For comet 71P/Clark on 2017 June 24 the MPC Minor Planet Ephemeris Service gives a value for m1 of 11.7. The JPL Horizons website returns 13.3 for the same input. Any ideas as to why they should be different ? Regards Roger Dymock www.britastro.org/projectalcock <http://www.britastro.org/projectalcock> Skype; rogerdymock67 Tel; 023 92647986