Sunday, 18 January 2015

Is the Moon a planet or a star?

It's rare that shopping channels grapple with complex problems in astronomy - but miracles do happen.  Here's the contributors to QVC discussing whether the moon is a planet.  An important consideration is apparently that "things live on it".

I'm posting this for your amusement, but you could make a serious discussion out of it.  After all, what does it mean to be a planet?  The word comes from the Ancient Greek meaning wanderer, because astronomers spotted that some heavenly bodies moved against the background of fixed stars during the year.  In that sense, the Sun was a planet too.

Ancient and medieval astronomers could see obvious differences between the Moon and the Sun and the other planets in the Solar System, but there were lively discussions about precisely what they were, and how we could know.  They also calculated the relative distances of the Sun and Moon with impressive accuracy, simply by observing eclipses!

In modern terms, we say the Moon is a moon because it orbits a planet, but it might be worth noting that the situation is more complex: in fact, because the Earth and the Moon exert force on each other, they both orbit a point between their centres (though, since the Earth is much larger, the centre of gravity of the Earth-Moon couple is still within the Earth).

And remember, it was a change to the definition of a planet by the International Astronomical Union in 2006 that caused Pluto to fall out of that category (the IAU has a good explanation of their reasoning here).

So a silly discussion can raise some serious questions.  One thing we can be pretty sure about, though: although the Moon has definitely had life on it in the past (12 Apollo astronauts), there's nothing there now.

Thanks to Christopher Graney and the HASTRO contributors for posting this video and discussing it so interestingly.

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