Because Comet 17P presents at a high Sun-Comet-Observer angle (approx. 78 degs), its physical structure is more difficult to appreciate. A 3D construction of the comet and its tail was attempted to better understand what is seen in the eyepiece. Using ephemeris data on the Sun-Comet-Earth and Sun-Earth-Comet angles and distances and a 2007-11-10 image by Thorsten Boeckel on CloudyNights and spaceweather.com, I have prepared a speculative 3D construction of Comet 17P on that date. The constructed model is dependent on the weak assumption that the tail extends back from the comet roughly in a straight line along the Sun-Comet vector. Thorsten Boeckel's original image can be viewed at: http://spaceweather.com/comets/holmes/10nov07/Thorsten-Boeckel1.jpg Computed physical parameters from the construction for 17P on 2007-11-10 are: Distance to comet - 1.622 a.u. Coma diameter - 23' - 1,623,405 km Outer halo diameter - 46' - 3,246,738 km Distance of near tail spot on Boeckel image - 34.5' - 12,379,417 km Distance to end of tail tip from center of coma - 57.5' - 19,577,970 km Diameter of tail end - 46' - 3,508,700 km The resulting construction looks similar to the Boeckel image: http://gallery.utahastronomy.com/v/kurt/solarsystem/Comet+17P+Holmes/040.jpg... http://gallery.utahastronomy.com/d/10715-2/040.jpg but unlike the real object, the model can be rotated in 3D: http://gallery.utahastronomy.com/v/kurt/solarsystem/Comet+17P+Holmes/041.jpg... http://gallery.utahastronomy.com/d/10718-2/041.jpg A self-loading VRML plug-in of the 3D construction can be found at: http://members.csolutions.net/fisherka/astronote/observed/C17P2007/Comet17P_... or the raw VRML wrl file can be downloaded directly from: http://members.csolutions.net/fisherka/astronote/observed/C17P2007/Comet17P.... for use with other VRML browswer plug-ins. Leveraging ephemeris data from the NASA-JPL Horizon online applet, I obtained the Sun-Observer-Target(comet) and Sun-Target-Observer angles and distances for the date of Boeckel's image. This simplified reduction of a 3D reconstruction of Comet 17P to plane trigonometry. The urls listed in the Addendum to this note illustrate the process used to solve various Sun-Comet-Earth plane triangles to arrive at an estimate of the length of the tail of Comet 17P. The resulting construction was used to build a 3D model of the comet using VRML 2.0. Although a speculative model that is probably not accurate, it is hoped that this 3D construction of 17P and its tail will help amateurs to better understand what is seen in the eyepiece. This is an amateur attempt at 3D construction, so any corrections and criticisms are welcomed and appreciated. - Kurt Addendum - Links to plane trigonometry construction panes http://gallery.utahastronomy.com/v/kurt/solarsystem/Comet+17P+Holmes/030.png... http://gallery.utahastronomy.com/v/kurt/solarsystem/Comet+17P+Holmes/031.png... http://gallery.utahastronomy.com/v/kurt/solarsystem/Comet+17P+Holmes/032.png... http://gallery.utahastronomy.com/v/kurt/solarsystem/Comet+17P+Holmes/033.png... http://gallery.utahastronomy.com/v/kurt/solarsystem/Comet+17P+Holmes/034.png... http://gallery.utahastronomy.com/v/kurt/solarsystem/Comet+17P+Holmes/035.png... http://gallery.utahastronomy.com/v/kurt/solarsystem/Comet+17P+Holmes/035A.PN... _______________________________________________ Sent via CSolutions - http://www.csolutions.net