Based on a review of many independent distance measurements using a number of different techniques, the International Astronomical Union has established 8,500 parsecs or 27,500 light years as the current best reference distance. It is believed to be a good estimate to about +/- 500 parsecs or so. More recent studies using a variety of methods are based on the statistical, weighted average of the following measurement methods which was published in a review article my Mark Reid in Annual Reviews of Astronomy and Astrophysics 1993, vol. 31, p. 345.
Method Value Uncertainty range ....................................................................... Primary Measurements 7,200 parsecs +/- 700 pc Calibrated by RR-Lyrids 8,200 +/- 900 pc Using solar motion 7,900 +/- 750 pc Using Oort's constants 9,000 +/- 1,700 pc LMC Calibration 8,000 +/- 800 pc Eddington Luminosity 7,400 +/- 2,000 Miscellaneous 7,800 +/- 2,000 ---------------------------------------------------------------------- Weighted Average 8,000 +/- 500 pc .......................................................................