O level
Yup but it was pure volume and depth..the mental calcs gave many sleepless nights.
funnily I was at the cruising association this morning and was briefed on Captain Cook and the venus expedition to calculate the size of the solar system using Jupiter's moons...so Nick Hoye's astro did come in handy as I understood the maths (geometry).
ps tephigrams...use them occasionally for paragliding.