Tuesday, 9 March 2010


Parallax is what allows you to view an object from two different positions and work out how far away it is. You use it unconsciously all the time, since you can see an object with both eyes and your brain automatically works out how far away it is. That's why people with one eye have much more difficulty estimating distances.

Now here's an interesting equivalent in time. A guy at Cornell University had some fossil corals which radiometric dating had shown to be 380 million years old, but he devised an alternative way of calculating their age.

It's based on two things I didn't know until I heard about the experiment. First, corals lay down their skeletons in such a way as to show not just annual rings, like trees, but also daily rings. Second, The spin of the earth has been gradually slowing down over time, largely as a result of the drag of the tides. It's a pretty minimal effect, two seconds in 100,000 years, but it does mean that the length of a day now is quite a bit bigger than it was when the fossil corals were alive in the Devonian period, and that means there are now fewer days in a year than there were then..

He calculated from the radiometric age of his fossils that there would have been 396 days in a year, and that each would have been 22 hours long. Then he counted the rings in his fossil corals, and behold! 400 days of 21.9 hours. That's pretty darned close!

And the reason I call this chrono-parallax is (I'm sure you're way ahead of me here!) that he used two different techniques to examine the same thing and work out how far (chronologically) it was away.

The sums are quite easy to do, though we have to rely on the accuracy of his ring-count.

(380,000,000 / 100,000) * 2 is the number of seconds the spin has slowed since then = 7,600.
7,600 / (60 * 60) is that in hours = 2.11.
So day length then was 21.89 hours.
And the number of days in the year was 365.25 * 24 / 21.89 = 400.46.

And that's why I like science! It's just so interesting and exciting! Don't understand the discrepancy between the calculated number of days and the measured number, though.


DJ Kirkby said...

You lost me when you got to the formula but I love science too! Do read any of Marcus Chown's stuff? I'm hosting him on my blog in the enxt month or so with a copy of his novel 'The Afterglow of Creation' to give away. x

Rob Clack said...

I haven't read any of his books, but I'll look out for them now you've mentioned him.