Body diameters to scale. Positions and surface artwork are illustrative; Saturn’s rings are excluded.
A change of perspective
3.4977× the diameter. See how the two worlds measure up.
Diameter is a distance across an object. Volume grows with the cube of that distance.
Choose a size to bring this comparison down to Earth.
Io: Derived by doubling JPL’s volume-equivalent mean radius of 1,821.49 km: diameter = 3,642.98 km. The listed radius sigma of 0.50 km becomes 1.00 km for diameter. This describes an equal-volume sphere, rather than an equatorial, polar or maximum surface-to-surface dimension.
Earth: Twice the source mean radius; body diameter only, excluding rings.
Derived by doubling JPL’s volume-equivalent mean radius of 1,821.49 km: diameter = 3,642.98 km. The listed radius sigma of 0.50 km becomes 1.00 km for diameter. This describes an equal-volume sphere, rather than an equatorial, polar or maximum surface-to-surface dimension.
Jupiter’s moon Io has a mean diameter of approximately 3,643 km. Earth’s Moon measures 3,474.8 km using the same definition. Dividing their diameters gives about 1.048, so Io is roughly 5% wider. These values are derived from the mean radii in JPL’s satellite parameter table.
For a scale model with the Moon 10 cm across, Io would be about 10.48 cm across. This compares linear size; it does not describe mass or orbital distance.
JPL defines mean radius through an equal-volume sphere and credits its Io radius to Archinal and colleagues’ 2018 report. Doubling that radius gives the diameter used here. The propagated diameter sigma is 1 km; the decimal places preserve the calculation, rather than imply centimetre-level accuracy.
Equatorial and polar measurements describe particular directions. A volume-equivalent diameter provides one consistent size for comparison, including when an object’s shape departs from a sphere. See the measurement methodology. The artwork is illustrative.