1. Describe a coordinate system that could be used to locate uniquely the seats in the classrooms of a multi-story building with one classroom on each floor. How many coordinates are needed in this case?

2. What is the orientation of the celestial equator for observers at the Earth's equator?

3. The Julian calendar, instituted by Julius Caesar in 46 B.C. and replaced by the modern Gregorian calendar beginning in 1582, averaged 365.25 days in length. How many annual events, such as the vernal equinox, move through the calendar while the Julian system was in effect?

4. We see Mars undergo retrograde motion when the Earth passes Mars. How does the motion of the Earth look to hypothetical Martian astronomers at that time?