<p>Water level is flat with regard to the balance between several forces predominantly gravity (other smaller effects include baroclinic effects). The direction of <a href="https://en.wikipedia.org/wiki/Gravity_of_Earth" rel="nofollow noreferrer">Earth's gravity</a> is a combination of gravitational pull and inertia (centrifugal &quot;forces&quot;). The gravitational pull is toward the relative <a href="https://www.scientificamerican.com/article/football-center-of-mass/" rel="nofollow noreferrer">center of mass of the earth-moon system</a> depending on the relative position of the Moon with respect to the local position on Earth's surface.</p>
<p><a href="https://i.sstatic.net/AYBcB.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/AYBcB.png" alt="simplified gravity figure" /></a> Simplified sea level for a stationary uniform Earth covered by water. <a href="https://blogs.egu.eu/divisions/cr/2016/10/19/sea-level-for-dummies/" rel="nofollow noreferrer">Source</a></p>
<p>Gravity does change slightly in different areas based on Earth's density (more info in <a href="https://earthscience.stackexchange.com/questions/7449/does-gravity-differ-from-place-to-place-on-earth/7454">this answer</a>). A good explanation of why water level is &quot;flat&quot; is found <a href="https://blogs.egu.eu/divisions/cr/2016/10/19/sea-level-for-dummies/" rel="nofollow noreferrer">here</a>.</p>
<p>The ocean is not like a bathtub. At shorter time scales the water level surface responds as a balance between multiple forces (<a href="https://oceanservice.noaa.gov/education/tutorial_tides/tides01_intro.html" rel="nofollow noreferrer">tides</a>, wind stress, friction, baroclinic torque, ...). The balance is complex and highly local:
<a href="https://i.sstatic.net/cIxXR.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/cIxXR.png" alt="Hycom global sea level Nov 20, 2018" /></a> <a href="https://www7320.nrlssc.navy.mil/GLBhycom1-12/" rel="nofollow noreferrer">Source</a></p>

CC BY-SA 4.0
2023-01-25T03:05:32.367Z
Gravitational pull is _not_ towards the Earth-Moon barycenter unless you are much farther away than the distance between the Earth and Moon. From the close-up vantage point of Earth's ocean level Earth's gravity is more dominant over the Moon's and the pull is thus more nearly towards the center of Earth alone.
Oscar Lanzi
CC BY-SA 4.0
2023-01-25T12:11:05.523Z
The gravitational pull is always relative to the relative position with regard to BOTH Earth and Moon. Of course at the Earth surface the pull of the Earth is much larger, but the gravitational pull is still with regard to the Earth-Moon system. Otherwise you will be missing the tides.
arkaia
CC BY-SA 4.0
2023-01-25T12:15:05.880Z
Still not directed towards the bartcenter when you are on the surface of Earth. Work out the force resultant.
Oscar Lanzi
CC BY-SA 4.0
2023-01-25T14:05:11.437Z
True. It will vary depending on the relative position of the point on Earth surface and the Moon. Not the barycenter. Corrected.
arkaia
CC BY-SA 4.0
2022-12-25T16:00:32.423Z
2023-01-25T14:05:09.007Z
2023-01-25T14:05:09.007Z
arkaia
<p>The Earth is has a globular shape and sea water level is a plane.</p>
<p>We are on the upper surface of the Earth, then how is sea water level a plane?</p>

CC BY-SA 4.0
2022-12-25T20:23:56.033Z
Short answer: It only seems to be flat in the area you can see, because the Earth is a lot bigger than you are.
Spencer
CC BY-SA 4.0
2023-01-25T19:31:34.313Z
it's not really a plane in the large scale, it's a spherical (well, oblate-spheroid) shell.  But locally appears like a plane just like the horizon and walking in a path appears a flat line.  Where did someone call sealevel a plane?
JeopardyTempest
<oceanography><earth-observation>
How is sea water level on the Earth a plane?
015ra
# How is sea water level on the Earth a plane?

(Asked by: username_0 on 2022-12-25)

The Earth is has a globular shape and sea water level is a plane.

We are on the upper surface of the Earth, then how is sea water level a plane?

**Tags:** oceanography, earth-observation

> **Comments:**
> **username_1**: Short answer: It only seems to be flat in the area you can see, because the Earth is a lot bigger than you are.  
> **username_2**: it's not really a plane in the large scale, it's a spherical (well, oblate-spheroid) shell.  But locally appears like a plane just like the horizon and walking in a path appears a flat line.  Where did someone call sealevel a plane?  

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## 1 answers

### Answer by username_3 on 2022-12-25. Score: 4

Water level is flat with regard to the balance between several forces predominantly gravity (other smaller effects include baroclinic effects). The direction of [Earth's gravity](https://en.wikipedia.org/wiki/Gravity_of_Earth) is a combination of gravitational pull and inertia (centrifugal "forces"). The gravitational pull is toward the relative [center of mass of the earth-moon system](https://www.scientificamerican.com/article/football-center-of-mass/) depending on the relative position of the Moon with respect to the local position on Earth's surface.

[![simplified gravity figure](https://i.sstatic.net/AYBcB.png)](https://i.sstatic.net/AYBcB.png) Simplified sea level for a stationary uniform Earth covered by water. [Source](https://blogs.egu.eu/divisions/cr/2016/10/19/sea-level-for-dummies/)

Gravity does change slightly in different areas based on Earth's density (more info in [this answer](https://earthscience.stackexchange.com/questions/7449/does-gravity-differ-from-place-to-place-on-earth/7454)). A good explanation of why water level is "flat" is found [here](https://blogs.egu.eu/divisions/cr/2016/10/19/sea-level-for-dummies/).

The ocean is not like a bathtub. At shorter time scales the water level surface responds as a balance between multiple forces ([tides](https://oceanservice.noaa.gov/education/tutorial_tides/tides01_intro.html), wind stress, friction, baroclinic torque, ...). The balance is complex and highly local:
[![Hycom global sea level Nov 20, 2018](https://i.sstatic.net/cIxXR.png)](https://i.sstatic.net/cIxXR.png) [Source](https://www7320.nrlssc.navy.mil/GLBhycom1-12/)

> **Comments:**
> **username_4**: Gravitational pull is _not_ towards the Earth-Moon barycenter unless you are much farther away than the distance between the Earth and Moon. From the close-up vantage point of Earth's ocean level Earth's gravity is more dominant over the Moon's and the pull is thus more nearly towards the center of Earth alone.  
> **username_3**: The gravitational pull is always relative to the relative position with regard to BOTH Earth and Moon. Of course at the Earth surface the pull of the Earth is much larger, but the gravitational pull is still with regard to the Earth-Moon system. Otherwise you will be missing the tides.  
> **username_4**: Still not directed towards the bartcenter when you are on the surface of Earth. Work out the force resultant.  
> **username_3**: True. It will vary depending on the relative position of the point on Earth surface and the Moon. Not the barycenter. Corrected.  

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