the amount of time the sun takes to cross the meridian twice
No well of course it is you num-num spanner spoon dohnuty poo
Very close to 24 hours, or 86400 seconds.
The same way they define an Earth day. They calculate the time it takes the Sun to make one complete turn on its axis.
Imagine that you dug a hole and planted a pole pointing straight up. At precisely noon, the shadow of the pole would point due north (or due south, if you are south of the Sun's 'sub-point"). It...