This very parallax effect. We know how far the Sun is from the Earth, and the Earth has moved twice this distance from January 1 to July 1. This in turn allows us to extrapolate distance to some of the nearest stars by their apparent motion relative to the other, "fixed" stars.

https://en.wikipedia.org/wiki/Cosmic_distance_ladder

https://www.youtube.com/watch?v=7ne0GArfeMs

OK but then how do we know how far the sun is from the earth? Is it like "bounce a signal off it and count time of flight?"

(Another serious question.)

> The first rigorous and accurate scientific measurement of the Earth-Sun distance was made by Cassini in 1672 by parallax measurements of Mars. He and another astronomer observed Mars from two places simultaneously. A century later, a series of observations of transits of Venus provided an even better estimate.

http://curious.astro.cornell.edu/about-us/41-our-solar-syste...

We can't bounce signals off the sun, but we can do "interplanetary radar" bouncing signals off at least the nearby planets.

Knowing those distances, the distance to the sun becomes a trigonometry problem.

The video of Terence Tao I linked addresses that and many other questions about climbing the cosmic distance ladder.

I read this book, "Measuring The Cosmos", a few years ago, and it talks about the various ways we use to establish distances to such far objects. The book describes how we grew our "measuring tape" so to speak. We started with learning accurate distances to things closer, the distance from the Sun to Earth, an AU, being one of those. That's useful for local solar system distances, but the numbers become too big for human consumption when measuring further objects. Light years became the next rung on the ladder. Parsecs next, etc.

Now, we use certain types of stars like pulsars that have known brightness for the particular frequencies. Once we find a new pulsar that matches, we can compare how bright it is to others of the same type. If it is brighter, it is closer. If it is dimmer, it is further away. I'm way way oversimplifying it, but hopefully it gets the concept across.

https://www.amazon.com/Measuring-Cosmos-Scientists-Discovere...