The Solar System – ASTR 121

First Midterm – October 19, 2015, 9:00-9:50

Instructions:  Print your name and student identification number at the top of this page.  Answer each of the questions A through J in the space provided.  Be clear and thorough in your answers using sentences, labeled diagrams, and formulas with symbols defined, where appropriate.  Each question is worth two points.

A.  Define a light year.

B.  Explain what is meant by the tilt of the Earth's axis.

C.  Explain why we can only measure angular distances between objects in the sky.

D.  What do we mean by apparent retrograde motion of some planets?

E.  What is an ellipse?

F.  State Kepler's third (harmonic) law of planetary motion, explaining any technical terms you use.

G.  Why are astronauts weightless in the space station?

H.  Give an example illustrating conservation of angular momentum.

I.  Explain why we have two high tides per day.

J.  The Moon has an angular size of about half a degree and is about 3.6 x 10⁵ km away.  Use this data to calculate its diameter.