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1. [5] If the Earth was formed with a population of 512 bazillion radioactive uranium 235 atoms, and assuming U-235 has a half-life of 0.7 billion years and the age of the Earth is about 4.2 billion years,  how many U-235 atoms remain in the Earth today?
   
   
2. [5] The moon is on your eastern horizon at midnight.  What is the phase of the moon?
   
   
3. [5] You observe that high tide occurs at about 3:00 in the afternoon. What is the approximate phase of the moon?
   
   
4. [4] Based on radioactive dating, what are the ages of [4 answers required] the oldest a) rocks found on Earth, b) mineral grains found on Earth, c) known moon rocks, d) meteorites?
   
   
5. [3] If the Moon were 30 times as far from the Earth as it is now, how would the force of gravity between the Earth and Moon compare to its current strength?
   
   
6. [5] If the Moon were 30 times as far from the Earth as it is now, what time(s) of day would low tide occur?
   
   
7. [5] Comet Halley orbits the Sun in about 80 years; Uranus orbits the Sun in about 80 years. Uranus' orbit is nearly circular, with a radius of 19.2 Astronomical Units.  At perihelion (closest approach to the Sun), C/Halley just reaches about 0.6 AU.  Using the table, below, determine which and how many planets' orbits C/Halley crosses.

Mercury	0.4 AU
Venus	0.7 AU
Earth	1.0 AU
Mars	1.5 AU
Jupiter	5.2 AU
Saturn	9.5 AU
Uranus	19.2 AU
Neptune	30.1 AU
Pluto	39.5 AU

8. [3] What is the principle source of heat inside the Earth?
   
   
9. [3] Which has a length of 24 hours, a solar day or a sidereal day?
   
   
10. [3] Which is longer, a solar or sidereal day? Is the difference best described as a few seconds, a few minutes or a few hours?
    
    
11. [3] Is the time from full moon to full moon shorter, longer or the same as the time it takes the moon to orbit the Earth?
    
    
12. [4] The day, month and year have obvious astronomical origins. What is the astronomical origin of the unit of time we call a "week"?
    
    
13. [6] The length of a year is not identical with the length of time required for the Earth to complete exactly one orbit around the Sun.  Why not?
    
    
14. [6] What is the most important physical reason that we have seasons?
    
    
15. [6] Two conditions are necessary in order for a solar eclipse to occur. What are they?  Why don't we have solar eclipses every month?
    
    
16. [6] Tides have produced changes in three different aspects of the Earth-Moon system. What are these three tidally induced changes?
    
    
17. [4] Where, in the solar system, have these three aspects of tidal evolution had the most profound effect? Briefly point out how this system has evolved further, or more profoundly, than the Earth-Moon system.
    
    
18. [6] Stable isotopes of carbon exist with 6 (Carbon-12) and 7 (Carbon-13) neutrons. All, of course, have 6 protons. Why doesn't Carbon-10 exist as a natural isotope in nature?
    
    
19. [6] Given only an image of the surface of a cratered moon or planet, how would you differentiate between craters of volcanic and impact origin?
    
    
20. [6] What have we learned about the history of the Moon from "crater counting"?
    
    
21. [6] In round numbers, the present distance to the Moon is about 400,000 km (i.e., the semi-major axis of the lunar orbit, which is fairly circular, rather than extremely elliptical) and the orbital period of the moon is 27.3 days. If the average rate at which the moon has been spiraling outwards from the Earth has been 10 cm (0.0001 km) per year for 3 billion years, by what distance has the moon's orbit expanded over 3 billion years? What was the Moon's distance from the Earth 3 billion years ago? What was the orbital period of the Moon 3 billion years ago?