Question: Regarding the Parallax method for measuring the distance to a star: Do you use 186 million miles? Seems to me that in six months the sun has carried the entire solar system a few extra miles that should be included in the equation. — Douglas
Answer: I think that you are referring to the fact that the Sun, and the solar system with it, moves in its path around the center of the Milky Way Galaxy in the 6 months that the Earth orbits from one side of its orbit around the Sun to the other. Although it is correct to take account of the relative motion of the solar system and the star being measured, in fact the trigonometric parallax method is limited to measurements of relatively nearby stars, so this relative motion is rather small. For further background on the parallax method for determining distances to stars, check out the Las Cumbres Observatory Global Telescope network description of parallax and distance measurement.