axially-averaged cross section of deviations of the geoid.
Deviations are in averaged metres. From p.166 of Fowler, 1990.Dave unfortunately has not provided a specific reference for the observation of the shape of the Earth, but as near as I can tell, he is referring to the discovery, back in the late 1960s, of the departure of the equipotential gravitational surface known as the "geoid" from an ideal oblate spheroid (a sphere flattened along the rotational axis) shape. The geoid corresponds approximately to sea level, and, on land, the position where sea level would be if a trench were dug across the continents and allowed to fill with ocean water. Geoid shape is measured mainly by satellite observations. The deviations of the geoid from an oblate spheroid are depicted below as an axially averaged cross section, taken by averaging all the deviations of the geoid along a given line of latitude. This is the only shape that seems to correspond to the "pear shape" that Dave was describing. For example, it seems unlikely Dave was referring to topography, which does not have the shape he has described (e.g., the north pole is a "hole" if dealing with topography).
axially-averaged cross section of deviations of the geoid.
Deviations are in averaged metres. From p.166 of Fowler, 1990.
Note that this is not the real shape of the Earth. It is an axial average of deviations of the geoid, and the shape is an artifact of the data processing to "flatten" it into a single, 2-dimensional cross section. The actual shape of the deviations is a more complex 3-dimensional shape, and is depicted on the contour map below:
map of geoid deviations. Deviations are measured in positive or
negative metres from the ideal oblate spheroid. Lowest areas have been
shaded blue, highest areas are shaded orange and red.
Although the averaged geoid deviations may be "pear shaped" there is no radially-arranged "north polar bulge" obvious in the map of the geoid deviations. Bob Grumbine and others have also pointed out that in order to be tidal, the structure would have to be bipolar (a north *and* south polar bulge, just like modern lunar tides have a podal and antipodal bulge). None of this is evident even if the "pear shape" were accurately reflecting the 3-dimensional shape of the Earth.
Ian Tresman (ianTresman@easynet.co.uk) suggested that other axial orientations might be a possibility (in talk.origins article <320c5a5f.32783249@news.easynet.co.uk>). It is difficult to evaluate this possibilty in a map projection, so I have wrapped the map around a sphere and rendered it in 3 dimensions from a variety of orientations. Note that the map projection is not quite appropriate, so there is some inaccurate distortion, but it does allow qualitative comparisons.
northern hemisphere
Atlantic Ocean area
southern hemisphere
Personally, I do not see anything in terms of podal-antipodal "bulges" at any conceivable axial orientation, and the lack of a "north polar bulge" is obvious. The shape is distinctly non-radially symmetric, with the largest negative (near India) and largest postive (western Pacific Ocean) anomalies close to eachother, and no corresponding positive or negative on the opposite side of the Earth.
Since the initial claim and identification of some of these problems, Dave has suggested electromagnetic forces may also have been involved, although it is not at all clear how or why this would have the desired effect.
By contrast, there are conventional geological explanations for deviations in the geoid. They are mostly due to tectonic processes in the mantle and crust and/or isostatic readjustment from the last glaciation. See Fowler for details.
A citation for the "polar bulge" on Mars has not been provided (as of August 11, 1996), but, as noted in talk.origins, it is expected that on a smaller body with lower gravity and thicker crust like Mars, larger non-isostatic-equilibrium geological structures could be maintained than on Earth.
Andrew MacRae macrae@geo.ucalgary.ca
Africa
Indian Ocean
Pacific Ocean