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We describe a novel method to determine direct geometrical distances toquasars that can measure the cosmological constant, Lambda, with minimalassumptions. This method is equivalent to geometric parallax, with the`standard length' being the size of the quasar broad emission line region(BELR) as determined from the light travel time measurements of reverberationmapping. The effect of non-zero Lambda on angular diameter is large, 40% atz=2, so mapping angular diameter distances vs. redshift will give Lambda with(relative) ease. In principle these measurements could be made in the UV,optical, near infrared or even X-ray bands. Interferometers with a resolutionof 0.01mas are needed to measure the size of the BELR in z=2 quasars, whichappear plausible given reasonable short term extrapolations of currenttechnology.Comment: 13 pages, with 3 figures. ApJ Letters, in press (Dec 20, 2002

Quasar Parallax: A Method for Determining Direct Geometrical Distances to Quasars