r><br>But the number of nights on which you can see all the Messier objects is limited due to the way the objects are distributed around the sky. Tom Polakis, an Arizona amateur astronomer and <I>Astronomy</I> contributing editor, has calculated the window of opportunity for a complete Messier marathon. Polakis's numbers are for 33&deg; north latitude, and they get more favorable as you work your way south. The beginning of the season is about March 17. The Messier marathon season ends on April 3. Remember,