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Earth's orbit is about 99% circular. How circular is Earth around the equator? I know it bulges around the equator and is a spheroid. I also know it is not smooth and has oceans and mountains and all that.

Is there a similar measure of eccentricity to describe how circular it is around the equator?

Is it stretched into an ellipse by the sun/moon, and if so by how much?

Just for comparison, does the Earth's equator deviate from a circle more than the Earths' orbit around the Sun deviates from a circle?

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    $\begingroup$ Did you do any prior research, like the obvious one... wikipedia? $\endgroup$ – AtmosphericPrisonEscape Feb 12 '19 at 20:18
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    $\begingroup$ Oops, my previous link is wrong for the Figure of the Earth, here it is. Also, another interesting article: the Equatorial bulge $\endgroup$ – FSimardGIS Feb 12 '19 at 23:05
  • $\begingroup$ @FSimardGIS I think that eccentricity is from the difference in the equatorial axis and the polar axis. I did find more info about what I was wondering though, in the Figure of the Earth on wikipedia under model and other shapes, it suggests a triaxial ellipsoid or possibly an osculating sphere. Apparently there is some controversy on this subject. $\endgroup$ – jreese Feb 13 '19 at 13:45
  • $\begingroup$ A very simple answer seems to be "out by 100 meters or so here and there". $\endgroup$ – Fattie Feb 14 '19 at 17:45
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My answer won't be complete, from lack of time and resources, but I still wanted to share some interesting aspects here that could be helpful.

The difficulty in aswering this question revolves around the complex and irregular shapes involved here. Also, finding a "best-fit" ellipse for comparison is not as easy as it seems, because it depends on what and how you want to model. The actual shape of the equator is rather complex and irregular. Generally speaking, it is pretty much circular, but indeed topography and the geoid complicate matters. The Earth's movement in the solar system is not a perfect ellipse either, because of the gravitational interactions with other celestial bodies. Let's review the different irregularities involved here.

The Earth is usually modeled as an ellipsoid of revolution (oblate spheroid), and the equator as a circle. A good example of that is the WGS 1984 geodetic reference system used by the Global Satellite Navigation System. Of course, the equator is not a perfect circle, it has irregularities mainly because of topography, and even sea level itself is a little irregular too. We can approximate sea level with a geoid, for example, here is a map of the EGM2008, a geoid used with WGS 1984 to transform ellipsoidal heights to geoid heights:

enter image description here

Basically, this map shows the height of the geoid (the idealized sea level without the effects of tides and currents) with respect to the WGS84 reference ellipsoid of revolution (semi-major axis 6,378,137 m, semi-minor axis 6,356,752.314 m). The differences are mostly less than 100 meters, and are caused by the irregular distribution of mass inside the Earth itself.

Now, some studies show that the Earth's shape could be slightly better modeled by a triaxial ellipsoid, and one could try to model the equator as an ellipse, and the Earth as a triaxial ellipsoid, however, even with a best-fit triaxial, we would still need geoid corrections for the irregular mean sea level, let alone topography, and geodetic computations would be more complex on a triaxial. Other funny models and names have come up over time, like a pear-shaped (because of a slight bulge in mid southern latitudes) model as a best-fit shape. But if you look at the map above, good luck visually finding the pear shape in there, or other mathematically modelizable aspects of these bumps. We are talking about very subtle differences here, that do not necessarily need to be taken into account for most purposes when describing the general shape of the Earth.

So depending on how you consider the shape of the equator (i.e. by topography and ocean floor, or sea level) you will arrive at a shape that is mostly circular with irregular bumps along the way. There is no authoritative agreement that I know of about an eccentricity of the equator. For instance, This study proposes a flattening of about 70 meters for the equator's ellipse, This article on Encyclopaedia Britannica proposes 80 meters.

For the Earth's orbit, for the sake of this comparison, we can use an best-fit ellipse of 149.598 million km by 149.577 million km. Of course, that is only a idealized ellipse, the real movement of the Earth in the Solar System is more complex.

Finally, say we scale down and superimpose the Earth's orbit's ellipse on the equator to compare. The eccentricity of the Earth's orbit is 0.0167, the semi-major axis is 149.598 million km and semi-minor axis is 149.577 million km. Scaled down by a factor of 23,455 to the equator's size, this corresponds to a difference of about 900 meters in both orbit axes. So I think we can agree that a best-fit ellipse of sea level along the equator is more circular than the Earth's orbit.

However, topography-wise, there are bumps up to over 4,000 meters in the Andes, and the sea floor reaches 5000 meters below sea level in several places. So the "topographic surface" equator would, for one thing, definitely appear more bumpy than the Earth's orbit. Unfortunately, I haven't found a example or study showing what a best-fit ellipse of the equator (including topography and bathymetry) could look like, mainly because we tend to approximate sea level, not topography itself, but with more time, tools and data, it might be possible to work out the answer with a "topographic best-fit" ellipse.

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