Concept Drift

When building ontologies its often convenient to assume that concepts and their instances don't change over time. Obviosly this is false. If the time is long enough, everything except maybe a few very abstract concepts changes - nevertheless this is just a great example for such a change: In a few billion years the moon will stop beeing a moon and become a planet. Try modelling that in OWL ;-)

The new definition [for planet], proposed this week by the International Astronomical Union (IAU), basically says every round object orbiting the Sun is a planet, unless it orbits another planet. But there is a big caveat: If the center of gravity, called the barycenter, is outside the larger object, then the smaller object is a planet. That wording elevates Pluto's moon Charon to planethood, an idea some astronomers have criticized.
But here's the thing. Earth's Moon was born in a catastrophic collision more than 4 billion years ago. It started out very close to the planet but has been moving away ever since. It's currently drifting away about 1.5 inches (3.74 centimeters) every year.
For now, the system's barycenter is inside Earth. But that will change.
"If the Earth and Moon do survive, then the barycenter will eventually move outside the Earth as the Moon recedes," Laughlin told SPACE.com. "At that point the Moon would be promoted to planetary status." [What would we call it?]

Rest of the article here.