it helps if its the equinox
January 4, 2019 7:53 AM Subscribe
The talk/workshop I give about computing the distance to the Moon[PDF] uses, it claims, nothing more than a pendulum and a stopwatch. And while it's sort of true that it uses nothing else, it's not really true, because it also uses the period of the Moon, and the size of the Earth. Now it might be possible to persuade you that it's OK to use the period of the Moon, since you can simply look out the window and measure that for yourself, but to use the size of the Earth seems a bit of a stretch. Surely there's no way to compute that from your back garden. Of course, we could claim that since the original definition of the metre was:Colin Wright calculates The Radius Of The Earth, from his back garden."One 10 millionth of the distance from the North Pole to the Equator on the longitude that runs through Paris"then perhaps we can quite reasonably claim that the Earth's circumference is, by definition, 40 million metres, but even so, perhaps that's not really fair, not really playing the game. But when I showed Distance to the Moon calculation to a friend of mine, it turned out he (a) was really interested, and (b) suggested a method to compute the radius of the Earth from your back garden.
This is not the Method of Eratosthenes
Radius Of The Earth, Part 2
Some nine and a bit weeks ago I posted about a method of calculating (or estimating) The Radius Of The Earth using a stopwatch and watching the Sun at sunrise (or sunset). When "Mike the Sundial" told me the idea I was just stunned at the simplicity and elegance. Colin Beveridge took up the challenge, and you can read his account of his method here:The Size Of The Earth, A Correction
When I read that I was interested to see just how different his take was from mine, but then I realised - it comes down to the phenomenon of "chunking". ColinB was working - as most people would - from first principles.
But I have a piece of knowledge that ColinB doesn't have, and I hadn't realised just how much it affected my approach.
Recently I've been discussing the details of measuring the size of the Earth by watching a shadow descending a wall at sunrise, or ascending at sunset. The sums are easy enough if you do it at either Equinox, and at the Equator, and you can read about it here:
The Radius Of The Earth
Radius Of The Earth Part Two
The challenge then is to do it somewhere other than at the Equator. After all, you can choose your time of year, so you can choose to do it at Equinox, but you can't necessarily travel to the Equator. So how can we do it at a non-Equator location, a non-zero latitude?<>>