Caption: "The equatorial (a), polar (b), and mean Earth radii as defined in the 1984 World Geodetic System revision." (Somewhat edited.) These Earth radii roughly specify the figure of the Earth.

The image shows the Western Hemisphere and Americas---somewhat stylized.

Features:

1. The Earth is actually somewhat oblate: i.e., it is approximately an oblate spheroid (see Wikipedia: Spheroid; Wikipedia: Reference ellipsoid).

2. Note:
   1. Earth Equatorial Radius R_eq_⊕ = 6378.1370 km.
   2. Earth polar radius R_po_⊕= 6356.7523 km.
   3. R_eq_⊕ - R_po_⊕ = 6378.1370 km - 6356.7523 km = 21.3847 km.
   4. Earth mean radius R_me_⊕ = 6371.0088 km by the Earth conventional mean radius formula (2a+b)/3 (see image also), whose rationale is hard to find.

3. The Earth radii quoted in the last item are for the modern reference ellipsoid for the Earth---which is a more precise specification of the figure of the Earth than just the Earth radii. The reference ellipsoid is a simple geometric shape that approximates the shape of the Earth to high accuracy: to be exact the shape of an idealized sea level that is continued through the continents. So much for geodesy.

4. The Earth's atmosphere is probably somewhat oblate too---but that's another story.

5. The oblateness of the Earth is due to the centrifugal force caused by the Earth's rotation. The centrifugal force just causes a rearrangement of the bulk Earth so that the Earth's self-gravity, the Earth's pressure force, and the centrifugal force itself balance to create hydrostatic equilibrium everywhere.

6. The combination of centrifugal force and to some degree the variation in distance from surface to center due to the oblateness causes the effective Earth's gravitational field g (i.e., true Earth's gravitational g_true plus centrifugal force at the Earth's surface) to decrease from the Earth's poles to the equator: 9.832 N/kg to 9.780 N/kg or a decrease of ∼ 0.5 % (see Wikipedia: Gravity of Earth: Latitude). The variation in effective Earth's gravitational field is below human perception, but quite easily measurable by gravimeters.

   Note Earth's gravitational field g_average = 9.80665 N/kg (which is defined in some way) and Earth's gravitational field g_fiducial = 9.8 N/kg (as used by many including yours truly).

7. The Earth's gravitational field g causes the acceleration due to gravity near the Earth's surface a_⊕, and in fact a_⊕ = g exactly by Newton's laws of motion. Note also that the unit N//kg is algebraically equal to the unit m/s**2, but their meaning is different (i.e., they have different quantity dimensions). Note moveover that the actual acceleration in free fall (in the loose everyday sense of falling in air) is affected by air drag.

   Note in a strict sense, free fall means a object is moving only under the force of gravity, except for smallish non-gravitational perturbations.

8. There other causes for the causes for small variations in the Earth's gravitational field: altitude, depth below sea level, and local topography and geology (see Wikipedia: Gravity of Earth: Variation in magnitude).

9. The Earth Equatorial Radius R_eq_⊕ = 6378.1370 km is the best natural unit distance for astronomy for the Earth-Moon system and near-Earth astronomical objects (natural or artificial) since it is the Earth radius that is naturally used in parallax measurements.

   Near-Earth astronomical objects are principally the Moon, Earth-orbiting artificial satellites, space debris, and near Earth objects (NEOs) (asteroids and comets) on close flybys of Earth.