Every now and again I ask my sisters to
come up with astronomy questions that they never thought to ask anyone before
and, out of a list of weird and wacky questions, there is always at least one
that sparks something and leaves me thinking “you know what, I’m not sure.”
The beauty of the world today is that we
have access to all the data we could want to answer these questions it is just
the case of hunting out the right sources and filtering out the credible
information. So I went out in search of an answer to the beautiful and simple
question of,
What is a moon?
to get a more in depth definition or series
of parameters other than the logical definition of a natural satellite. Here is
what I found out.
A moon is a natural satellite orbiting
around a planet that is itself presumably orbiting a star.
That is it. Even the IAU (International
Astronomical Union) doesn’t have an official definition other than natural
satellite. That said they didn’t have one for a planet until 2006, and that on
a technicality excludes all exoplanets, so maybe it is best not to ask them to
define it just yet, though they do have sole control over the naming of all
natural satellites and planets.
Unsurprisingly the first moon discovered
was our own, though it was originally thought to be a planet; it sets the
generic image in our minds for what a moon should look like. A large spherical
rocky body smaller than the planet it is orbiting. And while, yes a moon must
be smaller than the object it is orbiting, it does not need to be significantly
large nor spherical, and though most are primarily rocky, unlike our moon, many
others exhibit geological activity and even maintain dynamic atmospheres. One
moon of Saturn, Rhea, is even thought to have a moon of its own.
In our solar system there is a grand total
of 171 natural satellites orbiting the 8 main planetary bodies, and that number
has almost doubled since 2003 alone.
Mercury – 0
Venus – 0
Earth – 1
Mars – 2
Jupiter – 66
Saturn – 62
Uranus – 27
Neptune – 13
The four outer solar system planets
(Jupiter, Saturn, Uranus, and Neptune) all also have ring systems. These are
set apart from moons as they are not a distinct whole object and are instead
comprised of dust and ices forming a continuous band around a planetary body.
![]() |
Mars and its two moons; Phobos (left) and Deimos (right) |
Perhaps, like with the planetary
definition, any definitive distinction between these object and their rounder
neighbors will result in a reduction of the number of classified moons in the
solar system. However, until then there will continue to be confusion and
disagreements between the exact definitions until it can be clearly and
concisely defined, most likely by the IAU.
What’s next?
For some fantastic information of the soar system
and all of its natural satellites check out
The discovery of the Galilean moons in 1610
is brilliantly explained in Galileo’s own word at