The Naked Scientists

The Naked Scientists Forum

Author Topic: What's affecting Seattle's tide times?  (Read 3712 times)

Offline Leo Shaw

  • First timers
  • *
  • Posts: 3
    • View Profile
What's affecting Seattle's tide times?
« on: 24/05/2011 01:01:02 »
Leo Shaw  asked the Naked Scientists:
   
Question:
 
Seattle WA tides.

Over the past half dozen years or so, the lowest tides in the Spring and Summer months have occurred during the NEW MOON phase, while the Fall and Winter lowest tides occurred during the FULL MOON phase.  Spring and Summer lowest tides occur near noon, while Fall and Winter lowest tides occur at close to midnight.

Now in 2011 there has been a shift, while both the Spring/Summer lowest monthly tides and the Fall/Winter lowest monthly tides still occur at basically the same times during day/night respectively.

The Spring/Summer lowest monthly tides occur on the FULL MOON phase while the Fall/Winter lowest monthly tides occur on the NEW MOON phase. (this kinda sucks because we use to have nice moonlight for our winter intertidal exploration J)

So this switch of moon phases has occurred in a single year and will continue next year and I haven't checked past that.

Can you tell me exactly what is causing the shift?    

I know it's the relative positions of Earth-Moon-Sun but can't figure out the details.  I also notice that the elevation of the moon is the same for New Moon and Full Moon for both last year and this year: about 67degrees for the New Moon and 17degrees for the Full Moon so don't see a change there.
 
Thanks
Leo Shaw
Seattle, WA
What do you think?
« Last Edit: 24/05/2011 01:01:02 by _system »


 

Offline graham.d

  • Neilep Level Member
  • ******
  • Posts: 2208
    • View Profile
What's affecting Seattle's tide times?
« Reply #1 on: 24/05/2011 13:12:39 »
It is very complicated. There are a lot of factors at work other than the astronomical ones.

1. Seattle is not on the open ocean and so the tides are affected considerably by the flows in and out between Vancouver island and the mainland. A clearer picture would be to look at tides for Westport, though even here the tides are not "simple".
2. The currents and shape of the coastline also affect tides a lot. In particular, the Coriolis force can vary the tidal height and times on coastlines even quite a short distance apart.
3. Astronomically, the moon's orbit is a few degrees off the eliptic and also has a slightly eccentric orbit. In other words, it's position and distance vary with a different timescale to that of days, months or years. This affects tidal heights and times.

I took a quick look at various tidetables and the tides are certainly influenced by many factors in that region and I don't know the specific reasons for the local variations. The simple description of tides only really works well away from land masses, where, as a rule of thumb, high water (Springs) occurs a little time after noon or midnight (local time).
 

Offline Leo Shaw

  • First timers
  • *
  • Posts: 3
    • View Profile
What's affecting Seattle's tide times?
« Reply #2 on: 26/05/2011 02:22:37 »
Lots of factors for Seattle Tides which I have followed over the years, but I let NOAA handle all the complicated stuff  ;D
It's a great place for hitting the beaches at low tide as the lowest tides in Spring and Summer are midday so you can sleep in.
Winter is different with lowest tides occur at night....but in recent years we have had that great full moon overhead during low tide hours....not this year...now it's going to be real dark with that silly new moon....of course when the full moon is around at lowest Summer tide it's already set long before the low tide time.....
I liked it the old way....Full Moon for Winter Low tides and New Moon for Summer low tides.  Funny how that flip-flopped in a single year.
 ???
 

Offline CliffordK

  • Neilep Level Member
  • ******
  • Posts: 6321
  • Thanked: 3 times
  • Site Moderator
    • View Profile
What's affecting Seattle's tide times?
« Reply #3 on: 26/05/2011 03:46:51 »
I had worked up a response earlier... but got lost in the middle of the response.
There are a few factors affecting tides.

Both the Earth and the moon travel in elliptical orbits. 

The Earth's orbit goes through apsidal precession every 22,000 years or so.  So, it does a half precession every 11,000 years.  Currently the earth is closest to the sun in the winter, so one would expect to see the largest tides during the winter too.  In 11,000 years that will be in the summer.

The Moon's orbit precesses every 8.85 years.  So, every 4 to 5 years the lunar orbit will shift from being closest to the earth at full moon to being closest to the earth at new moon.

On March 19, we had a "Super Moon" which was a full moon where it was closest to Earth in its elliptical orbit.  Hmmm, that is part of your answer too, because March 19 essentially landed on the Spring Equinox (March 20).  So, the moon has now precessed to a different phase with respect to Earth's seasons.

There is also a Saros cycle (18.031 years) which is the time it takes for the moon to return to essentially the exact same position with respect to the Earth and Sun. 

Ahh, perhaps apsidal Precession is with respect to the stars, and Saros is precession with respect to the sun.

What I got stuck on was trying to figure out whether a moon closest to the Earth (and thus more gravity) or a moon furthest from the Earth (thus the center of gravity furthest from the center of the Earth, and more wobble) would cause a higher tide. 

I also have troubles envisioning why one gets a high tide on the back side of the earth, but I assume it has to do with the wobble, and perhaps less gravity due to the displaced center of gravity.

Perhaps that is the key.

Moon closest to the earth --> more lunar gravity and higher tide on the moon side.
Moon furthest from the earth --> more wobble, center of gravity farthest from the center of the earth, and higher tide on the opposite side of the earth.

Anyway, this is a bunch of jumbled thoughts, but your answer has to be in there somewhere.   :)
 

The Naked Scientists Forum

What's affecting Seattle's tide times?
« Reply #3 on: 26/05/2011 03:46:51 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums