How do they?

Question

How do they know that the sun is 93 million miles away? I read that the next star is 25 trillion miles away. How do they know? Is this one of the things where people can say this and other people say "It must be true, he's smart"?

Answer 1

They use parallax to measure the distance.  Pretty basic geometry and well understood.  As long as you take careful measurements your distance estimates can be quite accurate.

See link below to see how it works.

Second link contains a nifty Java applet (will just load in your browser, you do not need to download anything) which lets you play with how it works and animates the process.

Answer 2

parallax plus kepler's third law. if you have one distance in the solar system, you have them all.

Answer 3

The distance from Earth to Sun (average) is called the AU. By the middle ages astronomers had woked out the relative diastances of the planets from the sun, but did not know what the scale was -- in other words, the size of the AU.

Parallax measurements (apparent shift in position due to observers' at different locations) were carried out for transits of Venus and Mercury -- when these planets appear to move across the face of the sun. To achieve reliable parallax, observers were sent specially to far off locations. This was one of the reasons for the famous voyages of Capt. James Cook in the 18th century.

The estimated error on the size of the AU was finally brought down to nearly nothing following WWII when radar was bounced off Venus and the signal return time measured to a high degree of accuracy, which enabled the scale distances to be determined and the AU became very accurately known.

Nearby stars ' distances are measured directly by annual parallax, that is, images of the nearby star are made at one date, and then again six months later. In this time the Earth has moved halfway around its orbit (2 AU diameter baseline) and the tiny tiny shift in apparent position of the nearby star against the background stars much farther off can be measured reliably. REcently the satellite dubbed Hiparchos has measured stellar parallaxes much more accurately than ever before, so we have a very good handle on distances of stars in our stellar neighborhood.

Answer 4

Zerowant is right!

Its parallax method of measuring distances.

You need to know in detail about Physics and some optics to understand that.