Declination

Tides: A Deep Dive

Part 6

To review: Tides have diurnal, monthly (syzygy), monthly-yearly (perigee), and yearly (perihelion) components. The components can coincide and have additive effects.

In our cast of characters, the sun and moon both have heavy influence. Their effect on tides has to do with their proximity to, and therefore how much gravity they exert on, Earth. But aside from distance, there is more to it, including angles. Here again we might think a lot about our position and perspective on Earth. This story must be rewritten with differences from any given location, in any hemisphere, but the principles applied will be the same.

Seasons happen because Earth is tilted on her axis, and as she revolves around the sun, the angle of the relationship between our equator and the sun changes. Let’s be geocentric for a second, and we’ll talk in terms of declination, which is the angle of a celestial body with respect to our earth’s equator. (Here’s a site with diagrams that seemed understandable).

Declination is a term I learned while practicing shipboard celestial navigation. I’m fortunate to have been taught the rudiments of this dying art/science while I was a semester-at-sea student. If I knew the angle of the sun above the horizon wherever I was in space on a schooner, leaning against the rail measuring said angle with my sextant, at exactly noon, this translated very accurately into my latitude. Which is good to know!

This conversion of my noon sun sight into latitude was possible because we knew the angle we’d expect the sun to be sporting with respect to the equator (latitude 0) based on the date, and we knew the sun’s angle with respect to the horizon relative to our position, thanks to the sextant. There are a few other mathematical gymnastics involved, but these are the basics.

At dawn or dusk, when a few stars and the horizon could be seen through the sextant, triangulating the latitudes indicated by the declinations of several of these navigational stars could be used for another accurate latitude estimate. We can talk about the declination of any celestial body—stars, sun, planets, moon—even though we realize they’re not all revolving around us. The nautical almanacs (we called them “pubs” and carried these books aboard the ships) list the declinations, down to the hour, of all the most likely navigational celestial bodies.

navigate like it’s 1998

From Earth’s equator, the declination of the sun can range from 0 degrees (at equinoxes) to 23.5 degrees north latitude, (on ~June 20 or northern hemisphere summer solstice) or to 23.5 degrees south latitude (on ~Dec 21 or northern hemisphere winter solstice). When the sun’s declination is 23.5° North, it is the closest it is going to get to my latitude of roughly 45° north, giving us our longest, warmest days we fondly refer to as summer.

I promise this all relates to tides. The moon also has a declination. The moon’s orbit is at about a 5 degree angle to Earth’s orbit around the sun (confusing, I know; we’re juggling three balls). The moon travels between its maximum extremes of declination north and south (and through zero declination) relative to our equator during each lunar cycle (aka month). Here in the north, the moon pulls on us hardest at its maximum northern declination. Any phase of the moon can correspond to any declination.

That bit is tricky. The moon orbits Earth every 27.3ish days. But a new moon happens every 29.5ish days due to the Earth also orbiting the sun so the moon has to catch up. Then, the time between maximum declinations is 27.2ish days. The time between perigees is 27.5ish days. All of these get slightly offset and lap and overlap each other in a very slow progression.

When considering all three orbs, and thinking of the tilt of Earth on its axis, and then also thinking of the tilt of the moon (relative to Earth) the 23.5 and 5 degree tilts can either agree with each other, or they can be at odds, and so the declination of the moon ranges anywhere between around 18.5 and 28.5 degrees. This cycle takes 18.6 years to complete; that’s how slow the progression is.

So while each month has a maximum lunar declination, that maximum value differs across months and years, achieving its maximum maximum value only every 18.6 years.

I am not even playing, reader: this year is the year. It’s happening right now in 2024-2025. We’re at the max max, which is sort of a two-year slow-motion event. I am realizing, in real time, that this is also part of why this year is a great tidepooling year.

At times of year when the moon’s monthly maximum northern declination (when it pulls on us up north here most and makes our higher tides higher) coincides with syzygy (when our high tides are already higher because of full or new moon), these effects are additive as well, so we get extra big tides. And because this year’s maximum monthly values are also at their 18.6-year maximum potential magnitude, it’s extra big.

More on lunar declination next week.