To quote Mythbusters: Warning! Science Content I posted a link yesterday to this plot of the separation between Mars and the Moon over the span of a few hours Sunday: <http://members.cox.net/f7j9jwirjsljejic/separation.jpg> At the time, I mentioned that I was intrigued by but didn't know the reason for the difference in slope before and after minimum separation. Giving the question further consideration, I decided that it was most likely the result of the changing point of observation over the time period plotted, due to rotation of the Earth. To investigate further, I expanded the range of the plot to cover a 48-hour period. <http://members.cox.net/f7j9jwirjsljejic/separation_48t.jpg> In this plot, the X-axis shows Julian date - 2454450.0. A bit more interesting, and if you look carefully, you can see that there is some sign of a one-day component in the shape of the curve (note the bend at about 8.0 and again at about 9.0 on the X axis). To further test my speculation, I plotted the same data in the geocentric rather than topocentric reference frame. <http://members.cox.net/f7j9jwirjsljejic/separation_48g.jpg> Now the plot appears highly symmetrical, and I'm satisfied that I've answered my question. Now for anyone who hasn't moved on to other messages or gone to watch a rerun of The Apprentice, you may be thinking, "Right, but what the heck do geocentric and topocentric mean?" It's fairly simple, and except in special cases, the difference is usually negligible. The geocentric reference frame is that of a hypothetical observer located at the Earth's center, whereas the topocentric frame relates to us ordinary mortals at some particular point on the surface of the planet. Going back to my first plot, it showed the separation between the Moon and Mars as seen from my observatory a few kilometers southwest of Maricopa. As I mentioned in the initial posting, there was an occultation of Mars by the Moon for observers in the far north. The difference is that, as seen from the Moon, the Earth subtends an angle of about two degrees. Those two degrees are significant in the case of close apparitions involving the Moon, as well as the occasional Earth-crossing asteroid, spacecraft, etc. As for the difference in slope in the original plot, as the Earth rotates our vantage point changes by almost two degrees. This caused the apparent separation to decrees faster than it increased following closest approach Sunday evening. -- Mike -- -- cal_donley@xxxxxxx -- See message header for info on list archives or unsubscribing, and please send personal replies to the author, not the list.