17 January 2022

Calculating the circumference of the earth using timing analysis of the blast waves from the Tonga eruption arriving in Coventry, UK.

Whilst watching a youtube video containing satellite imagery of the Tonga eruption, I wondered if our weather station had detected the blast wave. It had, with a transient occurring at around 18:44UTC.  I mentioned this to EI7IG. He confirmed seeing the same thing, but also pointed out that there was a 2nd transient at around 01:44UTC the next day.

I did some quick spreadsheeting in Libre Offic Calc and managed to calculate the circumference of the earth to be 39427 km, within 1.53% of the average nominal value of 40041 km.

I ended up using a value for the speed of sound of 303.1 m/s, which is good for 30,000ft (9144m) aMSL. I thought that if airliners like to fly at that sort of altitude, then air might like to do the same thing.

You can see the data from my weather station on wunderground.

Updated, as I initially based my calcs on a t2 of 02:00 UTC on the 16th Jan, but it really does look more like 01:44 UTC as the beginning of the transient. I also changed the title from "TDOA analysis" to "timing analysis" as TDOA implies multilateration to determine the location of the source, and we already know that.