I suspect 5-10 miles would get er done.
South Carolina Ping
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Maybe somebody can tell me about all the oil spills that have fouled Galveston beaches...
I have been to the beach at Grand Isle Louisiana lots of times, deep see fishing, etc., and have never seen an oil rig. Apparently the Louisiana Gulf Coast is lousy with them, but I have yet to see one. Before anyone votes, protests, or writes an article against offshore drilling, I suggest they take a trip to the Louisiana Gulf Coast, get some gulf shrimp, go deep see fishing, whatever, and see what there is to see.
Depends on observer height, etc. Here is a good link.
http://mintaka.sdsu.edu/GF/explain/atmos_refr/horizon.html
I would also enjoy using them as citing marks while swimming.