"One had to be Newton to realize that the Moon is falling, when everyone sees that it doesn't fall." - P. Vallery, 1966
Instructions: There are 20 questions. The first set should be done on the bubble form, and the second and third sets on the test itself. If you are unclear on anything, please talk to me or Rob. The exam is due at the end of class, 12:35 PM.
1. A new asteroid has been discovered with a perfectly circular
orbit. Is this a violation of Kepler's first law?
a) No - it's still an ellipse
b) It can't really be a circle, and must be a measurement error
c) Kepler's laws should be fixed
d) It's probably two asteroids orbiting each other, like Ida & Dactyl.
2. Why are many observatories built near the equator?
a) It's warmer and the atmosphere is more stable there.
b) The nights are longer on the equator.
c) Throughout the year, all the stars can be seen from the equator.
d) There is less `light pollution' from street lights.
3. What holds the planets in their orbits?
a) Their momentum and energy.
b) The tug of gravity constantly pulling them toward the Sun.
c) Collisions with asteroids and comets
d) Kepler's second law.
4. Comet Hale-Bopp has an orbital period of about 4,000 years.
However, it was only visible to most people on Earth for a few months. Why could this be?
a) We can only see comets standing near the equator.
b) The comet melted as it got closer to the Sun.
c) Greenhouse gases in the comet (methane, carbon dioxide) prevented us from seeing it.
d) An application of Kepler's second law.
5. The Earth's atmosphere has been relatively stable over the age of
the Solar System. What is one reason why?
a) Its orbit has always been an ellipse.
b) It is small enough so that it cannot retain an atmosphere for very long.
c) People have recently tried to stabilize it by burning fossil fuels.
d) There has been feedback between its atmosphere and surface.
6. Put these objects in proper order of size (diameter)
a) Milky Way, planet, solar system, star, asteroid, universe.
b) Solar system, asteroid, star, planet, Milky Way, universe.
c) Universe, Milky Way, solar system, star, planet, asteroid.
d) Asteroid, solar system, planet, star, Milky Way, universe.
7. Put these times in proper order
a) Earth orbital period, Lunar orbital period, Earth rotational period, age of Earth,
age of Universe.
b) Age of Universe, age of Earth, Earth orbital period, Lunar orbital period, Earth rotational period.
c) Earth orbital period, Lunar orbital period, Earth rotational period, age of Universe, age of Earth.
d) Age of Earth, age of Universe, Lunar orbital period, Earth rotational period, Earth
orbital period.
8. Newton's law of gravity says that all bodies attract each other.
Why, then if there is a force of gravity between you and the star
Deneb, do you stick to the Earth and not orbit Deneb?
a) The Sun is the most massive star in the Galaxy, which is why we orbit it.
b) Gravity doesn't work between gasses, only planets.
c) The distance to Deneb makes its gravitational force unimportant.
d) Gravity works only for planets around the Sun.
9. In which of the following cases are you accelerating?
a) When in orbit around the Sun.
b) When slowing down at a stoplight.
c) When running around a corner at a constant speed.
d) All of the above
10. If you were standing on the South Pole, where would you see
Polaris?
a) It's not visible from the South Pole.
b) It rises in the East and sets in the West, like all stars.
c) You can only see it during the winter (southern hemisphere summer).
d) It would be visible, but only just barely above the horizon.
1. You are picnicing outside and see a moon overhead with
its east half illuminated (to your left). What meal are you eating?
The moon's east half is illuminated, so the sun is directly to your east.
It's sunrise, and you're eating breakfast
2. You are building sand castles on the beach and notice that the tides
are particularly high always at this time of the month, and regularly
destroy your creations. What phase of the moon is it? Do you believe
your friend who tells you there's a solar eclipse coming up tomorrow?
(Drawing a diagram may help!)
The Sun and the Moon are both tugging on the oceans, so they're
on the same side of the Earth. It must be new moon. A solar eclipse
is possible.
3. Explain what is meant by a `Universal Law' such as Newton's law of gravity.
Such a law applies everywhere, not just one planet or one galaxy.
4. The few top meters of the Earth's crust are called `topsoil' and are a
very fine powder (mixed with some water). What most directly causes this on the Earth?
Erosion -- wind, water, plant roots, etc.
5. The top few meters of the Moon are also a fine powder. Could the the
same process be at work, or a different one? If so, which?
There's no atmospheric erosion on the moon. The surface is due to
impact craters.
6. Assess the statement issued by one recent environmental protester: `We
really must stop the greenhouse effect!'
Ridiculous! If it weren't for the greenhouse effect keeping the
Earth warm, we'd all be frozen little ice cubes.
1. Describe retrograde motion of the planets in the sky. How could
you observe it, how long would it take to observe, and what causes it?
Describe how retrograde motion was modeled in both the Ptolemaic and
heliocentric models.
Retrograde motion is the apparent backwards motion of the planets against the stars. The planets each go in `retrograde loops' about once a year. For us, it's caused by the Earth overtaking the outer planets, which are moving slower (Kepler's 3rd law).
Ptolemy proposed that the loops were caused by the planets actually moving on looped paths in space: epicycles. Copernicus put the Sun (not the Earth) at the center of the Solar System, and thus introduced the concept that we could `overtake' the planets since the Earth was moving just like all the other planets.
Retrograde motion is discussed extensively in the text (e.g., p. 26
& 51).
2. Let's imagine astronomers have discovered a new planetary system
orbiting the star Beta Centauri, which a few times brighter than our
Sun. They can determine that it has four planets orbiting at 0.1 AU, 2
AU, 5 AU, and 100 AU. All planets have the same mass, size, and
density of the Earth. Describe what each of the the planets in this
system might look like. How would their atmospheres compare?
What about their surfaces? What features would you expect to see on
the surface of each one? Which ones might be the best place to look
for life like our own (which requires liquid water)?
0.1 AU: Probably not much happening here: the atmosphere has been lost and it'll be heavily cratered. Very hot (hotter than Mercury, at 0.3 AU). Might have volcanism & plate tectonics.
2 AU: Similar to the Earth? Probably some atmosphere, some erosion, perhaps some volcanism or plate tectonics. Impact craters are likely to be covered up by other processes.
5 AU: Depending on how bright the star is, this planet might also have an atmosphere. It might be similar to the Earth or Mars - although probably cooler than the planet at 2 AU.
100 AU: Cold! Atmosphere has probably frozen out to the surface, or at least
into polar caps. No greenhouse effect, lots of craters.
3. Compare and contrast the evolution of the surfaces on Mercury and
Venus. Be detailed: which processes are important or unimportant on
each one, and what determines which ones dominate? How do you think
the surfaces will continue to evolve for the next billion years?
Mercury: Not much but craters here! It's too hot for an atmosphere, so there's no erosion. The planet is so small that the surface (and interior) cooled off several billions years ago. Thus, there's not much in the way of volcanism or tectonism either. Mercury and the Moon are quite similar in all these regards.
Venus: Thick atmosphere, strong greenhouse effect. We saw several impact craters on Venus, but not a lot -- certainly not covering the surface. So, it's probably a much younger surface. We see volcanoes all over the place, which makes sense because the planet is relatively large (similar to Earth) and still has a warm interior. Although Venus doesn't have continents shifting around like Earth does, it still has some tectonic activity -- uplifts, fractures, and so forth.
As for future evolution, Venus would really be the interesting case
to look at. It might lose its atmosphere, or it might continue to
evolve with volcanism and tectonics similar to how it is now -- but in
a billion years, the features themselves might be completely shifted.
Mercury, on the other hand, will get a few new impact craters, but
probably very little interesting beyond that, since there's not much
opportunity for other processes at all on such a `dead' planet.
4. Let's say we were standing next to the Sun, and observing Pluto,
45 AU away. If the Sun were to suddently go out, how long (in hours)
would it take until Pluto disappeared from our view? In other words,
how long would it take the light to go out to Pluto, and return? Show
your steps.
Time = Distance / Velocity
Time = (90 AU) / (c)
Time = (1.35 1015 cm) / (3 1010 cm/s)
Time = 45,000 s
Time = 12.5 hrs
Since we know it takes about 8 minutes for light to get from the Sun
to the Earth, it makes rough sense that it should take a couple of
hours to get out to Pluto and back. Think back to the Model Solar
System to get a mental picture of the distances involved.
Last modified 20-Jun-2000