony. |
Question: Judging High Tide by the Moon
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. Cheers Jeff Morris wrote: "Navigator" wrote in message ... Wally wrote: Navigator wrote: What location has moon rise and set 12 hours apart? Virtually anywhere, I'd guess - just had a look at moon rise and set for some random locations (Glasgow, Jerusalem and Canberra), for today, and they're all a shade over 12 hours apart. If rise/set are close to due east/west, then the time for the moon to traverse the sky will be about 12 hours - think about the solar equinoxes, where the sun rises/sets due east/west. I dare say this princple won't hold at extreme latitudes. The moon passes the due east/west points twice in its mo(o)nthly cycle. The devil _is_ in the detail and is it ever 12 hours???? Why not? What's the problem? Or are you making the sophomoric argument that its very unlikely to be "precisely" 12 hours? It certainly isn't hard to find places and times where its within a couple of minutes. Certain latitudes are more favorable than others for this situation to exist, but there's nothing the precludes it. Have a go with: http://aa.usno.navy.mil/ select "Data Services" and then "Table of sunrise/sunset ..." |
Question: Judging High Tide by the Moon
N1EE wrote:
1 pt to you Wally. High Tide will be somewhat close to the mid point between moonrise and moonset. In theory. I have not researched many lcoations but I'd estimate +/- an hour for more locations. You need to do some more research. Use the following two resources to compare moonrise/set times and tides for various locations: http://www2.shore.net/~mcmorran/tide/tideform.html http://aa.usno.navy.mil/data/docs/RS_OneYear.html (be sure to correct for daylight savings time) For instance, try comparing Virginia Beach, VA and Solomon's Island, MD .. Moonrise/set times are within a couple of minutes of each other, but the tides are six hours out of synch. Why? Well, obviously it takes some time for the large bulge of water that is the tide to make its way up the Chesapeake Bay. Since water doesn't move instantly, moonrise is essentially useless for predicting tides on large bays. -- //-Walt // // Sigs suck. Oh, the irony. |
Question: Judging High Tide by the Moon
Sounds more like you don't know what you're talking about.
"Navigator" wrote in message ... 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. Cheers Jeff Morris wrote: "Navigator" wrote in message ... Wally wrote: Navigator wrote: What location has moon rise and set 12 hours apart? Virtually anywhere, I'd guess - just had a look at moon rise and set for some random locations (Glasgow, Jerusalem and Canberra), for today, and they're all a shade over 12 hours apart. If rise/set are close to due east/west, then the time for the moon to traverse the sky will be about 12 hours - think about the solar equinoxes, where the sun rises/sets due east/west. I dare say this princple won't hold at extreme latitudes. The moon passes the due east/west points twice in its mo(o)nthly cycle. The devil _is_ in the detail and is it ever 12 hours???? Why not? What's the problem? Or are you making the sophomoric argument that its very unlikely to be "precisely" 12 hours? It certainly isn't hard to find places and times where its within a couple of minutes. Certain latitudes are more favorable than others for this situation to exist, but there's nothing the precludes it. Have a go with: http://aa.usno.navy.mil/ select "Data Services" and then "Table of sunrise/sunset ..." |
Question: Judging High Tide by the Moon
Are you saying that the peiod between rise and set is fixed?
Cheers Jeff Morris wrote: Sounds more like you don't know what you're talking about. "Navigator" wrote in message ... 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. Cheers Jeff Morris wrote: "Navigator" wrote in message ... Wally wrote: Navigator wrote: What location has moon rise and set 12 hours apart? Virtually anywhere, I'd guess - just had a look at moon rise and set for some random locations (Glasgow, Jerusalem and Canberra), for today, and they're all a shade over 12 hours apart. If rise/set are close to due east/west, then the time for the moon to traverse the sky will be about 12 hours - think about the solar equinoxes, where the sun rises/sets due east/west. I dare say this princple won't hold at extreme latitudes. The moon passes the due east/west points twice in its mo(o)nthly cycle. The devil _is_ in the detail and is it ever 12 hours???? Why not? What's the problem? Or are you making the sophomoric argument that its very unlikely to be "precisely" 12 hours? It certainly isn't hard to find places and times where its within a couple of minutes. Certain latitudes are more favorable than others for this situation to exist, but there's nothing the precludes it. Have a go with: http://aa.usno.navy.mil/ select "Data Services" and then "Table of sunrise/sunset ..." |
Question: Judging High Tide by the Moon
"Walt" wrote
For instance, try comparing Virginia Beach, VA and Solomon's Island, MD . Moonrise/set times are within a couple of minutes of each other, but the tides are six hours out of synch. Why? Well, obviously it takes some time for the large bulge of water that is the tide to make its way up the Chesapeake Bay. Since water doesn't move instantly, moonrise is essentially useless for predicting tides on large bays. I ask Uncle NOAA for tide info. Scotty |
Question: Judging High Tide by the Moon
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. Now you can claim that you were "trying to get people thinking about something" but I claim you have no idea about lunar noon and it relationship to the tides, and that you had some bizarre notion that a 12 hour duration was not possible. "Navigator" wrote in message ... Are you saying that the peiod between rise and set is fixed? Cheers Jeff Morris wrote: Sounds more like you don't know what you're talking about. "Navigator" wrote in message ... 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. Cheers Jeff Morris wrote: "Navigator" wrote in message ... Wally wrote: Navigator wrote: What location has moon rise and set 12 hours apart? Virtually anywhere, I'd guess - just had a look at moon rise and set for some random locations (Glasgow, Jerusalem and Canberra), for today, and they're all a shade over 12 hours apart. If rise/set are close to due east/west, then the time for the moon to traverse the sky will be about 12 hours - think about the solar equinoxes, where the sun rises/sets due east/west. I dare say this princple won't hold at extreme latitudes. The moon passes the due east/west points twice in its mo(o)nthly cycle. The devil _is_ in the detail and is it ever 12 hours???? Why not? What's the problem? Or are you making the sophomoric argument that its very unlikely to be "precisely" 12 hours? It certainly isn't hard to find places and times where its within a couple of minutes. Certain latitudes are more favorable than others for this situation to exist, but there's nothing the precludes it. Have a go with: http://aa.usno.navy.mil/ select "Data Services" and then "Table of sunrise/sunset ..." |
Question: Judging High Tide by the Moon
Walt -
While the "theory" might imply high tide at lunar noon, it doesn't work out that way in practice. For instance, in the Gulf of Maine high tide precedes lunar noon by less than an hour. But then you go down to Block I and the tide is almost 4 hours before noon. Cape May is also almost 4 hours early. This continues all the way down the down the coast to the Keys. In fact, there isn't a place on the East Coast between Cape Cod and Key West where high tide is within two hours of lunar noon. Rather than randomly sampling tides, try looking at the tables I posted: http://www.sv-loki.com/NETides.jpg http://www.sv-loki.com/SETides.jpg "Walt" wrote in message ... N1EE wrote: 1 pt to you Wally. High Tide will be somewhat close to the mid point between moonrise and moonset. In theory. I have not researched many lcoations but I'd estimate +/- an hour for more locations. You need to do some more research. Use the following two resources to compare moonrise/set times and tides for various locations: http://www2.shore.net/~mcmorran/tide/tideform.html http://aa.usno.navy.mil/data/docs/RS_OneYear.html (be sure to correct for daylight savings time) For instance, try comparing Virginia Beach, VA and Solomon's Island, MD . Moonrise/set times are within a couple of minutes of each other, but the tides are six hours out of synch. Why? Well, obviously it takes some time for the large bulge of water that is the tide to make its way up the Chesapeake Bay. Since water doesn't move instantly, moonrise is essentially useless for predicting tides on large bays. -- //-Walt // // Sigs suck. Oh, the irony. |
Question: Judging High Tide by the Moon
Jeff Morris wrote:
Walt - While the "theory" might imply high tide at lunar noon, it doesn't work out that way in practice. Yes. I think that's what I was trying to illustrate with my "random" examples. Rather than randomly sampling tides, try looking at the tables I posted: http://www.sv-loki.com/NETides.jpg http://www.sv-loki.com/SETides.jpg Good point. While a random sample will disprove the connection between lunar noon and high tide, these charts give a better idea of the overal trend. Worth a second look. -- //-Walt // // Sigs suck. Oh, the irony. |
Question: Judging High Tide by the Moon
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. -- Wally www.forthsailing.com www.wally.myby.co.uk |
Question: Judging High Tide by the Moon
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 |
Question: Judging High Tide by the Moon
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 |
Question: Judging High Tide by the Moon
C'mon you can try better than that.
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. |
Question: Judging High Tide by the Moon
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 |
Question: Judging High Tide by the Moon
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 |
Question: Judging High Tide by the Moon
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 |
Question: Judging High Tide by the Moon
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. :-) -- Wally www.forthsailing.com www.wally.myby.co.uk |
Question: Judging High Tide by the Moon
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. :-) |
Question: Judging High Tide by the Moon
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 |
Question: Judging High Tide by the Moon
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. |
Question: Judging High Tide by the Moon
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). |
Question: Judging High Tide by the Moon
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 |
Question: Judging High Tide by the Moon
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 |
Question: Judging High Tide by the Moon
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 |
Question: Judging High Tide by the Moon
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. :-) |
Question: Judging High Tide by the Moon
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 |
Question: Judging High Tide by the Moon
"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. |
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