1-10-2007 00:48 UTC from 41 N 111.8 W Observed comet C2006P1 McNaught between 00:48 and 01:07 UTC on 1/10/2007, in the setting sun horizon at an altitude of 3-4 degrees. This comet had a bright compact nucleus with a tail about 3/4 degree with the nucleus very roughly estimated with a diameter of 5' and an apparent brightness between 0 and -2 mags based on order of twilight appearance of bright objects, Venus (v-3.9), alf Cyg (v0.0) and alf Aqu (v0.8) and using the Out-Out method. Since the comet set before nautical twilight against a red-orange Belt of Venus, there were no contemporaneous stars against which to make a brightness measurement. The combination of low altitude and low atmospheric smog also confounded making an estimate. My crude drawing of C2006/P1 fails to convey its four unique properties 1) ephemeral visibility, 2) unusual orbital geometry, 3) inner tail cone, and 4) illusion of dimensionality. http://members.csolutions.net/fisherka/astronote/observed/C2006P1/C2006P1McNaught.jpg 1) Ephemeral visibility For most comets, observers more often are accustomed to high dark-sky altitudes and several viewing days in which to leisurely enjoy its characteristics. This comet has a very low solar elongation. Ephermedes for C2006P1 gives a Earth-object distance of 0.9616 au; an object-Sun distance of 0.2048 au; and a solar elongation of 12°. The object Sun distance of 0.2 a.u. translates into a linear distance of 30,405,161 km, which is less than the mean orbital distance of Mercury at 0.387 a.u. or 57,909,176 km. Its low solar elongation gives C2006P1 an very narrow observing window during twilight. After local sunset around 00:30 UTC, I scanned the twilight sky around the target zone for about 15 minutes. I was convinced that comet would set before the bright twilight background sky contrast would dim sufficiently to make it visible. Nothing could be seen in my 10x50mm binocular scan of the target zone at 00:45 UTC. At 00:48 UTC, I checked one last time and the comet and a portion of its tail were then plainly visible. What a difference just a three minutes made. By 00:57 UTC, the background sky had dimmed considerably, making the comet a naked-eye object. From there, I was able to observe the comet until 01:07 as it descending the last 2 degrees into the heavy urban pollution band and set below the horizon. The entire observing window was only 19 minutes and perhaps 5 of those minutes a higher quality experience. From my observing position looking at C2006P1 to the west, a take-off route from the Salt Lake International Airport cuts north-south across the target field. During the observing window, several jets took-off, climbed north-south through my binocular AFOV and then turned west. Some of those flyers must have had an amazing view of the comet out their window ports. 2) Unusual orbital geometry C2006P1's ephemeral visibility is a result of its orbital geometry. The following schematics are from NASA/JPL's Near-Earth-Object 3-D orbital plotting online applet: Comet McNaught orbit from an oblique view http://members.csolutions.net/fisherka/astronote/observed/C2006P1/C2006P1McNaughtOblique.gif Comet McNaught orbit from North Ecliptic Pole view http://members.csolutions.net/fisherka/astronote/observed/C2006P1/C2006P1McNaughtNEP.gif C2006P1 is on its inbound leg, with a perihelion between the Earth and the Sun and the orbital track almost perpendicular to the eclipitic. Presently, the comet is about above the Sun relative to the eclipitic plane but closer than the orbit of Mercury. When the Sun sets, the Sun is perpendicular to an observer's local horizon. Graphic - http://members.csolutions.net/fisherka/astronote/notes/EarthIsMoving/img/SunsetA.jpg A result of the comet's 3-D geometry is that we are currently looking at it from "below" with the tail pointing towards, but at a slight oblique angle, above the Earth. 3) Inner tail cone and the illusion of dimensionality Since Comet McNaught is closer to the Sun than Mercury and the metals like lead melt on the Sunward side of Mercury, one can only image the intense temperatures currently on the comet's surface. The size of the coma has been reported between 3' and 5'. At a distance of 0.9616 astronomical units, this implies a coma diameter between 125,000 km and 210,000 km. Even at 1 arcmin, the coma has a diameter of 41,666 km - several times larger than the diameter of the Earth. These physical coma sizes are much larger than the probable size of the physical nucleus and is indicative of the temperatures involved. As shown in my crude drawing, a bright cone extended back from the coma and resolved into a brighter pipe that extended into the tail. The intense temperatures and sunlight also made for a well-defined, almost auroral curtain-like tail. The unique object-Sun-Earth orbital geometry created a shading-albedo effect that added an illustion of dimensionality to Comet McNaught. The side of the tail extending northward in the ecliptic plane was brighter than the south ecliptic plane portion of the tail. The impression I had was that from this angle of observation, the central coma was casting a shadow across the lower portion of the tail. This shadow contributed to an illusion of dimensionality. From this observing angle, we are looking at a cone shaped tube extending back from and pointing above the Earth. Recent observations of total coma magnitude and coma diameter from the ICQ website < http://cfa-www.harvard.edu/iau/icq/CometMags.html > : Jan. 10.35, -2.3:*, 4' (Y. Nagai, Gunma, Japan, 11x80 binoculars; 0.5-deg tail; alt. 2.2 deg; civil twilight); 10.26, -2.5:*, 3' (K. Hornoch, Vranov, Czech Republic, 10x80 binoculars; 0.2-deg tail; very low, late dawn); 10.01, -1.7:*, -- (R. A. Keen, Mt. Thorodin, CO, naked eye; alt. 4.3 deg, bright twilight; 0.5-deg tail); 9.73, -2.0, 5' (C. Labordena, Castellon, Spain, naked eye; 3-deg tail); Many photos appear at www.spaceweather.com . - K. Fisher fisherka@csolutions.net