What do the circles mean?

[getbac]

DESTROYER PATTERNS:
1/2 search 850 (1000) yrds/meters
3/4 search 4150 (4300) yrds/meters
Full, 180d search 8000 (8200) yrds/meters
(=safe distance)
According to what I be been taught visual distance at sea is around 14 miles, about 30k yds/meters. Then you hit the curvature of the earth. Anything after that becomes "over the horizon" The big ring is 8000 yds/meters So not sure if its a visual or not. It stays the same in anyweather which would change if it was a visual cue (imo)! At first I thought it might be radar but that became apparently wrong when I got radar. My radar goes to 80K yds/meters/40 Nmiles, so am at a loss. Seems a destroyer and other surface radars would go much further than what I would be capapble of doing!
The second circle is 4150 yds/meters and thats 2 Nmiles. Sounds right for a listening device. and of course Sonar would be the smallest of operational attributes (again imo) thus the little 1/2 circle. And yea boy do I use the blank spot in the rear of the destroyers, helps to knock em off
Also have you noticed the subhunters have a much larger 180d ring? I do stay out of reach on the surface but feel comfortable when sumberged to be much closer with no problems!
Take a look at this if you are in to some real confusion lol.
https://en.wikipedia.org/wiki/Horizon

us land lubbers are hard to deal with huh?