It's written in the dwarf planets

On this day in 2005 the dwarf planet Eris was discovered by astronomers at Mount Palomar Observatory. It was the event that led to the International Astronomical Union (IAU) having to define the term 'planet' for the first time. 

When I was younger, we were told that there were nine planets: (in order from the Sun outwards) Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Then Eris turned up and NASA initially described it as the Solar System's tenth planet because it was significantly bigger than Pluto (27% bigger).  However that raised the issue of how to classify Ceres - a small planet that lies in the Asteroid Belt between Mars and Jupiter. And some of the moons of the larger gaseous planets are actually bigger than Ceres, Pluto and Eris (and even Mercury).

And so, with the prospect of more objects of similar size being discovered in the future, the IAU redefined what a planet is and Pluto, Eris and Ceres became 'dwarf planets'. 

And, sure enough, they've now been joined by a whole bunch more.

This got me wondering ... how does all of this fit with astrology?

Now, cards on the table time, I'll admit that I don't get astrology. I may hang out with witches and druids and I do have a very open mind. But I can't quite get my head around the idea that the movements of planets and the patterns of stars in the night sky affect our destinies. 

For a start, those patterns are only patterns in our heads.

When our ancestors looked up into the night sky they saw a series of bright stars that formed a kind of saucepan shape. In some countries it's known as the Plough. In others it's Ursa Major - the Great Bear. Depending where you are it's also known as the Camel, the Shark, the Skunk, the Sickle, the Bushel and the Canoe. The Chinese know them as  The Government or Tseih Sing. In Hinduism they are known as the Seven Sages.

But here's the same view from the Hubble Space Telescope. And, as you can see, there are a lot more stars in that part of the sky that we can't really see with the unaided eye. Do none of the others count as part of the constellation? Some of the smaller, less bright stars are closer to Earth than the seven bright stars. And the bright stars are not even vaguely close to each other.

The first bright star on the left is called Eta Ursa Majoris (formerly Alkaid) and it's 104 light years away from Earth. A light year is the distance travelled by light - travelling at 186,000 miles per second (or 671 million mph) - in one year. When we look at the star, we're seeing it as it was 104 years ago - it's taken the light that long to reach us.

The second star from the left is Mizar. It's 82.8 light years away. So it takes nearly 83 years for its light to reach us and 21 years for it to reach Eta Ursa Majoris. Dubhe, the star at the top right of the 'box' is 123 light years away and Merak, the star below it is 79.7 light years away. Not only are they too distant to have any kind of effect on us, they are also all so far from each other that they can't possibly have any influence on each other either.

So how can your birth sign have any bearing on the kind of person you are?

Now, I do appreciate that astrology is much more complex than that and you would need to have a full astrological chart drawn up that takes into account the date and time of  your birth, where you were born, and other factors. But the fact remains that predictions are based on the position of planets within the constellations. 

Different cultures have employed forms of astrology since at least the 2nd millennium BCE and Western astrology can trace its roots to 17th century BCE Mesopotamia. But when it was first formalised people only knew of a few celestial bodies - the Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn. Uranus wasn't recognised as a planet until 1781. Neptune wasn't found until 1846. And Pluto was found less than 100 years ago in 1930. Surely each new discovery would have upset the systems used by astrologers? If, as they claim, they can explain aspects of a person's personality and predict significant events in their lives based on the positions of celestial objects then surely all predictions will be inaccurate unless you are aware of every object? Were predictions that were made prior to Pluto's discovery wrong?

And now we have all of these new dwarf planets to factor in - if Pluto is significant enough to feature in horoscopes then so too must Eris, Sedna, Haumea and the rest. Any system that works on tracking the predictable movements of a set of objects must surely be thrown into disarray by the introduction of a whole new set of objects? And why do Mercury and Pluto have an influence on astrology but not the moons of the outer planets - some of which are bigger than Mercury and Pluto? And what about the objects we have yet to discover - do they not have an influence already?

I will just reiterate that I endeavour to keep an open-mind and am always happy to discuss any subject in a reasoned, polite and interested way. I'm not setting out to alienate astrology believers or to trigger non-believers (I have friends in both camps).

But I am genuinely stumped and I'd be interested to hear how astrologers make their systems fit with constantly changing modern astronomical knowledge. 

Meanwhile, happy birthday Eris! 

Glad you got to join the party.