Skip to content

The moon, the moon’s nodes and climate change sea level rise – Part I

May 21, 2021

[Posted by Chuck Almdale]

Rising sea levels are in our present and in our future. Due to the little-known effects of the moon’s orbital tilt and the precession of the orbital nodes, we are in the middle of a 9.3-year dampening of the sea level rise, and currently see little-to-no rise. But in 2026-2035, we enter a 9.3-year-long enhancement of sea level rate-of-rise and will see an approximately four-inch permanent rise.

The lunar gravitational pull affects our oceanic tides and is the primary cause of our daily tides. The solar gravitation pull is a secondary factor, but when moon and sun are in alignment (as seen from earth) at new and full moons, they work together to create the cycle of highest high and lowest low monthly tides, the annual northern winter “king” tides, and other even longer tidal cycles.

There are other, local influences: latitude, shape of the offshore ocean floor, the magnifying effect of narrowing bays. Most locations have two high and two low tides every day, as we do in Southern California; other locations—such as the Gulf of Mexico—have one high and one low tide per day. The discussion that follows is of a general nature, not absolutely true for all locations at all times of the day or year.

The Bay of Fundy tidal funneling effect—the farthest, narrowest points have the greatest tidal height fluctuation.


There’s a “super full moon” and total lunar eclipse scheduled for May 26, 2021. Flooding may occur at some low-lying coastal areas. This will be due to predictable high tides, not to unpredictable and unlikely tsunamis. The total lunar eclipse starts at 4:11 AM PDT (11:11 UTC) and lasts 14.5 minutes. There will be another super full moon (but no eclipse) on 6-24-21.

The chart below shows predicted tides for Santa Monica Pier for May 22-29, 2021. Highest tide is +6.89 ft. at 9:44 PM on 5-26-21. Lowest tide is -1.74 ft. at 5:00 AM on 5-27-21. These are very high and very low tides for this location.

Super full moons (and super new moons) occur when the moon is at perigee (closest approach to the earth). The moon’s orbit is an ellipse, not a circle. The average earth center-moon center distance is 238,000 miles, but can be as close as 223,694 miles at perigee during supermoon or as far as 251,655 miles at apogee during micromoon. A super moon appears 14% larger and 30% brighter than a micromoon. (Super- and micromoon are not official astronomical terms, so there’s no hard definition concerning distances.) Supermoons, being closer to the earth, have an increased gravitational pull and tidal fluctations are greater, with higher highs and lower lows. The higher high tide can cause flooding. The lower low tide makes for nice tide pools.

Ellipse: Draw an ellipse by stretching a string from focus 1 to (x,y) to focus 2. Hold down the two string ends at the focii. Place a pencil tip at (x,y) and—keeping the string taut—make a line around the two focii. The result is the above ellipse. The farther apart the focii, the more elongated the ellipse. Kepler’s first two laws of planetary motion, formulated around 1610, state that 1) the orbit of a planet is an ellipse with the sun at one of the two focii, and 2) a line segment connecting the orbiting planet to the sun sweeps out equal areas during equal periods of time. Thus the moon in its elliptical orbit moves more quickly when near the earth.

The Tides

Daily high tides are caused by the moon’s gravitational pull on the ocean water closest to it, causing a bulge in the water. The earth rotates once per day and this bulge moves around the earth once per day. The water’s inertia causes a lesser bulge on the opposite side of the planet. Low tides fall between the two bulges. Thus, for most places, there are two high and two low tides per 24-hour period, with the higher of the two high tides on the side facing the moon. On average, it’s 24 hours and 50 minutes from one daily highest tide to the next. If high tide occurs at noon one day, then (roughly) seven days later it will be low tide at noon. The moon’s “monthly” cycle from new moon to new moon is 29.53 days long.

Earth’s orbit is an ellipse: the average earth center-sun center distance is 92,955,807 miles, but at perihelion (closest point) it can be 1.7% shorter, at aphelion it can be 1.6% longer. The year’s highest tides occur during our northern winter when the earth is at or close to perihelion, around January 4. The earth approaches and leaves perihelion slowly so—depending on the other factors already mentioned—the actual annual highest tide can occur from November to March.


Montage of moon in eclipse, showing “blood on the moon” (Sebastien Gauthier 5/14/14; NASA website)

Solar eclipses occur when the moon passes between the sun and earth and the moon’s shadow falls on the earth’s surface. The total eclipse shadow is small—at most 166 miles in diameter—and a total solar eclipse is at most 7.5 minutes long. Lunar eclipses often occur 14 days before or after a solar eclipse when the earth is between sun and moon and the moon passes through the earth’s shadow. A lunar eclipse can be several hours long. This is sometimes called “blood on the moon” as the darkened moon often has a reddish hue, due to the scattering of sunlight passing through the earth’s atmosphere on it’s way to the moon.

So far we have the following:

  • The moon’s gravitational pull creates a bulge in the ocean. Inertia in the water creates another bulge on the opposite side of the planet.
  • The rotation of the earth causes these bulges to move around the globe once per day.
  • The additional gravitational pull from the sun at new moon and full moon causes the monthly cycle of highest highs and lowest lows in the daily tide.
  • The elliptical orbit of the moon around the earth causes supermoon at perigee and micromoon at apogee.
  • The elliptical orbit of the earth around the sun causes annual highest and lowest tides centered on the January 4 perihelion, and annual most-moderate tides at aphelion six months later.

If these few orbital factors were all there were to the moon’s orbit, we’d have total lunar and solar eclipses every month. We don’t, as you’ve likely noticed. So there’s more.

Here’s a great short film from Inside Science, only 2:42 long:

Coming up in Part II
The moon’s tilted orbit and the nodes
The wobbling moon
The nodal cycle and the rising ocean
Sources and links

Comments are closed.

%d bloggers like this: