Io vs The Moon
Io (3,643 km) is 1.05× the size of the Moon (3,475 km).
The Moon is 95% of the height of Io. The gap between them is 168 km (552,493 ft).
The comparison in numbers
| Thing | Size | In feet | Measured as |
|---|---|---|---|
| Io | 3,643 km | 11,952,756 ft | diameter |
| The Moon | 3,475 km | 11,400,262 ft | diameter |
What the scale shows
Io (3,643 km) is 1.05× the size of the Moon (3,475 km). The gap between them is 168 km (552,493 ft). Both are planets & moons, so the figures describe the same kind of thing.
Draw both on one baseline and the smaller one fills 95% of the frame while the larger one runs to the top. The two are close enough that the difference is easy to see side by side.
Read the rest — 8 more sections
What each figure means
The figure for Io is its diameter (mean diameter). The figure for the Moon is its diameter (mean diameter).
Crossing them
At a jet's 900 km/h, Io's 3,643 km would take 4 hours and the Moon's 3,475 km would take 3.9 hours.
In people: Io is 2,143,059 adults of 1.7 m tall, the Moon is 2,044,000. By size Io is 12th of the 16 planets & moons on this site and the Moon is 13th, so 1 place separates them in the ranking.
How to read these two figures
Before trusting any comparison, check what each number measures. Io is given as a diameter, mean diameter. The Moon is given as a diameter, mean diameter. Both are diameter figures, so putting them on one baseline is a like-for-like comparison, although the exact individual or convention still matters.
Worlds are listed by diameter. Gas giants are not round, so equatorial diameter is used for them and mean diameter for the rest. All figures are rounded from published mission data.
Where the pair sits among all the sizes here
Across all 86 things on this site, Io is number 12 from the top and the Moon is number 13. No other item falls between them, so in this collection they are direct neighbours.
Because the collection spans 13 mm up to 1,392,700 km, a ratio of 107,130,769,231× from smallest to largest, the position of a pair says a lot: both sit in the upper third of the list.
What the length ratio hides
A length ratio of 1.05× is easy to picture, but area and volume grow faster than length. If two objects had exactly the same shape, Io would have about 1.1× the surface or footprint of the Moon, and about 1.15× the volume. These two are not the same shape, so treat the figures as an upper bound for an imaginary scaled copy, not as a measurement of either one.
The drawing above shows length only. The Moon is drawn 95% as tall as Io, but the equivalent share of footprint would be only 91%, which is why the smaller one looks even slighter than the numbers alone suggest.
Time to cross each one
Light travels at 299,792,458 m/s, so it crosses Io (3,643 km) in 12 milliseconds and the Moon (3,475 km) in 12 milliseconds. A jet at 900 km/h would take 4 hours for the larger and 3.9 hours for the smaller.
These times assume a straight line and constant speed, which no real crossing has, but they turn a number in metres into something the body understands. At any fixed speed the travel times keep the same 1.05× ratio as the sizes.
The same figures in other units
Io measures 3,643 km, which is 11,952,756 ft or 2,263.8 miles. The Moon measures 3,475 km, which is 11,400,262 ft or 2,159.14 miles.
The difference between them is 168 km (552,493 ft). In kilometres the smaller one is 3,474.8 against 3,643.2, so the 1.05× ratio holds in every unit; only the numbers on the ruler change.
Questions people ask about this pair
Which is bigger, Io or the Moon?
Io is bigger. At 3,643 km it is 1.05× the size of the Moon, which is 3,475 km.
How many times does the Moon fit along Io?
Less than twice: the Moon is 95% of the size of Io, so one fits with 168 km to spare.
Are the two figures measured the same way?
Yes in kind: both are diameter figures. Io is mean diameter; the Moon is mean diameter.
How does the gap look in people?
Taking an adult of 1.7 m, Io is about 2,143,059 people tall and the Moon is about 2,044,000. The gap of 168 km is 99,059 people.
Where do the numbers come from?
Each item has its own page that names the measurement and links to the Wikipedia article it was taken from. Some figures are rounded, and animals vary from one individual to the next, so treat them as representative values rather than records for every case.
How the sizes are taken. Each figure says what it measures — height, length, diameter or elevation — and for which individual or convention. Everything is drawn standing upright on one baseline so a single scale holds, including things that are long rather than tall. Values come from the linked Wikipedia article for each item.