<p>For (a) through (c), consider the differences between the <a href="http://en.wikipedia.org/wiki/Geoid" rel="nofollow noreferrer">geoid</a> and the reference ellipsoid, in addition to the barycentre.</p>

<p>For (d): A slightly higher <a href="http://oceanservice.noaa.gov/facts/perigean-spring-tide.html" rel="nofollow noreferrer">perigean spring tide</a> than usual, meaning in some geographical locations perhaps an amplitude difference of inches rather than millimetres.</p>

CC BY-SA 3.0
2016-11-25T15:37:53.297Z
2016-11-25T15:43:50.330Z
2016-11-25T15:43:50.330Z
Johan Wilmot
<p>(a) What, if anything, is the difference between Earths geographic center, <em>i.e.</em> mid-point of the spin axis, and the gravitational center?</p>

<p>(b) What is the average distance between the Earth's center of gravity, and the Earth-Moon system's center of gravity?</p>

<p>(c) The recent 'super-moon' is a reminder that the moon's distance has an orbital wobble. So, by how much does this vary the distance between the Earth <em>vs</em> Earth-Moon gravitational centres, and</p>

<p>(d) Is the moon's varying distance from the Earth sufficient to create any noticeable effect upon sea/ocean tidal amplitudes?</p>

CC BY-SA 3.0
2016-11-25T01:01:13.723Z
PS: By 'noticeable', I mean more than a few millimeters.
Gordon Stanger
CC BY-SA 3.0
2016-11-25T01:36:20.443Z
https://en.wikipedia.org/wiki/Barycenter
Spencer
CC BY-SA 3.0
2016-11-25T14:06:41.727Z
I think there's more than one question here. (c) and (d) are easy to relate and fit together (and FWIW, I'm pretty sure the answer to (d) is "yes"), but I think the others need asking separately. Might I suggest seperating them out?
Semidiurnal Simon
CC BY-SA 3.0
2016-11-25T16:23:14.867Z
Information on (d): http://earthscience.stackexchange.com/questions/8917/how-often-do-king-tides-perigean-spring-tides-occur
arkaia
CC BY-SA 3.0
2016-11-25T23:44:23.003Z
What exactly is the "mid-point of the spin axis"? The axis is infinitely long, it doesn't have a mid-point.
Wolfgang Bangerth
<tides><earth-moon>
Musing on Earth's gravity
Gordon Stanger
# Musing on Earth's gravity

(Asked by: username_0 on 2016-11-25)

(a) What, if anything, is the difference between Earths geographic center, *i.e.* mid-point of the spin axis, and the gravitational center?

(b) What is the average distance between the Earth's center of gravity, and the Earth-Moon system's center of gravity?

(c) The recent 'super-moon' is a reminder that the moon's distance has an orbital wobble. So, by how much does this vary the distance between the Earth *vs* Earth-Moon gravitational centres, and

(d) Is the moon's varying distance from the Earth sufficient to create any noticeable effect upon sea/ocean tidal amplitudes?

**Tags:** tides, earth-moon

> **Comments:**
> **username_0**: PS: By 'noticeable', I mean more than a few millimeters.  
> **username_1**: https://en.wikipedia.org/wiki/Barycenter  
> **username_2**: I think there's more than one question here. (c) and (d) are easy to relate and fit together (and FWIW, I'm pretty sure the answer to (d) is "yes"), but I think the others need asking separately. Might I suggest seperating them out?  
> **username_3**: Information on (d): http://earthscience.stackexchange.com/questions/8917/how-often-do-king-tides-perigean-spring-tides-occur  
> **username_4**: What exactly is the "mid-point of the spin axis"? The axis is infinitely long, it doesn't have a mid-point.  

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## 1 answers

### Answer by username_5 on 2016-11-25. Score: 1

For (a) through (c), consider the differences between the [geoid](http://en.wikipedia.org/wiki/Geoid) and the reference ellipsoid, in addition to the barycentre.

For (d): A slightly higher [perigean spring tide](http://oceanservice.noaa.gov/facts/perigean-spring-tide.html) than usual, meaning in some geographical locations perhaps an amplitude difference of inches rather than millimetres.


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