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We present 30 optical spectra and 49 photometric epochs sampling the first517 days after discovery of supernova (SN) 1999em, and derive its distancethrough the expanding photosphere method (EPM). SN 1999em is shown to be a TypeII-plateau (II-P) event, with a photometric plateau lasting until about 100days after explosion. We identify the dominant ions responsible for most of theabsorption features seen in the optical portion of the spectrum during theplateau phase. We find the distance to SN 1999em to be D = 8.2 +/- 0.6 Mpc,with an explosion date of 5.3 +/- 1.4 days before discovery.  We examine potential sources of systematic error in EPM-derived distances,and find the most significant to result from uncertainty in the theoreticalmodeling of the flux distribution emitted by the SN photosphere (i.e., the``flux dilution factor''). We compare previously derived EPM distances to 5 SNeII in galaxies for which a recently revised Cepheid distance exists from theHST Key Project and find D(Cepheids) / D(EPM) = 0.96 +/- 0.09. Finally, weinvestigate the possible use of SNe II-P as standard candles and find that for8 photometrically confirmed SNe II-P with previously derived EPM distances andSN 1999em, the mean plateau absolute brightness is M_V(plateau) = -16.4 +/- 0.6mag, implying that distances good to ~30% (1-sigma) may be possible without theneed for a complete EPM analysis. At M_V(plateau) = -15.9 +/- 0.2 mag, SN1999em is somewhat fainter than the average SN II-P. The general consistency ofabsolute SNe II-P brightness during the plateau suggests that the standardcandle assumption may allow SNe II-P to be viable cosmological beacons at z >2.Comment: 79 pages, 26 figures, accepted for publication in the Publications of  the Astronomical Society of the Pacifi

The Distance to SN 1999em in NGC 1637 from the Expanding Photosphere Method