More notes at http://tanguay.info/learntracker
C O U R S E 
Moons of Our Solar System
David Rothery, The Open University
C O U R S E   L E C T U R E 
What is a Moon?
Notes taken on February 16, 2015 by Edward Tanguay
what is a moon?
a planet is an object that orbits a star
a moon is an object that orbits a planet
some moons are shaped like asteroids
can be a quite small
we have not defined the cutoff point where a moon is a moon and not just debris
our solar systems number of moons has been steadily increasing
in the last 20 years we have found many new moons around Jupiter and Saturn
the number changes on an annual basis
Earth has one moon
its origin is based on the giant impact hypothesis
an object half as wide as the earth slammed into the earth very early on
this collision resulted in both objects being to a large extent vaporized and liquified
the core of this object combined with earth's core
the rest of the material from that object and from the outer part of the earth splattered into space into a big disk of debris that eventually consolidated to become the moon
Mars has two moons
a possibility that these moons are captured asteroids
Venus and Mercury don't have any moons
the gas giants, the large, bloated planets that orbit our Sun
their gravity and wide orbits have enabled them to pick up many moons
some of the moons formed in orbit around the planets
others are captured asteroids and comets