We have been able to compare with astrometric precision AstroDrizzle processed images of NGC 6720 (the Ring Nebula) made using two cameras on the Hubble Space Telescope. The time difference of the observations was 12.925 yr. This large time base allowed the determination of tangential velocities of features within this classic planetary nebula. Individual features were measured in [N II] images as were the dark knots seen in silhouette against background nebular [O III] emission. An image magnification and matching technique was also used to test the accuracy of the usual assumption of homologous expansion. We found that homologous expansion does apply, but the rate of expansion is greater along the major axis of the nebula, which is intrinsically larger than the minor axis. We find that the dark knots expand more slowly than the nebular gas, that the distance to the nebula is 720 pc ±30%, and that the dynamic age of the Ring Nebula is about 4000 yr. The dynamic age is in agreement with the position of the central star on theoretical curves for stars collapsing from the peak of the asymptotic giant branch to being white dwarfs.

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Published in The Astronomical Journal, v. 145, no. 6, 170, p. 1-8.

© 2013. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

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G.J.F. acknowledges support by NSF (0908877, 1108928, and 1109061), NASA (10-ATP10-0053, 10-ADAP10-0073, and NNX12AH73G), JPL (RSA No. 1430426), and STScI (HST-AR-12125.01, GO-12560, and HST-GO-12309). M.P. received partial support from CONACyT grant 129553. W.J.H. acknowledges financial support from DGAPA–UNAM through project PAPIIT IN102012. C.R.O.'s participation was supported in part by HST programs GO 12309 and GO 12543.