Is the Earth Spherical?

No, I’m not going to claim it is flat, but anyone who looks into this subject will easily find multiple references to the earth being an oblate spheroid.

As to what this means, the distance between the north and south pole is slightly less than the distance across the equator, making it an oblate spheroid or in other words, it has a slight bulge around the equator.

The reason for this is that the spin of the earth and a few billion years have given it the bulge. Depending on which reference source you use, this is given as a figure between about 20 and 40km.

This isn’t easy to measure as the earth isn’t perfectly smooth either with mountains like Everest at 8,850m high and even parts of the earth are below sea level like the Dead Sea which is about 413m below sea level (yes there are deeper parts but those below the sea don’t count as they are covered by the sea).

And whilst Everest is the highest point above sea level, it isn’t the highest point as measured from the centre of the Earth due to the bulge around the equator. Instead that point is the summit of Chimborazo in Equador at 6,263m above sea level but 2,163m further from the centre of the earth than Everest.

Now the next question is, can you see this difference on images of the earth?

To test it, I took a GK-2A image:

GK-2A Infra Red Full Disk

The image is 2200 pixels by 2200 pixels and if the earth was perfectly spherical then there would be the same size total gap between the edges on the left + right side as there are for the gaps to the top + bottom sides.

So to test it, I zoomed into each centre mid-point and started counting pixels by colouring alternate pixels in red to make it easier to count.

Left side – 13 Pixels
Right Side – 12 pixels

So left to right it is pretty well centred in the image with just 25 pixels total gap between the edges. So doing the same thing for the top and bottom edges:

Top Side – 17 Pixels
Bottom Side – 16 Pixels

So the top to bottom gap total is 33 pixels as opposed to the 25 pixels left to right, showing that the picture shows the earth is indeed an oblate spheroid.

As to how noticeable it is in the image, have a close look back at the full disk image and spot the red marks. They are there, just, although to get the image onto this page I had to scale it down to just 800 x 800 pixels but they are still visible when you zoom in really close.

Now for anyone with a good understanding of optics, I did make the assumption that the lenses used in the camera have been manufactured to be symmetrical, so there is no systematic error in distances measured as a function of the location on the image. Plus of course similar assumptions being made for the sensor used to capture the image along with the associated image processing.

But for all that, the difference is about 0.36%, which agrees well with the different geodesic models putting this figure about 0.33%, so our rather simple measurement using the image is actually in good agreement with significantly more complex measurements.

One Reply to “Is the Earth Spherical?”

  1. Hi Mike, very well explained through imagery, though we knew the cause but haven’t seen images or variation in images to depict the underlying concept of the centrifugal force caused by the rotation around its north-south axis in turn deforming the sphere into an oblate ellipsoid.
    Thanks for sharing.

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