Thursday, March 31, 2011

Seeing How Gravity Differs Around The Earth.

I would like to thank Phil at Bad Astronomy for bringing this video to my attention.  It turns out gravity is not felt quite the same around the whole earth.  Fortunately, for the last two years  "ESA's GOCE satellite has gathered enough data to map Earth's gravity with unrivalled precision."

From these gravity measurements the GOCE team was able to produce the above video.  What you see is known as a "geiod" and shows how gravity differs around the earth:
The geoid is the surface of an ideal global ocean in the absence of tides and currents, shaped only by gravity. It is a crucial reference for measuring ocean circulation, sea-level change and ice dynamics – all affected by climate change.
This means (I believe) that if the earth was just an ocean feeling the same varying gravity that exists on our earth, the above video shows how our "ocean earth" would be deformed by these differences in gravitational pull.  As you can see, the effects are very interesting and go a long way explaining why we have different currents and weather patters across the oceans.


  1. Cool. It would be nice to know the delta. A long while ago, I had to check Fortran code for the gravitational model for computing orbits (two weeks of work!). I believe it was Bessel function expansion or something, and, it somehow accomodated gravitational variations experienced by the sat.


    A spellchecker for the commentors would be a great help, especially for the ancient eyes like mine!

  2. Ancient1,

    Thanks for the comment and I am always looking for ways to improve the blog in small steps. (Like evolution. :))

    Oh, and don't get me started with Fortran codes that take weeks to understand. (Especially if it is FORTRAN 77!!!)

  3. Spherical Bessel functions, I hope :)

  4. I have a random FORTRAN question: why do so many FORTRAN programmers feel the need to put every key word in all caps? I'm not complaining, just trying to figure out why this community of programmers do when nobody else does. (Except astronomers using IDL every once in a while but lets not go with how backward astronomers usually are about everything.)

    I can think of two things:

    1. The culture evolved this way independent of the C and other language cultures for some reason I don't understand.

    2. Syntax highlighting didn't exist in the initial days of FORTRAN so and people have not changed now the syntax highlighting has been around now. (Again, unlike C and virtually all other programmers.)

    I'm not saying this is bad, just wondering...

  5. Modern Fortran programmers don't do that, but it stems from the fact that Fortran compilers are not case-sensitive, so anciently using all caps was just sort of the standard practice to give an additional level of commonality to codes.

  6. Nick, you should see my Fortran programs. I intentionally break all the traditional rules. All key works lower case. I start indenting since that's not needed anymore. IE...

    program Hello

    ! No indention needed since not in a loop, function, etc...
    print *, "Hello World!"

    end program Hello

  7. So, just to clarify, the yellow and red areas are where the "ideal ocean" would be higher, so gravity is weaker there? And the dark blues (like southern India) would be where the ocean level would be lower, so gravity is stronger there? I just want to make sure I'm interpreting their visualization correctly.

    On a side note, gravity variation maps can be extremely interesting and useful for geological purposes. I'll try and look up some things and comment again later.

  8. It looks like I may have gotten the coloring backwards. See APOD:

  9. Something like that! I was checking the accuracy of Fortran statements - some errors, but nothing that control systems would not compensate... Sometimes, I wonder if we could ever do anything without good feedback control systems.


    I gave up doing Fortran in about 1990; Powerpoint replaced it! Seriously, doing stupid presentations...


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