[A post for a friend of a friend who may be able to help, but obviously anyone can read and comment if you have any thoughts. Yes, comments are enabled for once :-)]
Executive Summary
Searching for items on my home town of Williamstown , Victoria on archive.org, I got an interesting hit. An 1863 scientific paper that gives a very precise location of the “Government Observatory, Williamstown, Victoria”
He’s an Astronomer and I know some Astronomy, so was able to convert his Longitude – in Hours etc – to Degrees (see below for Optional quick maths). The values in decimal degrees are:
Longitude: 144.91375 degrees E
Latitude: 37.8685 degrees S
I plugged these into Google Earth and – as I’d assumed – it didn’t work as I’d hoped. It put the Observatory in the sea, not far from the Timeball Tower:
Note: I should point out it’s not difficult to find the actual historical locations of the Observatories (there were at least two in Williamstown). I’m interested in how to convert the 1863 Lat and Long to the ‘correct’ modern values.
I said I’d assumed it wouldn’t work as I knew enough about maps – from my hiking (mis)adventures – to know the 1863 document probably used a different Datum and Projection.
What I’d Like To Find Out
The questions are:
- What Datum and Projection would a British scientific publication have used in 1863?
- And so, what transformation(s) do I need to apply to convert to WGS84 etc? I’m not afraid of maths; so not put off by tan() or even arctan() 🙂 And I can use Excel quite well.
Optional: Longitude in Hours? The Maths.
I assumed that because he’s an Astronomer he’s giving the Longitude as the equivalent to a celestial/space-based Right Ascension (RA). Instead of giving an angle in degrees, RA is given in what amounts to time (Hours, Minutes, Seconds).
The reason is that the Earth rotates and returns to the same point in 24 hours. In that time it does a complete circle; 360 degrees. So in one hour it rotates (360/24) = 15 degrees.
To convert RA (hours) to Longitude (degrees) we simply multiply by 15.
Note: for Astronomy tasks, using RA (in hours) is quite useful. Even as an alternate clock (!), which we’ll definitely return to soon on this Blog.
Quick estimate: his RA is just under 9 hours and 40 minutes. That’s 9 and 2/3 hours, so 9.6666 hours, you’d hopefully agree.
9.6666 x 15 = 144.999 degrees.
My balcony here in Williamstown has Longitude 144.8922 degrees E.
QED. In a back-of-the-envelope #ballpark sense 🙂