So the earth was “buzzed” by asteroid 2005 YU55 the other night. A roughly 1000-foot long bit of cosmic debris passing within the moon’s orbit at a distance of 202,000 miles. At the time that I was trying to catch the object it’s motion against the background stars was almost 500 arcseconds a minute, moving towards the east-northeast. Since the apparent east-towards-west motion of the stars across the sky due to the earth’s almost 24-hour rotation period (called diurnal motion) is about 897.5 arcseconds per minute, that meant that as far as tracking the asteroid was concerned it would appear to be moving from east to west at somewhat less than half the speed of diurnal motion. Well, the CGE mounts I use for the telescopes are not able to track something like that. So that meant I had to set the telescope at a location a few minutes ahead of the asteroid and wait for it to enter the field of view. Since the telescope would be driven at a speed necessary to keep the star field steady (compensate for the diurnal motion) that meant the object would enter the field of view from the west and track towards the east through the field of view.
My initial attempts to locate 2005 YU55 were failures. I’d forgotten that the positions I’d downloaded were geocentric positions; that is, where the object would appear against the background sky as seen by an imaginary viewer located at the earth’s center. In almost any other instance that would get me close enough to assure the object was in the camera’s 8.5-arcminute field of view. Not this time! Because the object was so close the location of the observer on the earth’s surface is an important consideration. In this case the difference in apparent position of 2005 YU55 as seen by an observer on the earth’s surface (the topocentric coordinates) could be different from the geocentric coordinates by a degree or more. Doh! So I quickly converted a few of the coordinates to topocentric coordinates (thank you again Jean Meeus!) and viola!