I want to know how big the earth is. I have heard of double sun set experiments, but it appears that you need to know you latitude, which I do not. I want to do this without using any outside measurements or values. What I can do is measure lengths and times. I can't be in more than one place at once. I can be near a large body of water. I also know the basics of astronomy (the earth is a round thing going around another giant round thing.) and trignometry are correct. Any formulas need to be explained though, no magic internet formulas.
(If you want a reason for why I might want to do this, maybe I think the airline industry has bribed all scientists everywhere to say that transcontinental flights are longer than they actually are to increase fares, but mathematicians have remained uncorrupted/I went through and checked all the proofs and believe the axioms.)