Attractive scenery

This story might be apocryphal; I haven’t been able to verify it, but it is certainly plausible.

Mount Egmont National Park forms almost a perfect circle around Mount Taranaki. Given its status, its bush remains intact, unlike the rest of Taranaki. You get a great view of this dark green circle with a mountain poking out of it on a flight from Hamilton to Christchurch, assuming, of course, the weather is good, which is a different matter. Now, I’m told, if you look carefully at a map, you’ll see it isn’t quite a circle – the boundary has a small kink in it.  (I’m not talking about those annoying extra bits that extend west over the Kaitake ranges that detract from the otherwise perfect cone-shape – it’s the circular bit I’m refering to.)

Now, on the Te Papa map, I had a good look at the park, and I couldn’t really see a kink in the circle. However, the story goes that when the park was surveyed, they forgot to take into account the gravitational pull of the mountain itself. That meant that the surveyer’s plumb line, which supposedly hangs vertically, was attracted very slightly to the mountain and the ‘vertical’ they worked with wasn’t quite true.  After working their way around the volcano, marking off the park boundary at a supposedly constant radius, they found that they didn’t quite end up at the same point as they started from. Hence a kink. 

Maybe someone can tell me if this story is actually true. But what I can do is give an estimate of how big the effect would be. Knowing Newton’s law of Gravitation (objects attract with a force proportional to the product of their masses and inversely proportional to the square of the distance between them) and the rough dimensions of the mountain (2.5 km high) I reckon the effect would have been about one part in ten thousand. In otherwords, a plumb line of one metre in length would hang a tenth of a millimetre ‘sideways’, off the vertical.  But having walked around a circle of radius six miles around the mountain (circumference of  about 60 km), this effect may have added up a bit.

Gravitational fluctuations are not always unhelpful – they are extensively used by geologists as one of the tools to determine just what lies beneath our feet – different rocks have different densities, which means they attract things slightly differently. These are small effects, and we wouldn’t notice them, but they are quite measurable with scientific equipment, and very useful.

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