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Welcome to Full-Spectrum Science Shorts! I'm your host, Ron Hipschman. Today, King Tides. If you missed the previous video detailing the reasons for the tides, you might wanna check that out. Don't worry though, we'll review the important parts again in this segment. I wanna note that the diagrams in this presentation representing the sizes and distances of the sun, moon, and earth are wildly out of scale. For a better picture of this, see the Full-Spectrum Science Shorts segment titled, "That's Why They Call It Space." Let's review the effects of the moon's gravity on the waters of the earth. If we consider only the moon, its effect is to stretch the oceans, creating two high tides, one pointing at the moon and one pointing away from the moon. It also creates two low tides at right angles to these. The moon isn't the only actor on our stage though. We also need to consider the sun. Although the gravity of the sun is much stronger, the sun is also much farther away. What's important though, is that the difference in the sun's pole from one side of the earth to the other side is much smaller than the moon's. This results in solar tides being less than half as high as lunar tides, like you see here. Of course we must consider both lunar and solar tides together. As the moon revolves around the earth, we see various phases depending on the relative positions of the sun, earth, and moon. These phases range from new moon when the moon is between the sun and the earth, to first quarter, when we see half the moon illuminated, to full moon, to last quarter, when we see the other half illuminated right there, and back again eventually to new moon. At new and full moon, the alignments are like you see here, the sun, moon, and earth are all along a straight line. And the sun and moon pull together along that same line. By the way, this alignment has a wonderful name associated with it. It's called a syzygy. So the moon's effect is to stretch the waters like you see here. And the sun pulling along the same line, enhances the pull of the moon, stretching the waters of the ocean even more. This creates higher high tides and lower low tides at new and full moon. These are called spring tides, and this has nothing to do with the season spring, but rather because the water springs up towards the moon. We're looking at large tides only in this talk. So we're not going to deal with the tides at other lunar phases since they're all smaller. What's important is that the highest and lowest tides occur at new and full moon. It takes the moon about two weeks to orbit the earth between new and full moon positions. So there's about two weeks between spring tides. They occur about twice every month. Generally speaking, the tidal bulge follows the moon from new moon to new moon, takes about 29 and a half days. So the earth rotates 29 and a half times within the tidal bulge as the moon orbits around the earth. As passengers on the rotating earth, we're carried through the high and low tides. If you observe the little person in our diagram, as she's standing on the rotating earth, the rotation carries her through low and high tides. Sometimes her head is above water and sometimes she's below the surface. Most places on earth experience two high tides and two low tides every day. Though there are some exceptions. Here we see a panorama of the bay water from the pier of the Exploratorium at low tide. Tides in our area can vary over nine feet from low to high. Here's the view at high tide, quite a dramatic difference. It might be easier to see if we had both photos on the screen at the same time. At low tide, the pilings are sticking perhaps six feet or more out of the water. At high tide, they're hidden well below the surface. Remember that spring tides occur twice every month, at new and full moon, creating larger than normal tides. But every so often there are other factors that can enhance the spring tides, making an even bigger difference from low to high tide. Let's see what those factors are. The moon revolves around the earth in an ellipse, sometimes farther from the earth called apogee, and sometimes coming closer to the earth called perigee. At perigee, the tidal effects are greater. At apogee, the effects are less. So once a month when the moon is closer to the earth, we get enhanced tides. Likewise, the earth revolves around the sun in an ellipse, sometimes farther from the sun called aphelion, and sometimes coming closer to the sun called perihelion. At perihelion, the solar tidal effects are greater. At aphelion, less so. Once every year, somewhere near January 4th, we'll experience enhanced solar tides. Now we combine all these effects. Enhanced lunar tides once a month when the moon is near perigee. Enhanced solar tides once a year in December or January when the earth is near perihelion. And spring tides at new or full moon. Added up, we get a measurably higher tide once or twice a year, we call king tides. Here's a tide chart for December, 2020 provided to us by our wonderful tide volunteer, Alan Davis. Notice that the highest tides at 7.3 feet and the lowest tides at minus 1.6 feet occur on Monday the 14th. Not coincidentally. This date also is the date of the new moon. And one day after the moon's closest approach to the earth at perigee, and a little more than three weeks before the earth's closest approach to the sun for the year perihelion on January 5th. Note that the sun-earth distance would not change much during the three plus weeks. These are the makings for king tides. Let's look at some. Here we see Pacific waves crashing over the sea wall and into the Sea Breeze Motel parking lot in Pacifica, just South of San Francisco, photographed by Alan Grinberg. I've waved-watched many times on this spot, but not during king tides. I like staying dry. Dave Rauenbuehler took this beautiful action shot at the fishing pier in Pacifica in December, 2015. I'll bet that unsuspecting person never forgets the power of king tides. You don't have to be Otis Redding to enjoy sitting on the dock of the bay, watching the tide roll away. This is a normal day at Pier Three, a short walk from the Exploratorium. This is a little before a normal 5.2 foot high tide. However, four days earlier on February 9th, 2020, we were experiencing a king tide along with windy conditions. You might not wanna sit here for long, at least without galoshes and a raincoat. In another view that same day, I admit I did get a wee bit wet recording this. We can use these king tides to give us a peek into the future. Because of global climate change and the rising of the oceans, the king tides of today are the normal high tides of tomorrow. Here you see the historical records for mean sea level going back before 1900. Notice the slow rise of the ocean in the jagged red curve. This is the actual data. There are several possible future outcomes or predictions, depending on how we decide to deal with climate change. Even the intermediate prediction in green has sea level rising one meter or over three feet by 2100. If king tides reached much higher than this, it could require the Exploratorium to retreat from its beautiful waterfront home. You wouldn't want this. We certainly wouldn't want this. So let's get busy and see if we can fix the problem. If you're unaware of the Coastal Commission's California King Tides Project, I encourage you to go to their website and check it out, and even participate. Thank you for viewing Full-Spectrum Science Shorts! presented by the Exploratorium in San Francisco. In our next Shorts program, we'll explore why the tides will be responsible for eventually eliminating total solar eclipses. This program like all Exploratorium programs is only possible because of donors like you. We know that this time is challenging, but if you can, help us keep educational content like this free and accessible to all by donating today at www.exploratorium.edu/connect. Thank you.