In the celestial sphere model, Earth is stationary and the stars are carried on a sphere that rotates about an axis that points at Polaris, the “North Star”. (Recall that this is not what is actually happening.) In the figure below, two stars, Star A and Star B, are each shown at four different positions (1, 2, 3, and 4) through which each star will pass over the course of one revolution of the celestial sphere. You (the observer) are located on the Earth in the Northern Hemisphere, and the portion of the celestial sphere that is above your horizon is shown. 1.
Is Star A ever in an unobservable position? If so, which position(s)? If not, please explain why it has no unobservable positions. Star A is unobservable at position 4 because it crosses the observer’s
horizon plane. 2.
Is Star B ever in an unobservable position? If so, which position(s)? If not, please explain why it has no unobservable positions. Star B is a circumpolar star because its position never crosses the observer’s horizon plane. 3.
Which star, Star A or Star B, has a larger altitude angle when both are at their position 2s. Star B 4.
When Star A is at position 1, what is it said to be doing? Hint: It may help you to draw the cardinal directions on this figure.
Star A is rising in the East.
