The Astrophysical Journal, 666:694 Y715, 2007 September 10 # 2007. The American Astronomical Society. All rights reserved. Printed in U.S.A. A OBSERVATIONAL CONSTRAINTS ON THE NATURE OF DARK ENERGY: FIRST COSMOLOGICAL RESULTS FROM THE ESSENCE SUPERNOVA SURVEY W. M. Wood-Vasey,1 G. Miknaitis,2 C. W. Stubbs,1,3 S. Jha,4,5 A. G. Riess,6,7 P. M. Garnavich, R. P. Kirshner,1 C. Aguilera,9 A. C. Becker,10 J. W. Blackman,11 S. Blondin,1 P. Challis,1 A. Clocchiatti,12 A. Conley,13 R. Covarrubias,10 T. M. Davis,14 A.