“Earth is an oblate spheroid with an equatorial radius of about 6,378 kilometres, slightly flattened at the poles by its rotation.”

• NASA's Earth Fact Sheet lists an equatorial radius of 6,378.137 km and a polar radius of 6,356.752 km, giving a flattening of 0.00335 (≈1/298.26) — exactly the 'slightly flattened at the poles' shape described.
• The World Geodetic System 1984 (WGS 84), the reference ellipsoid used by GPS and global geodesy, defines Earth's semi-major (equatorial) axis as 6,378,137 m with inverse flattening 298.257223563, i.e. an oblate spheroid.
• The equatorial bulge is physically explained by rotation: centrifugal effect in a self-gravitating, deformable rotating body produces hydrostatic flattening, a result dating to Newton's Principia and confirmed by the 18th-century French geodetic arc expeditions to Lapland and Peru.
• Satellite geodesy (e.g., GRACE, LAGEOS, satellite laser ranging) measures the J2 gravitational coefficient (~1.0826×10⁻³), which directly quantifies the rotational oblateness, independently confirming the ~21 km equator-to-pole radius difference.
• Direct imagery and altimetry from spacecraft, plus everyday phenomena (ship hulls disappearing over the horizon, lunar eclipse shadow, differing star fields by latitude), confirm the overall spheroidal shape.
• Rounding 'about 6,378 km' matches the accepted 6,378.137 km to better than 0.003%.

• Strictly speaking, Earth's true figure is the geoid, an irregular equipotential surface that deviates from the best-fit ellipsoid by roughly −106 m to +85 m; 'oblate spheroid' is an excellent approximation, not an exact description.
• Earth is very slightly triaxial — the equator itself is not perfectly circular (equatorial radii differ by roughly 70–140 m), so a biaxial oblate spheroid is an idealization.
• Topography (Chimborazo's summit is the point farthest from Earth's centre; Everest is the highest above sea level) means the physical surface departs from any smooth ellipsoid by up to ~20 km peak-to-trough.
• Rotation is the dominant but not sole influence on the figure: mantle convection, glacial isostatic adjustment and ice-mass changes cause measurable secular variation in J2 (the flattening is not perfectly static).
• None of these caveats contradict the claim; they are refinements at the sub-kilometre or wording level.

99/100 — the lab’s own confidence in the score above, not a second reading of the claim.

NASA Goddard / NSSDC Planetary Fact Sheetgov · 98 · NASA Science — Earth Factsgov · 96 · NOAA National Geodetic Survey (GEOID models)gov · 95 · Wikipedia — World Geodetic System (WGS 84 parameters)other · 80 · Wikipedia — Earth radiusother · 78 · Wikipedia — Equatorial bulgeother · 75 · NASA JPL — GRACE/GRACE-FO gravity missiongov · 94

• NASA’s Earth Fact Sheet lists Earth’s equatorial radius as 6,378.137 km and polar radius as 6,356.752 km, showing the equator is about 21.4 km farther from the center than the poles—consistent with an oblate spheroid.
• NOAA explains that Earth is not a perfect sphere but is slightly flattened at the poles and bulges at the equator because of its rotation, matching the claim’s causal explanation.
• The claim’s phrase “about 6,378 kilometres” is appropriately rounded from the standard equatorial radius value of 6,378.137 km.

• Minor caveat: Earth is not a mathematically perfect oblate spheroid; its real shape is more precisely described as an irregular geoid/reference ellipsoid because of topography and variations in gravity. This does not materially undermine the claim as written.
• Minor caveat: the exact radius depends on the reference model used, but the cited equatorial radius of about 6,378 km is the standard accepted value.

99/100 — the lab’s own confidence in the score above, not a second reading of the claim.

NASA Goddard Space Flight Center / National Space Science Data Centergov · 98 · NOAA National Ocean Servicegov · 97

• Geodetic reference systems, notably the World Geodetic System (WGS 84) used by GPS, define Earth's equatorial radius as exactly 6,378.137 kilometers and its polar radius as 6,356.752 kilometers.
• Physical laws and satellite measurements confirm that Earth's rotation creates a centrifugal force that pushes mass outward at the equator, resulting in an equatorial bulge and a slight flattening at the poles, making the planet an oblate spheroid.

• From a highly pedantic standpoint, Earth is not a mathematically perfect oblate spheroid. Because of unequal mass distribution, mountains, and ocean trenches, its true and exact shape is slightly irregular and bumpy, technically referred to as a 'geoid'. However, an oblate spheroid is the standard, accurate geometric approximation used in science.

100/100 — the lab’s own confidence in the score above, not a second reading of the claim.

NASAgov · 100 · NOAAgov · 100

• Authoritative geodetic models define Earth as an oblate ellipsoid: the National Geodetic Survey states that Earth’s equatorial radius is larger than its polar radius.[https://geodesy.noaa.gov/research/geopotential-datums/geopotential-surface.shtml]
• NASA lists Earth’s equatorial radius as 6,378.137 km and polar radius as 6,356.752 km, confirming slight polar flattening and an equatorial radius of about 6,378 km.[https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html]
• The U.S. National Geospatial-Intelligence Agency’s WGS 84 reference system specifies a semi-major axis of 6,378,137 meters and an Earth flattening factor of 1/298.257223563.[https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84]
• NASA explains that Earth’s equatorial bulge results from its rotation, with the equatorial radius exceeding the polar radius.[https://pwg.gsfc.nasa.gov/stargaze/Srotfram1.htm]

• The statement is scientifically accurate as written, so there is no substantial contrary evidence. The only qualification is that an oblate spheroid is an idealized reference figure: Earth’s physical shape is more irregular and is more precisely represented by the geoid, which reflects variations in gravity and topography.[https://geodesy.noaa.gov/research/geopotential-datums/geopotential-surface.shtml]
• The phrase 'flattened at the poles by its rotation' identifies the principal physical cause, but Earth’s exact shape also reflects mass distribution, internal structure, tides, and other geophysical effects; these are refinements rather than contradictions.

99/100 — the lab’s own confidence in the score above, not a second reading of the claim.

NOAA National Geodetic Surveygov · 98 · NASA National Space Science Data Centergov · 99 · U.S. National Geospatial-Intelligence Agency, Office of Geomaticsgov · 99 · NASA Goddard Space Flight Centergov · 96

• Geodetic measurements (satellite laser ranging, GNSS/GPS, VLBI, and gravimetry) give a mean equatorial radius of 6378.137 km (WGS 84), matching the claimed ~6378 km.
• Polar radius is ~6356.752 km, so Earth is an oblate spheroid (flattening f ≈ 1/298.257) produced by rotational centrifugal force, as predicted by Newton and confirmed by modern observations.
• This reference ellipsoid is the standard first-order model used by NASA, NOAA/NGS, IERS, and mapping agencies worldwide.

• The physical surface is a geoid that departs from a perfect mathematical oblate spheroid by tens of meters (topography, ice, and mass anomalies).
• The equator itself has a very small ellipticity, so the figure is not a pure spheroid of revolution.

95/100 — the lab’s own confidence in the score above, not a second reading of the claim.

NASA Goddard Space Flight Centergov · 98 · Wikipediaother · 80 · Encyclopædia Britannicaother · 90