Earlier today we all had great fun over the video of QVC presenter Shawn Killinger and designer Isaac Mizrahi wondering whether the Moon is a planet or a star. Rachel Maddow made great fun of it. Now it looks like Isaac may well have been right all along - the Moon fits the definition of a planet.
The question how many moons Earth has comes up quite a lot on the BBC Two comedy panel show "QI" presented by Stephen Fry. In it contestants loose points if they give an incorrect answer. By coincidence it was asked again tonight.
Fry points out that the International Astronomy Union defined what a planet it in 2006, demoting Pluto to the status of a "dwarf planet" along with Ceres and other objects. The current definition of a planet is:
a celestial body that
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
(c) has cleared the neighbourhood around its orbit.
The last condition is important as it makes the difference between a planet and a dwarf planet which is defined as:
a spherical celestial body revolving about the sun, similar to a planet
So to qualify as a planet the Moon would have to
a) Orbit the Sun [check]
b) Have sufficient mass to remain almost round [check]
Which is when we come to the final condition. There are no other objects that are permanently in the vicinity of the Earth apart from the Moon. The question is which did the clearing and the answer seems to be both (all those craters on the Moon bear witness to the impact of other objects). So astronomers argue that the Earth and Moon are a binary planetary system. Wiki rather helpfully tells us the center of gravity:
Considering the Earth–Moon system as a binary planet, its centre of gravity is within the Earth, about 4,624 km from its centre or 72.6% of its radius. This centre of gravity remains in-line towards the Moon as the Earth completes its diurnal rotation. It is this mutual centre of gravity that defines the path of the Earth–Moon system in solar orbit. Consequently the Earth's centre veers inside and outside the orbital path during each synodic month as the Moon moves in the opposite direction.
In the other planets with much smaller moons relative to their size, it is the center of the planet that defines the path of its orbit. So remember, the Earth does not have a moon. What we call the Moon is our companion in our binary system!
The big question is will we have to change the lyrics to all those songs:
"When our companion planet hits your eye, like a big pizza pie, that's amore"
"Shine on harvest binary twin"