Moon/High Tide Issue

[darrellx (Darrell)]

Hi All

I need some help to clarify the relationship between the Moon and the High Tide.

My other hobby is sailing, and over the years I have noticed the relationship between the moon and the tides. But I have never really paid much attention. My interest really only went as far as "Moon (when full) = poor viewing; High tide = good sailing". With all our recent poor weather, and after reading a number of books related to this issue, I have had a more detailed look at this relationship, and I am a bit confused.

In all the diagrams I have seen on this, the bulge of the high tide is always shown directly under the moon. I have attached two graphics that I think are typical. So when the moon transits my location I would expect the high tide. My observations do not support this. For the last month, I have recorded the time the moon transits and compared it to the tides. There does appear to be a cyclic variation, but generally, the moon transits between two and three hours after my local high tide. Or to put it the other way round, high tide occurs between two and three hours before the moon is overhead. Why???

I understand that the tides are influenced by the topography of the ocean floor; our latitude; and there is a friction component (between the water and the land) in there as well. But I cannot see how that would account for the high tide preceeding the moon by upto three hours.

By way of example, take Sunday last week. There was a high tide at the Gold Coast Seaway at 7:21pm. The moon rose at 3:37pm and transit at 10:36pm. So on the day, the high tide happened 3hours and 15minutes before the moon was overhead. That also means the moon transits 3 hours and 8 minutes before low tide. If you were in Sydney, high tide was about 7:00pm, and if you were at the Brisbane Bar high tide was 8:40pm. Rockhamption experienced the high tide at 8:26pm. There doesn't seem to be anywhere on the east coast that experienced a high tide while the moon transits the meridian (lets say 153degrees), or was even close.

So, how does this happen? I have attached a third graphic which shows the location of where I think the moon is relative to the tide.
Some Google searches have revealed;
"The shape of the shoreline and the ocean floor changes the way that tides propagate, so there is no simple, general rule that predicts the time of high water from the Moon's position in the sky." (from Wikipedia)
And "However, for a given location the relationship between lunar altitude and the time of high or low tide (the lunitidal interval) is relatively constant and predictable, as is the time of high or low tide relative to other points on the same coast. For example, the high tide at Norfolk, Virginia, predictably occurs approximately two and a half hours before the Moon passes directly overhead."

This all seems so counter-intuitive - the high tide occurs two or three hours BEFORE the moon passes overhead at the Gold Coast and Norfolk Virginia. Just don't seem right.

Given the Moon is the major contributor to the tides, I can understand how the high tide would follow the moon. But I can't understand how
it would preceed the moon. If there is no explanation, I might just have to accept it as "thats the way it is".

Thanks
Darrell

[noeyedeer (Matt)]

I'm no expert, but I think as the moon gradually moves away from the earth the instance of the tides accuring at the moons transit will fall further behind. I'm assuming millions of years ago high tide did coincide with the transit.

Matt

Oh and the pics I think just make it easier for people to understand the gravitational effect the moon has. If they put the moon where you have placed it, I think would confuse confuse more people then just yourself

[Steffen]

The tidal bulge is about 10˚ ahead of the position of the Moon, due to the Earth's own rotation. It gives the Moon an extra gravitational tug, which causes the Moon to speed up and increase the height of its orbit.

Cheers
Steffen.

[noeyedeer (Matt)]

Originally Posted by Steffen

The tidal bulge is about 10˚ ahead of the position of the Moon, due to the Earth's own rotation. It gives the Moon an extra gravitational tug, which causes the Moon to speed up and increase the height of its orbit.

Cheers
Steffen.

that makes sense ... more than mine
matt

[Dave2042 (Dave)]

I agree with Steffen, but here's another way of thinking about it is this.

The earth rotates much faster (once a day) than the moon orbits (once a month). This means you can think about it as the moon just being stationary while the earth rotates. So, we have a tidal bulge that is stationary while the earth rotates under it.

Now the oceans are disrupted by the continents. Think about the Americas particularly, running north to south between the poles. As the earth rotates under the tidal bulge, they act as a big vane pushing the water forward (like the inside of a washing machine). The result is that the bulge (ie high tide) is well ahead of where you'd expect it.

[barx1963 (Malcolm)]

Dave
Very clear explanation.

Malcolm

[darrellx (Darrell)]

Guys

Thanks. That makes sense. With Steffens solution, the rotation of the earth under the water would probably "pull" the water bulge alone as well. But I hadn't thought of the interference by the continents. Good one Dave.

Darrell