While plotting Comet 8P/Tuttle this morning I came across an oddity (at least it was odd to me) at the Minor Planet Center. Since it involves finding Comet Tuttle I thought it might be helpful to post about it here. After plotting the comet on my computer's virtual sky I went to the Minor Planet & Comet Ephemeris Service ( http://cfa-www.harvard.edu/ iau/MPEph/MPEph.html ) and downloaded the position of the comet as seen from my location for the next few hours. While the altitude of the comet in my Mac matched with what MPC reported, the azimuth was off by 180 degrees. Where the Mac said the comet is due north, MPC reported the comet's azimuth as 179. To me that meant due south. I posted a question about this on the Minor Planet list and heard from several people that MPC follows an old convention which has 0 being south and 180 being north. So if you plan on using MPC to find Tuttle (or any other object for that matter) keep the 180°-out-of-phase thing in mind. patrick