The Earth is a sphere
Not wrong, but our understanding became significantly more precise.
TaughtThe Earth is a sphere, like the globe on the teacher's desk.
NowIt is an oblate spheroid, bulging at the equator by about 21 kilometers, and lumpy on top of that. GRACE satellite gravity mapping from 2002 produced the true shape, the geoid, which is irregular enough that its exaggerated rendering is nicknamed the potato.
What actually happened
Rotation causes the bulge. The equatorial radius is about 6,378 kilometers and the polar radius about 6,357, a difference of roughly 21 kilometers, which Newton predicted from rotation before anyone measured it. This is why Chimborazo in Ecuador, not Everest, is the point on the surface farthest from Earth's center, despite being over 2,000 meters shorter, and why launch sites sit near the equator to borrow the extra rotational speed.
The deeper refinement is the geoid, the surface of equal gravitational potential that sea level would follow. Because mass is distributed unevenly, mountain roots, dense mantle, ice sheets, the geoid undulates by up to about 100 meters above or below the reference ellipsoid. The GRACE mission, launched in 2002, mapped it by flying twin satellites and measuring minute changes in the distance between them as gravity tugged them.
The honest caveat is that on the scale of a classroom globe, this is invisible. Relative to its size the Earth is smoother than a billiard ball, and a globe is an excellent model. But GRACE turned that abstraction into a working instrument: by watching gravity change month to month it now tracks ice loss in Greenland and Antarctica and groundwater depletion in aquifers, which is a shape measurement doing climate science.
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Sources
- NASA: GRACE and GRACE Follow-Onscience.nasa.gov
- NOAA National Geodetic Survey: What is the geoid?ngs.noaa.gov
Who was taught this
Still standard through 2005, so anyone who finished school between 1950 and 2002 learned the earlier version.