earth cant be flat, that's dumb
Educate yourself
I educated myself, and here is what I found:
A transit with a powerful telescope, combined with observations 91 days apart, observations on the sun and on a horizon star, can show the 93 million mile distance to the sun (plus or minus 10%) by trigonometry. Nothing can refute this, since it can be repeated over and over to give a nearly accurate answer. Using a better telescope, and making similar measurements gives the 93 million miles rather accurately.
Now I'm happy.

Fuck off, using a sextant and plane trigonometry the distance to the Sun is measured at about 3,000 miles.