Week 1 Lecture 2
Solutions at the bottom.

1)  In-video quiz at timepoint 05:22.

Celestial Sphere: an imaginary sphere of infinitely large radius enclosing the universe so that all celestial bodies appear to be projected onto its surface
For more detail, see http://en.wikipedia.org/wiki/Celestial_sphere

Meridian: a great circle passing through the two poles of the celestial sphere and the zenith of a given observer.
For more information, see http://en.wikipedia.org/wiki/Meridian_(astronomy)

2)  In-video quiz at timepoint 08:45.

Oops! Ronen said "celestial horizon. He meant "celestial equator."

3)  In-video quiz at timepoint 10:44.

If two stars on the celestial sphere are separated by 75 degrees of longitude, what is their separation in hours of right ascension (RA)? 

a) 4 hours
b) 5 hours
c) 6 hours
d) 7 hours

Solutions:

3) Correct answer is b) 5 hours