How was the Earth to sun distance measured. How do we know how far from the Earth the Sun actually is.
There are a bunch of ways. The simplest is triangulation.
Consider a transit of Mercury or Venus, when the planet gets between the Earth and the Sun. If you observe the planet's path across the face Sun from two different places, measuring the chords across the Sun's disk and the timings, using no more than trigonometry, you can get an accurate distance to the Sun.
(It's worth noting that it is easy to measure the distance to, say, Mars in terms of the ratio of the distance to Mars compared with the Earth-Sun distance, than it is to measure the Earth-Sun distance in miles. Hence the Astronomical Unit. We accurately knew many astronomical distances in AU before we knew the length of the AU equally accurately.)
Here is the best answer I have seen to this:It works through the calculations using simple observations.
Cassini (the guy, not the spacecraft) calculated the distance to the Sun. See http://curious.astro.cornell.edu/about-us/41-our-solar-system/the-earth/orbit/87-how-do-you-measure-the-distance-between-earth-and-the-sun-intermediate
Edmond Halley realised the transit of Venus across the Sun could be used to calculate the distance. NASA has a page explaining that: https://pwg.gsfc.nasa.gov/stargaze/Svenus1.htm
Nowadays things are easier. We can easily find the distance to the Moon to centimeter accuracy by bouncing laser light from the reflectors left on the Moon during the Apollo program. We can use that to calculate the distance to the Sun.