anchor

Took today off, somehow I struggled to take all my annual leave this year. Well, it was a glorious day to take off. Yesterday, at coffee we had an interesting conversation about an experiment to determine the size of the difference between the vertical and a plumb line.. A plumb line doesn't show the vertical because it is deflected by the surrounding mass distribution. So, in the case of Arthur's Seat, there is the mountain itself, but more significantly the absence of mass in the Firth of Forth and the extra mass of the Pentland Hills. So in 1856 they essentially repeated the experiment at Shiehallion in 1771 where the mean density of the Earth was found at Arthur's Seat. To do so they observed the transit of stars using an Airy Zenith Sector a telescope that looks vertically up. I was curious if I could find where the stations were. The one on the summit is pretty obvious, the other two on the meridian passing through the summit not so much. I think I will have to do some more research. I suspect the ring is for anchoring the trig point station. Anyway, I ended up going for a great walk around Holyrood Park and home via Tollcross to pick up some dinner.

For those interested you can get a pdf of the paper describing the results. More pictures of today's walk on flickr.