SCIE 1920U Chapter 12: 12 Galaxy Distances and Hubble
Document Summary
The precise value of the au has been determined through a technique called radar ranging. We bounce radio waves from earth to venus, round-trip travel time tells us distance from. Using kepler"s laws and little geometry to calculate the length of an au. We can calculate a star"s distance if we know the precise amount of the star"s annual shift due to parallax. More distant stars have smaller parallax angles. By definition, the distance to an object with a parallax angle of 1 arcsecond is 1 parsec (pc), which is equivalent to 3. 26 light years. Parallax is our most accurate technique for measuring stellar distances, but parallax angles are only measurable for relatively nearby stars. We can determine the lamp"s distance more accurately if we can measure its apparent brightness. An object such as a street lamp, for which we are likely to know the true luminosity, represents what astronomers call a standard candle.
