Explained: Why the Earth does not look oblate in photos from space

creatonez

New Member
Because the rotation of the Earth forces mass near the equator against gravity, the Earth is a shape known as an oblate spheroid, a shape mathematically equivalent to taking a sphere and scaling it down on one axis.



Many things in science and engineering have to take into account the true shape of the Earth, called the reference ellipsoid. Most notable is GPS, which uses the WGS standard for determining correct coordinates on a slightly squashed Earth. Modern land surveying and computer GIS systems would be crushed if assumptions about the oblate spheroid shape of the Earth were wrong. (They would be even more crushed if it turns out the Earth is flat.)

Anyways, I saw a comment on a flat earth debunking video asking why you can't see the oblateness of the earth in photos of the earth. I was curious so I grabbed an image from the Himawari 8 satellite, rotated the right hemisphere 90 degrees and transposed it on the bottom of the image such that it matches up with the equator.

The oblateness is a very small effect, but can be seen clearly in these high resolution images:

pop.PNG
Here is a less compressed version of the above image https://cloudflare-ipfs.com/ipfs/QmegMQSwmDS6X7aHXH3r832uS86zFbTjy1ABYQfd3BjmwG (12 MiB instead of 5 MiB)
It seems Metabunk automatically cached and recompressed my extremely large image

QmR14w7y1CdnnWJFdG7eVv1xev4we313VQE2RajwQZLyzH.png

If you want to do this yourself, go to https://seg-web.nict.go.jp/wsdb_osndisk/shareDirDownload/bDw2maKV?lang=en and look for full-disk images with a timestamp close to 11:32 JST, which is when the sun is directly between the geostationary satellite (positioned at 140.7° E) and the Earth, so the shown hemisphere is completely lit up. The images are centered exactly on the earth, so you can make the cut at 5500 pixels on either axis. Interesting aside... because of the pitch black pixels during the night, the PNG-compressed image size fluctuates wildly over a 24 hr period.

The most oblate planet in the solar system is Saturn, with a polar diameter of around 90.203% the equatorial diameter. We can do the same thing to photographs of Saturn and see a much more pronounced oblateness.

 
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Darold

New Member
I read about the shape of the earth many years ago and what you address here in satellite images makes it more awesome for me. What had attracted my attention, way back when, was an article that stated with the title (or something similar): "Mount Everest is not the highest mountain on Earth". Of course, the title was to grab the eyes of the ignorant (heh heh). The article went on to explain that Mount Everest was the highest from sea level, but not from the center of the earth. Then explained in more detail about the earth's shape. So, a matter of perspective, and math, and... anyhow, thanks for the post! Not my original reading piece, but an interesting tag-in link (I hope): Highest point on earth
 

porkpie

New Member
I read about the shape of the earth many years ago and what you address here in satellite images makes it more awesome for me. What had attracted my attention, way back when, was an article that stated with the title (or something similar): "Mount Everest is not the highest mountain on Earth". Of course, the title was to grab the eyes of the ignorant (heh heh). The article went on to explain that Mount Everest was the highest from sea level, but not from the center of the earth. Then explained in more detail about the earth's shape. So, a matter of perspective, and math, and... anyhow, thanks for the post! Not my original reading piece, but an interesting tag-in link (I hope): Highest point on earth
Yeah it all depends what you mean by highest mountain:
Mauna Kea, one of six volcanoes that have formed the island of Hawaii, is the tallest mountain on Earth at 9,966 meters (32,696 feet, 6.2 miles). This is 1,116 meters (3,661 feet, 0.7 miles) taller than Mount Everest and roughly the same height in the atmosphere where commercial airplanes fly. With 4,205 meters (13,796 feet, 2.6 miles) above sea level, more than half of Mauna Kea’s height falls below the surface of the ocean, with its base reaching 5,761 meters (18,900 feet, 3.6 miles) deep.
https://www.nationalgeographic.org/media/mauna-kea/
 

JMartJr

Senior Member
This is neat as all get out .. one correction, you want the moment when the satellite is directly between the Sun and the Earth -- if the Sun tries to get between the satellite and the Earth, that would cause problems... ;)
 

Amber Robot

Active Member
This is neat as all get out .. one correction, you want the moment when the satellite is directly between the Sun and the Earth -- if the Sun tries to get between the satellite and the Earth, that would cause problems... ;)
Another way to say this is that the sub-satellite point and the sub-solar points are coincident.
 

Ravi

Active Member
Interesting to note that there was an ESA mission a few years back, called GOCE, which measured Earth's geoid. Indeed the best way to create an accurate topographical reference for everything on Earth.

The mission​

The geoid is also used as a reference surface from which to map all topographical features on the planet. An improved knowledge of gravity anomalies will contribute to a better understanding of Earth's interior, such as the physics and dynamics associated with volcanism and earthquakes and also further our knowledge of land uplift due to post-glacial rebound.
source: https://www.esa.int/Enabling_Support/Operations/GOCE
 
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