mhabich Posted February 17, 2023 Share Posted February 17, 2023 Big tides today, and big variation between larger and smaller tides. We're at the new moon, so we'd expect big spring tides...but why the variation? Looking at the whole month, the two tides in a day are unequal now, but equal a week ago or a week in the future. The moon's orbit plane around the earth is at an angle to the earth's axis. When the moon is at a high angle of declination above or below the equator, one tidal bulge at our latitude is large and the other is small, so we get a big tide and a little one each day. We're also at a peak of declination at this time of month, and near zero declination a week before or after today. Quote Link to comment Share on other sites More sharing options...
alcoons Posted February 17, 2023 Share Posted February 17, 2023 Wish I could answer your question. Observation: when I walked at Castle Island yesterday I was thinking I was seeing the lowest tides I have ever observed. Particularly easy to judge by how far out I could see exposed rocks off the south east corner of the fort beyond the South Korean War Memorial. Guess I am not so crazy after all. Quote Link to comment Share on other sites More sharing options...
mhabich Posted February 17, 2023 Author Share Posted February 17, 2023 Al, I thought I'd answered my question with lunar declination. But, as you well know, my purpose in life is to make things more complicated than they need be and confuse hell out of everybody. Quote Link to comment Share on other sites More sharing options...
JohnHuth Posted February 27, 2023 Share Posted February 27, 2023 (edited) One main issue with the tides is the concept of resonance. It's like pushing a swing on a playground with someone on it. The swing has a natural frequency if you just displace it and let it swing back and forth. Now think about someone pushing that swing. If you push the swing well below its natural frequency, it doesn't go so high. If you push it well above its natural frequency, it doesn't go as high. But if you push it at its natural frequency, it goes really high. Bodies of water that are partially trapped have different natural frequencies, depending on their volume, depth etc. The 'pushes' come from when we rotate under both the lunar bulge and the solar bulge. Some places have a twice daily high tide - like most of the east coast. Some places have a once daily high tide. Some places, like Puget Sound have unequal high tides - maybe twice a day, but very different heights. The Bay of Fundy has a natural frequency that's very close to the frequency of the lunar bulge, which is why the tides are so dang (an industry term) high. When considering tides and making predictions, each body of water has its own susceptibilities to the different driving frequencies or pushes. The way the tides are calculated is that they're broken down into "how much of this and such frequency - like the lunar bulge" affects the tide in a given place. Once those susceptibilities are known, NOAA or whoever can crank out the predictions. In the case of the East Coast, although we're mostly in the "twice a day" mode (yes, I know it's 12 hours plus some change....but for this discussion) - but we aren't precisely in a twice-a-day mode, and there's some uneven high and low tides, depending on the body of water, and then you also get into whether the moon is at a closest approach or it's at a different declination etc. Maybe this is more complicated that you wanted to hear, but that's what's going on. Edited February 27, 2023 by JohnHuth Quote Link to comment Share on other sites More sharing options...
mhabich Posted February 27, 2023 Author Share Posted February 27, 2023 Yup, a lightly damped second order system. Quote Link to comment Share on other sites More sharing options...
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