On Tides

Tuesday, September 20, 12022 HE

Why does the moon raise and lower the tides over and over again?
It has to make shore.

Source: https://upjoke.com/tide-jokes

I love a good pun as well as a corny joke. For more tide jokes, check out this link to upjoke.com. The one about the three women and a man on the beach is a classic. Offside, but funny.


Despite growing up close to the coast (Squamish, British Columbia) I didn’t do a lot of watersports growing up. I never mastered the marvels of marine miracles, so my segue into SUP was a steep study. The following post covers basic and more complex aspects of the tidal phenomenon that I feel are useful knowledge for aspiring waterfolk.


  1. What are Tides?
  2. Cosmological View of Tides
  3. Life Without Tides?
  4. Timing of Tides: Lunar Vs. Solar Day
  5. Tide Types
  6. The Moon’s Axis
  7. Spring and Neap Tides
  8. King Tide Confusion
  9. Here Comes the Sun
  10. Other Factors: Who is the King?
  11. Perigean Spring Tide
  12. Tides Versus Currents
    1. Citations

What are Tides?

According to dictionary.com, the tide is “the periodic rise and fall of the waters of the ocean and its inlets, produced by the attraction of the moon and sun, and occurring about every 12 hours.” Astronomically speaking, tides are a result of tidal force, “the gravitational pull exerted by a celestial body that raises the tides on another body within the gravitational field, dependent on the varying distance between the bodies” (Dictionary.com).

Cosmological View of Tides

A less geocentric, more cosmological description of Earth’s tidal phenomena is below by Neil deGrasse Tyson. I had the same reaction that his co-host Chuck Nice had 🤯!

This more accurate account offers a change in perspective. The third rock is turning inside the liquid bulge caused by the attractive forces of the Sun, the Moon, and the Earth. More generally, our perception is that the ocean’s water is rising and falling or approaching and receding along the shorelines rather than the shorelines moving within a relatively static water mass.

Life Without Tides?

A friend recently sent me a link to “Episode 2: Snowball Earth” of Naked Science‘s five-part documentary series Catastrophe relating to a conversation on climate change. A few minutes into the second episode, I realized I needed to start from the beginning of the series. I was that intrigued. The series “investigates the history of natural disasters, from the planet’s beginnings to the present, putting a new perspective on our existence and suggesting that we are the product of catastrophe” (PioneerProductionsUK, 2014). And low and behold, “Episode 1: Birth of the Planet” presents the hypothesis that a catastrophic event created the Moon, the so-called Giant Impact Theory. Essentially, an early solar system planet, Theia, collided with another planet, proto-Earth, resulting in the formation of our Moon. The episode proposes the theory that life on Earth may not have developed without the Moon’s tidal effects on the Earth. Violent at first, due to the Moon’s proximity, the gradual lunar retreat eventually slowed the Earth’s rotation, creating calmer conditions, allowing cyanobacteria to flourish, oxygenating the planet. A near primogenial ordo ab chao, or ‘out of chaos, comes order,’ scenario. The ouroboros of our Earth.

Ouroboros: the eternal cycle of destruction and rebirth.

Timing of Tides: Lunar Vs. Solar Day

From this perspective, the dual diurnal (my term, not the official term) highs and lows of the tide make much more sense (more on this below). The Earth takes slightly less than 24 hours to complete its rotation, 23 hours, 56 minutes, and 4.09053 seconds, to be precise, the so-called sidereal period. Two major attractive forces acting on the Earth that most people consider are the Moon and Sun. Conceptually, we think of gravity acting on the centre of mass of an object, rather than its surface. When dealing with celestial-sized bodies the distances are so large that the body’s gravitational fields act on themselves with significant results! This effect is due to the inverse square law. Simply the law states that the effect of gravity decreases with distance. The Earth has a radius of 6,371 km, so at its equator it is roughly 12,742 km wide. But as you will see in the video below, the distance can actually change due to tidal force. Not only does gravity attract the water on Earth it warps the shape of the Earth. The tidal force makes the Earth more ellipsoidal rather than spherical (note that this deformation is also a result of the centrifugal force of the Earth’s rotation). The Earth is literally stretched by the tidal force acting upon it.

The side of the Earth closest to the Moon is pulled toward the Moon. But at the same time, Earth’s centre is also shifted toward the Moon. The shift is equivalent to pulling the side of the Earth furthest from the Moon away, which is why the ellipsoid forms and why there are two high tides. One high tide occurs in the bulge on the same side of the Earth as the Moon, and another, approximately 12 hours and 25 minutes later, when the Earth turns into the other bulge. The second high tide occurs when the same part of the Earth has rotated around to be opposite the Moon and enters the opposite tidal bulge (see the animation below the video). It takes approximately six hours and 12.5 minutes for the water at the shore to go from high to low or low to high tide. Six hours 12.5 minutes is roughly the time it takes a point on the Earth to rotate to be at a right angle to the Moon or a quarter turn. You’ll notice that it is not a quarter of a day, but slightly longer.

One reason for the lag-time in the tide results from the movement of the Moon. As the Earth completes its roughly 24-hour rotation, the Moon is also moving in its roughly 27.3-day journey around the Earth. A small side note here is that this is the tropical or sidereal month and is relative to the Vernal Equinox, whereas one orbit of the Moon relative to the Sun, a synodic month, takes about 29.5 days. During the 24 hours the Earth takes to complete a revolution, the Moon moves ahead of the Earth by about 50-minutes. The Moon’s motion during this time is one major reason why the tidal cycle is slightly longer than the solar day. For more on tidal curiosities, check out the eponymous page, “Tidal Curiosities,” on the NOAA SciJinks website.

The practical application of this knowledge is that here in the Pacific Northwest, high and low tides are approximately six hours apart, and at the same time, high or low tides will shift by about one hour every day. Without access to a tide chart, you can roughly estimate the tide times with this knowledge once you’ve observed a high or low tide.

Tide Types

The timing of tides brings me to another curiosity. While researching more, after I had already christened my term “dual diurnal” tide, I came across the true technical term for this phenomenon. There are in fact three distinct tide cycle types, diurnal, semidiurnal, and mixed semidiurnal (US Department of Commerce). Generally speaking, most places on Earth experience two high and two low tides every lunar day. In fact, if the Earth were a perfect sphere without large continents, everywhere on Earth would experience two equally proportioned high and low tides every lunar day. But the large continental masses and irregular surface of the Earth provide resistance and block the westward passage of water around the Earth. Remember that the Earth is turning around inside the tidal bulge. Since the Earth is spinning eastwardly the water mass is moving relatively westward. The restricted movement of water causes complex interactions within the various ocean basins that result in dramatic local and regional differences within basins (US Department of Commerce).

Three basic tidal patterns emerge. In places where there are two relatively equal high and low tides, the pattern is called semidiurnal. If the high and low tides differ in height, it is called mixed semidiurnal. This is the pattern that we see here in the Pacific Northwest. Finally, some areas of the world, such as the Gulf of Mexico, experience only one high and low tide per day, and this is called a diurnal cycle. The image below shows where the various tidal patterns occur around the globe.

The picture below shows examples of tidal graphs for the various types of tidal cycles.

Source: https://oceanservice.noaa.gov/education/tutorial_tides/tides07_cycles.html

On a practical level, knowing the tide pattern locally, in this case, the Pacific Northwest, helps with planning. For example, at Spanish Banks, the water will be far away at the lowest of the low tides and may not be an ideal time to head to the water with your SUP. Better to pack your skim board! Furthermore, it is worth noting that larger tidal range changes will be accompanied by stronger currents. The large movement of water can result in stronger more rapid currents. 

The Moon’s Axis

Another factor that affects the tidal pattern type is the tilt of the Moon’s orbit around the Earth. Many people are familiar with the tilt of the Earth’s axis, 23.5 degrees from the plane of its orbit around the Sun. It is the tilt of the Earth that gives us the current seasons seen regionally around the globe. The Moon’s orbit around the Earth is tilted about five degrees from the plane of the Earth’s orbit around the Sun.

The tilt means that the maximum tidal bulge will be slightly above or below the equator. As a result, some areas of the Earth only pass through the tidal bulge once in a lunar day creating a diurnal tidal cycle. Check out these animations from the NOAA SciJinks site for a visual schematic of this phenomenon.

Spring and Neap Tides

Other tidal terms that you often hear are spring and neap tides. Spring tides are semimonthly tides of increased range (i.e., increased and decreased height) that are a result of the Moon being New or Full. New and Full Moons occur when the Sun, Earth, and Moon are all nearly in alignment. The technical term for this is the phenomenally fantastic word syzygy (/ siz-i-jee /). New Moons (i.e., a dark moon) occur when the Moon is in-between the Sun and Earth, in syzygy, and thus the surface visible to us is shadowed. Ironically, the dark side of the Moon is lit, we just can’t see it. A Full Moon occurs when the Moon is on the opposite side of the Earth to the Sun. In this case, the light from the Sun is able to reach the Moon’s surface and is reflected back toward us illuminating the lunar surface.

So simply stated, the spring tide is the highest and lowest two tides during the month that occur at Full or New Moon. Spring tides are higher high tides and lower low tides. Spring tides are separated by roughly a fortnight (i.e., approximately fourteen days, which is where the “for” in fortnight comes from). It takes approximately two weeks for the Moon to complete half of its orbit around the Earth to go from the syzygial alignment of a New Moon to the equally syzygial Full Moon alignment, or vice versa (I’ll stop with the syzygies now, I just couldn’t get enough of the word at first). The increased tidal range during these Moon phases is a result of a compounding of the Moon and Sun’s gravity effects on the Earth’s tidal bulge (see the schematic below).

graphic showing orbit of moon around Earth, illustrating spring and neap tides
Source: US Department of Commerce, National Oceanic and Atmospheric Administration. “Why Do We Have Spring Tides in the Fall?” NOAA’s National Ocean Service, 1 Aug. 2014, oceanservice.noaa.gov/facts/springtide.html.

Conversely, neap tides are the two smallest tidal ranges of monthly tides and result from the Moon being at quadrature (i.e., First or Third Quarter). Thus, neap tides are lower high tides and higher low tides than average. The spring and neap tides only partly match the calendar’s monthly cycle since the lunar orbit is 27.3 days. Generally, the spring and neap tides are separated by seven days, as per the change in the Moon’s demi-phases. The term “spring” does not refer to the season. Spring tides occur perennially. Rather, the term “spring” was used as the tide was seen to be springing forth during the New and Full Moons. The etymology of “neap” is uncertain

The image below is a schematic of the difference between spring and neap tides.

Tidal ranges - differences between neap tides and spring tides - graph
Source: https://www.sailingissues.com/navcourse6.html#:~:text=Spring%20tides%20have%20higher%20high,than%20in%20a%20low%20tide.

King Tide Confusion

The “King Tide” does not have a formal definition, at least not a consistent one, and is used to describe several different but related phenomena. For me, and I am sure others, this caused confusion since, to start, the spring tide is popularly referred to as the “King Tide.” The term King Tide is also used to describe exceptionally high tides. Furthermore, King Tide has also been used to describe the “highest of all high tides during the year” (Sutherland). Can you see why there is confusion with all these related uses? We have spring tides or exceptionally higher tides that occur at regular semimonthly cycles at New Moon and Full Moon. So, are they really an exception? I guess they are, in that there are roughly 24 of them in a year out of 365 days. In any case, for some, these are referred to as King Tides.

In addition to the regular semimonthly cycles of exceptionally higher tides, the tide is also (exceptionally) higher when the Moon is at its perigee. The Moon’s orbit is elliptical and somewhat irregular so it has points where it is closer and further from the Earth. The lunar perigee is the point in the Moon’s orbit when it is closest to the Earth and the apogee is when it is furthest. As we saw with the inverse square law earlier, gravity is proportional to distance, and thus the tidal effects of the Moon are stronger at its perigee. In my mind, I see the “King Tide” as an exceptional term that should be reserved for an exceptional tide. At a minimum, the “King Tide” title should be reserved for the rarer occasions where a spring tide overlaps with a perigean tide, the so-called perigean spring tide. The Moon’s perigee overlaps with a Full or New Moon typically six to eight times during the year causing a perigean spring tide. A much more exceptional (in)frequency than the standard spring tide.

But in addition to the Moon’s orbital orientation effects on tides, you also have the Earth’s.

Here Comes the Sun

The Earth’s path around the Sun is also elliptical, and this can alter the Sun’s gravity effects on tides. When the Earth is closest to the Sun, it is called perihelion, and when it is furthest, it is aphelion. An easy way to remember the difference between the peri’s (perigee and perihelion) and the ap’s (apogee and aphelion) is to associate the “p” with proximal and the “a” with away. Then you should always remember and associate the correct distances for each term.

As discussed earlier, the syzygial interactions of the celestial bodies can compound their gravitational effects. Thankfully, the Earth’s elliptical path around the Sun is quite consistent. It is at perihelion around January 4th (approximately two weeks after the December solstice) and aphelion near July 4th (approximately two weeks after the June solstice). I think an easy way to remember this is to note that it is the reverse of what you would expect from the temperature (albeit in my northern hemisphere-centric world view). I think it is often assumed that the Earth is closest to the Sun in the summer and furthest in the winter. However, it is not the Earth’s distance from the Sun that causes the temperature differences of the seasons. Rather, it is the angle of its rotational axis that results in more or less insolation during the respective seasons.

Other Factors: Who is the King?

If our definition of King Tide is the highest tide of the year, which I am in favour of, then there are other things to consider. Complicating things further, exceptionally high tides can occur for other reasons. For example, seasonal effects like higher water levels due to the thermal expansion of warmer water can alter tidal range/height. And storm surges resulting from strong offshore winds and lower barometric pressure can result in higher water levels and cause coastal flooding. These effects can overlap a perigean spring tide to cause extreme elevations in water levels and coastal flooding. Last year in November, the Jericho Pier was subjected to the effects of a King Tide and was submerged underneath the higher waters. Another King Tide coupled with strong winds in January this year was the final straw for the pier, which is still closed at present. The first event in November last year coincided with the lunar perigee on November 5, 12021HE. The second event, in January, occurred around both the Moon’s perigee (January 9, 12022) and the Sun’s perihelion (January 4, 12022HE). The November 9, 12021 HE tide was predicted to be approximately 4.8 metres, but was observed to be 5.3 metres. While, the tide on January 7, 12021 HE was predicted to be approximately 4.9 metres, but was observed to be 5.5 metres. Thus they both likely lived up to the true title of King Tide.

Perigean Spring Tide

As discussed above, this is often a King Tide, but not always. Essentially it is a high tide that occurs when the Moon is closest to the Earth (i.e., perigee) and the Sun, Moon, and Earth are in syzygy (i.e., there is a New or Full Moon and thus a spring tide). However, a perigean spring tide is not necessarily the highest tide of the year. Thus to me, it is a high tide, but not always a King Tide.

Tides Versus Currents

While tide and current are related, they are not the same thing. A simple way to conceptually distinguish ocean tide and current is to think of the tide as the vertical movement of water and the current as the horizontal movement of water. In this view, the tide causes water levels to raise and fall, whereas the current is the water movement toward or away from a place. More formally, “ocean currents are the continuous, predictable, directional movement of seawater driven by gravity, wind (Coriolis Effect), and water density” (National Geographic). Thus, the tide has a massive effect on currents as it is the relative movement of a massive bulge of water around the Earth caused by gravity. As the bulge moves, it forces water around, over, and through the geological formations of the Earth. The tide is one factor in creating currents. Currents are also affected by wind (ultimately differential heating and the Coriolis Effect) and water density (i.e., the thermohaline circulation). In many cases, the tide and current can be conceptualised as the same phenomena. However, in places with narrow channels, currents must be considered separately from tides, since they move at rapid rates and occur at different times. So much so, that the Government of Canada produces separate charts/tables for the prediction of tides and currents.

If you made it this far, thank you for reading. And I hope that you found this post interesting and informative. I sure learned a lot looking into it.


Citations

“Tide Definition & Meaning.” Dictionary.com, Dictionary.com, https://www.dictionary.com/browse/tide.

US Department of Commerce, National Oceanic and Atmospheric Administration. “Types and Causes of Tidal Cycles – Tides and Water Levels: NOAA’s National Ocean Service Education.” Types and Causes of Tidal Cycles – Tides and Water Levels: NOAA’s National Ocean Service Education, 1 June 2013, https://oceanservice.noaa.gov/education/tutorial_tides/tides07_cycles.html#:~:text=Unable%20to%20move%20freely%20around%20the%20globe%2C%20these%20tides%20establish%20complex%20patterns%20within%20each%20ocean%20basin%20that%20often%20differ%20greatly%20from%20tidal%20patterns%20of%20adjacent%20ocean%20basins%20or%20other%20regions%20of%20the%20same%20ocean%20basin.

Sutherland, Scott. “The Science behind King Tides: What Are They and How Do They Happen?” The Weather Network, https://www.theweathernetwork.com/ca/news/article/king-tides-reveal-the-subtle-but-powerful-moon-sun-influence-on-our-planet.

“Ocean Currents.” National Geographic Society, https://education.nationalgeographic.org/resource/resource-library-ocean-currents.

YouTube. (2014). YouTube. Retrieved September 20, 2022, from https://youtu.be/Ghd8H-KvwVA.

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