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# Solar System and Motions of the Planets

2. 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?

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

14. 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?

1. 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?

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#### Solution Preview

2. We can use the simple Cartesian coordinate system for this purpose. Let us imagine that the X-Y plane defines the students in the classroom while the Z axis defines the floors. For example let us consider a student in the first position in the first floor of the building. He is assigned with an (x,y) value of (1,1) since he defines the first point in the x-y plane. Now since this classroom is in first floor he has an z value of 2 since a value of 1 is reserved for ground floor. Hence the coordinates which define this student is given by (1,1,2). So the z value will be constant for students in each floor while their x and y values change.

6. The celestial equator is defined as a extension of earth's equator on to the celestial sphere. Hence for an observer on earth's equator, the pole star will be near the horizon and celestial equator will be directly overhead. If you stand on Earth's equator, the celestial equator ...

#### Solution Summary

This in-depth solution of 674 words explains the concepts on the coordinate system, orientation of the celestial equator, Julian calendar, and the retrograde motion of Mars.

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