# What is Sea Level? - Every 19 Years **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [August 24, 2009, 2:39pm UTC](https://boards.straightdope.com/t/what-is-sea-level-every-19-years/507503 "2009-08-24T14:39:20Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![qazwart](https://avatars.discourse-cdn.com/v4/letter/q/5fc32e/32.png) [@qazwart](https://boards.straightdope.com/u/qazwart)\ **Post date:** [August 24, 2009, 2:39pm UTC](https://boards.straightdope.com/t/what-is-sea-level-every-19-years/507503/1 "2009-08-24T14:39:20Z")
> [@](#): > > From [What is Sea Level](http://www.straightdope.com/columns/read/148/what-is-sea-level) > Sea level is based on the assumption that because the oceans are interconnected and water tends to find its own level, the ocean surface around the world forms an ellipsoid (or flattened sphere, which is what shape the earth is) of more or less uniform geometry, once you average out the tides and seasonal variations over a long period, **typically 19 years.** I found that 19 years quite interesting. It is an unusual number for something like that: Could it be related to the 19 year cycles in the Jewish calendar? Let me explain: The Jewish calendar is a solar/lunar calendar. Months are determined by the phases of the moon with new months beginning on the new moon. However, the months themselves must occur in a particular season, so every few years, a _leap month_ is added. This leap month cycle is a 19 year cycle. I believe there are seven leap months every 19 years. Thus the 19 years somehow relates to a complete cycle between the sun and the moon. Since both are responsible for tides, there appears to be a need for this complete cycle. Does this make sense? ---
**Author:** ![janeslogin](https://avatars.discourse-cdn.com/v4/letter/j/df788c/32.png) [@janeslogin](https://boards.straightdope.com/u/janeslogin)\ **Post date:** [August 24, 2009, 2:55pm UTC](https://boards.straightdope.com/t/what-is-sea-level-every-19-years/507503/2 "2009-08-24T14:55:25Z")
One site says \*\*traditionally \*\* it has to take into account a 19 year cycle (Then it rambles on into mention of a cycle 5 months shorter so I won’t bother you with a link.) The original SD response said \*\*typically \*\*19 months. I can’t find how satellite data is calculated. It is now done with satellites isn’t it? ---
**Author:** ![glowacks](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/glowacks/32/5548_2.png) [@glowacks](https://boards.straightdope.com/u/glowacks)\ **Post date:** [August 24, 2009, 5:11pm UTC](https://boards.straightdope.com/t/what-is-sea-level-every-19-years/507503/3 "2009-08-24T17:11:27Z")
> **[Metonic cycle](https://en.wikipedia.org/wiki/Metonic_cycle)** > > The Metonic cycle or enneadecaeteris (from Ancient Greek: ἐννεακαιδεκαετηρίς, from ἐννεακαίδεκα, "nineteen") is a period of almost exactly 19 years after which the lunar phases recur at the same time of the year. The recurrence is not perfect, and by precise observation the Metonic cycle defined as 235 synodic months is just 2 hours, 4 minutes and 58 seconds longer than 19 tropical years. Meton of Athens, in the 5th century BC, judged the cycle to be a whole number of days, 6,940. Using these w... 19 tropical years differ from 235 synodic months by about 2 hours. A synodic month being the average period of the Moon’s revolution. Thus if one wanted to average sea level over a certain number of years, 19 years provides a good sample size and is neutral with respect to tidal variation. ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [August 24, 2009, 6:13pm UTC](https://boards.straightdope.com/t/what-is-sea-level-every-19-years/507503/4 "2009-08-24T18:13:16Z")
**qazwart** , the answer appears to be that they are related in that they have the same cause, the relationship between Moon orbits of Earth and Earth orbits of Sun to provide even numbers. ---
**Author:** ![John\_W.Kennedy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_w.kennedy/32/1031_2.png) [@John\_W.Kennedy](https://boards.straightdope.com/u/John_W.Kennedy)\ **Post date:** [August 24, 2009, 6:28pm UTC](https://boards.straightdope.com/t/what-is-sea-level-every-19-years/507503/5 "2009-08-24T18:28:13Z")
The number 19 turns up over and over again in Earth-Moon-Sun relations. Easter involves a 19-year cycle, and there is a physical 19-year cycle in which the Earth wobbles a bit. (This is not the same as the much larger 25,800-year wobble that causes the celestial north pole to wander. The whole universe is one big spirograph.) ---
**Author:** ![pipper](https://avatars.discourse-cdn.com/v4/letter/p/c2a13f/32.png) [@pipper](https://boards.straightdope.com/u/pipper)\ **Post date:** [August 24, 2009, 8:04pm UTC](https://boards.straightdope.com/t/what-is-sea-level-every-19-years/507503/6 "2009-08-24T20:04:54Z")
But the underlying assumption is that if we were to take away the tides and seasonal variations, the water level would be the same everywhere- which is apparently not entirely correct. During some recent reading abut El Nino, it was stated that the trade winds in the pacific pile up water along its western edge, so much so that the water level in the Coral sea is 1/2 meter higher than the water level along Ecuador (and El Nino is caused by a weakening of the trade winds that allows the warm surface water to “slide downhill” back towards Ecuador). So do sea level measurements account for this? And is the actual average surface of the ocean near ecuador technically below sea level? Or is this accounted for as part of the catchall “seasonal variations”? ---
**Author:** ![Freddy\_the\_Pig](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@Freddy\_the\_Pig](https://boards.straightdope.com/u/Freddy_the_Pig)\ **Post date:** [August 24, 2009, 9:08pm UTC](https://boards.straightdope.com/t/what-is-sea-level-every-19-years/507503/7 "2009-08-24T21:08:09Z")
No, the 19-year averaging of tides is not related to the Jewish calendar. They are two different effects with two different causes, which both happen to involve the number 19. The sea-level factor is due to the 19-year “regression of the nodes”, over which the Moon’s orbital tilt relative to the Earth’s equatorial plane varies from 18 to 29 degrees and back. When the tilt is lesser, Earth and Sun tend to pull more in the same direction and tidal variation is a little greater. The Jewish calendar cycle is due to the coincidence that 19 years are almost exactly equal to 235 synodic months. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 24, 2009, 10:36pm UTC](https://boards.straightdope.com/t/what-is-sea-level-every-19-years/507503/8 "2009-08-24T22:36:54Z")
I don’t think that’s a coincidence, though. The regression of the nodes is ultimately due to interactions between the Sun, Moon, and Earth, and it’s not surprising that a resonance (or near-resonance) of those bodies would leave its stamp. ---
**Author:** ![Freddy\_the\_Pig](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@Freddy\_the\_Pig](https://boards.straightdope.com/u/Freddy_the_Pig)\ **Post date:** [August 25, 2009, 2:09pm UTC](https://boards.straightdope.com/t/what-is-sea-level-every-19-years/507503/9 "2009-08-25T14:09:04Z")
The true period of the regression of the nodes is 18.6 years, though, so it would be out of sync after a few cycles. I think 19 orbits and 235 orbits are too large a period over which to set up a meaningful gravitational resonance. I think the double-19 is a coincidence.