Astronomy 10 - Summer 2000, Session A
Midterm Exam - Solutions

Multiple Choice

  1. Which of the following statements about the sky is FALSE?
    1. A given star rises about 4 minutes earlier each night because of Earth's revolution about the Sun.
    2. As viewed from the equator, the North Star (Polaris) is on the northern horizon.
    3. From any given point on Earth, the entire celestial sphere is visible over the course of a year.
    4. The number of stars that are "circumpolar" (i.e., always above the horizon) is larger in Alaska than in California.
    5. Stars appear to circle around the celestial poles because of the rotation of Earth about its axis.

  2. You're in a boat on the Atlantic Ocean 30° West of Greenwich, England. You're at 60° North longitude traveling east at a speed of 50 km/hr. (Earth's radius = 6,371 km). The Sun is on your meridian. What is the local time?
    1. 9:00 AM
    2. 10:30 AM
    3. 12:00 PM
    4. 12:42 PM
    5. 3:00 PM

  3. The Moon's angular diameter is roughly equal to that of a dime held at arm's length because
    1. the Moon has the same physical diameter as the dime.
    2. the ratio of the Moon's distance to the Moon's physical diameter is equal to the ratio of the arm's length to the dime's physical diameter.
    3. the ratio of the Moon's distance to the Moon's physical diameter is equal to the ratio of the dime's physical diameter to the arm's length.
    4. the Moon's angular diameter is inversely proportional to the square of its distance, and the Moon is much farther away than the dime.
    5. light takes a much longer time to travel from the Moon than from the dime.

  4. Which of the following statements about planets is FALSE?
    1. Their apparent paths through the sky lie close to the ecliptic and thus define the plane of the Solar System.
    2. They always move in a west to east direction with respect to the stars.
    3. Venus and Mercury are never visible at a position on the Celestial Sphere opposite to the Sun.
    4. The word "planet" is of Greek origin meaning wanderer.
    5. Planets do not, in general, twinkle.

  5. The gravitational force exerted on a small mass (m) by a large mass (M) is
    1. greater than the force exerted on M by m.
    2. equal to the force exerted on M by m.
    3. less than the force exerted on M by m.
    4. either greater or less than the force exerted on M by m depending on the distance between them.
    5. independent of the distance of separation.

  6. Which of the following is NOT true about acceleration due to gravity near the surface of a planet?
    1. It is directly proportional to the mass of the planet.
    2. It is directly proportional to the mass of the falling object.
    3. It is inversely proportional to the square of the planet's radius.
    4. It is roughly uniform over the surface of the planet.
    5. It describes the rate at which the speed of a falling object increases.

  7. The Moon has a radius about 1/4 that of Earth's and a mass about 1/80 that of Earth's. An astronaut whose mass is 100 kg on Earth, would have what mass on the Moon? (Neglect the mass of his spacesuit.)
    1. 17 kg
    2. 25 kg
    3. 42 kg
    4. 50 kg
    5. 100 kg

  8. Which one of the following statements about electromagnetic radiation is FALSE?
    1. Electromagnetic radiation behaves as though it has properties of both waves and particles.
    2. Visible light is one form of electromagnetic radiation, with wavelengths roughly between 4000 Å and 7000 Å.
    3. An electromagnetic wave consists of oscillating electric and magnetic fields that are self-propagating.
    4. Electromagnetic radiation travels with the same speed through a vacuum and through transparent substances like glass and water.
    5. The correct order of electromagnetic radiation, from shortest to longest wavelength, is as follows: gamma rays, X-rays, ultraviolet, visible, infrared, radio.

  9. Which of the following photons has the highest energy?
    1. visible light
    2. X-rays
    3. radio
    4. gamma rays
    5. infrared

  10. Which of the following is FALSE about blackbodies?
    1. Blackbodies are always black.
    2. Blackbodies are perfect radiators of light.
    3. Blackbodies are perfect absorbers of light.
    4. Hotter blackbodies give off more light at all wavelengths than cooler blackbodies from each square centimeter of surface per second.
    5. Hotter blackbodies give off more blue light than cooler blackbodies.

  11. Star Scully is 4 times hotter than star Mulder. If the wavelength at which most of Scully's light is emitted is 500 nm what is the wavelength at which most of the light is emitted by star Mulder?
    1. 200 nm
    2. 420 nm
    3. 500 nm
    4. 125 nm
    5. 2,000 nm

  12. When an electron moves to a higher energy level in an atom it             a photon of specific energy.
    1. emits
    2. absorbs
    3. alters
    4. excites
    5. orbits

  13. A heated gas of a pure element with no background light source will give off
    1. a continuous spectrum.
    2. an emission spectrum.
    3. an absorption spectrum.
    4. no observable spectrum.
    5. gamma rays.

  14. For astronomers today, the primary function of telescopes is to
    1. make distant objects appear closer.
    2. make small objects appear larger.
    3. make fuzzy objects appear clearer.
    4. gather light.
    5. let them peep into nearby houses.

  15. Which of the following statements about the two Keck 10-meter telescopes, built on Mauna Kea volcano in Hawaii by Caltech and the University of California, is FALSE?
    1. They consist of many hexagonally-shaped mirrors properly aligned with various sensors.
    2. They are currently the largest optical telescopes in the world.
    3. They are reflecting telescopes.
    4. They are able to make observations at infrared wavelengths better than telescopes built at sea level.
    5. They are able to observe at ultraviolet wavelengths as well as the Hubble Space Telescope.

  16. Which of the following was NOT believed by the ancient Greek philosophers?
    1. The heavens are perfect and unchanging.
    2. All heavenly bodies move on paths composed of circles.
    3. Earth is flat.
    4. Earth is the center of the Universe.
    5. The planets are closer to Earth than are the stars.

  17. Retrograde Motion
    1. is impossible to explain with a geocentric (Earth-centered) model of the Solar System.
    2. occurs when the physical orbit of a planet around the Sun reverses its direction.
    3. is a term applied to the few objects in the Solar System which physically orbit the Sun in a direction opposite that of all other planetary orbits.
    4. is easily explained in a heliocentric (Sun-centered) model of the Solar System.
    5. proves that the orbits of planets are ellipses, not perfect circles.

  18. Kepler's Second Law implies that
    1. planets move in elliptical, not circular orbits, with the Sun at the center of the major axis.
    2. planets travel equal distances in equal time.
    3. the cube of the period of an orbit is proportional to the square of the semimajor axis.
    4. planets travel faster in their orbit when they are closer to the sun than when they are farther from the sun.
    5. AT&T's One-Rate Plan really does make sense.

  19. NASA decides to place a satellite in a circular orbit around the Sun with an orbital period of 8 years. What should be the semimajor axis of the orbit?
    1. 16 A.U.
    2. 8 A.U.
    3. 4 A.U.
    4. 2 A.U.
    5. None of the above.

  20. Suppose a satellite has been sent out of the Solar System. If it takes a year for a radio signal to travel from Earth to the satellite and back again, what is the distance of the satellite? (Note: a year is about 3.16 x 107 seconds.)
    1. 5 x 1012 meters
    2. 100,000 A.U.
    3. 0.5 light-years
    4. 1 light-year
    5. 1012 km

  21. In what way is Earth NOT different from all of the other terrestrial planets?
    1. Earth's surface is mostly covered by liquid water.
    2. Earth's atmosphere has a high percentage of oxygen.
    3. Earth has a large Moon.
    4. Earth has mountains and volcanos.
    5. Earth's surface is abundant in biological lifeforms.

  22. Which of the following planets shows evidence for the runaway greenhouse effect?
    1. Mercury
    2. Venus
    3. Earth
    4. Mars
    5. Jupiter

  23. The terrestrial planets
    1. all have atmospheres.
    2. all have the same distance from the Sun.
    3. all have moons.
    4. all have about the same density.
    5. all have the same surface temperature.

  24. Jovian planets differ from the terrestrial planets in all the following ways EXCEPT
    1. Jovian planets are more massive.
    2. Jovian planets are denser.
    3. Jovian planets have rings.
    4. Jovian planets are mostly hydrogen.
    5. Jovian planets are bigger.

  25. Tidal forces are NOT the primary cause of
    1. the volcanos on Io.
    2. collisions of asteroids with planets.
    3. the breakup of Comet Shoemaker-Levy 9, which eventually crashed into Jupiter.
    4. the formation of the asteroid belt.
    5. the formation of the rings of Saturn.

  26. Which of the following statements about comets is FALSE?
    1. The tail of a comet consists of dust and gas that reflects sunlight.
    2. The tail of a comet can sometimes point roughly in the direction of the comet's motion.
    3. Most comets come from the asteroid belt, and brighten as they approach the Sun.
    4. When Earth passes through the orbit of an old, disintegrating comet, a meteor shower can occur.
    5. A comet is basically a "dirty snowball" that evaporates as it gets heated.

  27. Comets are mainly composed of
    1. carbon and silicon
    2. ices and dust
    3. rock and sulfer
    4. iron and nickel
    5. nougat and cherries

  28. Which one of the following is NOT an aspect of the "Nebular Hypothesis" for the formation of the Solar System?
    1. shortly after its formation, the Sun's strong wind blew away most of the remaining particles in the Solar System.
    2. The nebula from which the Solar System formed spun progressively faser as it collapsed.
    3. The Sun formed in the central, densest part of a collapsing cloud of gas and dust, whereas the planets formed farther out.
    4. The Solar System formed a disk-like structure because in a disk, the "centripital acceleration" on a particle balances the acceleration caused by gravity.
    5. A passing star gravitationally pulled material out of the Sun, producing a nebula from which the planets subsequently formed.

  29. Sunspots are
    1. clouds of dense gas and dust in the Sun's photosphere.
    2. regions of strong magnetic fields that inhibit the rise of hot gas from below the photosphere.
    3. regions of the Sun's photosphere where it is much too cold for nuclear reactions to occur, unlike the rest of the photosphere.
    4. holes in the Sun's photosphere through which we can see the dark interior of the Sun.
    5. what they call dogs on the Sun (whoever "they" are).

  30. The Solar wind
    1. is generated by sunspots.
    2. flows around the Sun, in the same direction as the planets orbit.
    3. consists of electrons and positive ions which can ultimately get trapped by Earth's magnetic field and produce aurorae
    4. can be used to measure the Sun's rotation period, since it comes from a particular region of the Sun's photosphere that is periodcially blocked and unblocked from our view.
    5. is produced only during total solar eclipses.

  31. Tides
    1. generally occur only once per day (i.e., one high tide and one low tide) because the Moon exerts a pull on the water closest to it.
    2. are generally least high when the Sun, Earth, and Moon are aligned.
    3. gradually make Earth's day shorter.
    4. are produced mostly by the Sun rather than the Moon because the sun is so much more massive.
    5. are the reason we always see the same side of the Moon.

  32. How many complete spins about its axis of rotation does Earth make in one full orbit about the Sun?
    1. 1
    2. 360
    3. 364
    4. 365
    5. 366

  33. Which of the following is the main reason why Earth has seasons?
    1. Earth's orbit is elliptical, we have summer when we are closer to the Sun and Winter when we are farther from the Sun.
    2. The tilt of Earth's axis causes one hemisphere of the planet to be closer to the Sun during the day than the other. Because it is closer to the Sun, it receives much more solar energy.
    3. The tilt of Earth's axis causes one hemisphere to receive more direct sunlight than the other, for a longer period of time, increasing the energy deposited there.
    4. The Earth changes its axial tilt angle relative to the Ecliptic plane during each orbit in such a way as to bring more light in the summer.
    5. During part of the year more volcanos erupt depositing debris in the atmosphere blocking out some light from the Sun.

  34. During an eclipse of the Sun by the Moon
    1. only a small spot on Earth is covered by the Moon's shadow.
    2. the Sun is completely invisible to the Earth for a period of up to 7 hours.
    3. the eclipse can be seen anywhere on Earth where it is daytime (weather permitting).
    4. the Sun actually is brighter than normal due to the refraction of the Moon's atmosphere.
    5. it must be close to noon for a viewer on Earth to be able to see it.

  35. During an eclipse of the Moon by the Earth
    1. only a small spot on the Moon is covered by Earth's shadow.
    2. the Moon is completely invisible at any given place on Earth for a period of about 7 minutes.
    3. the eclipse can be seen anywhere on Earth where it is nighttime (weather permitting)
    4. The Moon is actually brighter than a full Moon due to the refraction of Earth's atmosphere.
    5. it must be close to midnight for a viewer on Earth to be able to see it.

Short Answer

  1. The radio telescopes that make up the VLA (Verry Large Array) in New Mexico each have a diameter of 25 m. What is the smallest angle they can resolve when observing at a wavelength of 1 cm? If Mars has a diameter of 30", can Mars be resolved? The telescopes of the VLA can be strung out to have a maximum separation of 27 km. How much better is the resolution if the signals from the antennas are combined using the techniques of radio interferometry compared to the resolution of a single dish?

    The formula for the minimum angular resolution of a telescope is 1.22/D, where is measured in radians. Plug in our values for the size of the radio telescopes and the wavelength of radio waves:
    = 1.22(1x10-2 m/25 m) = 5 x 10 -4 radians

    5 x 10 -4 radians * (206,265"/1 radian) = 100"

    Since 100" is larger than the angular size of Mars (30") it CANNOT be resolved.

    The images made by the telescopes in interferometry mode will have the effective resolution of a telescope the size of their spacing. Thus, since the maximum separation of the telescopes is ~ 1,000 times bigger than the individual telescopes the resolution will also be ~ 1,000 times smaller.

    2/1 = D1/D2 = (25 m/27 x 103 m) 1 x 10-3

    2 = 1 x 10-3 1 = 0.1"

  2. Ionized atoms are atoms that have lost one or more electrons. Photons can ionize atoms. Are ionized atoms easier to make with red light or blue light? Explain.

    To ionize an atom requires the input of energy. It is easiest to ionize an atom with the most energetic photons. Using the formula E = hc/, the bluest photons (shortest wavelengths) have the higher energy. Thus, it is easier to ionize an atom with blue photons rather than red.

  3. Explain how astronomers can use the doppler shift of light to detect planets around other stars.

    If a star has a planet orbiting it, the star must feel the same force of gravity on it that the planet feels toward the star. Because the star is much more massive than a planet this force causes only a very small acceleration toward the planet. Thus, as the planet orbits the star it tugs and pulls the star toward and away from our line of site. The motion of the star can then be detected by astronomers on Earth via the doppler effect of light. As the star moves toward us its light waves get bunched up in front of it making the wavelength shorter (blueshifted), likewise as the star moves away from us the light waves get spread out to longer wavelengths (redshifted). The amount of doppler shift tells us the amount of change in velocity and indicates the mass of the planet going around it when taken together with the period of the change in velcotiy via Kepler's 3rd law.

  4. In Galileo's time what were some observations that supported the heliocentric (Sun-centered) model of the Universe? What were some arguments made by supporters of the geocentric (Earth-centered) model of Ptolemy against the heliocentric model?

    Galileo observed that Jupiter had moons that orbited it rather than Earth, proving that not everything in the Heavens revolved around Earth. Galileo also observed the planet Venus to go through a full cycle of phases, just like the Moon. This is only possible if Venus orbits the Sun and not Earth. Kepler had also found that with Tycho Brahae's best data the Earth-centered model of Ptolemy was completely inadequate to explain the motions of the planets as well as his Sun-centered model with elliptical orbits for the planets. Proponents of the geocentric Universe argued that if the Earth were in motion about the Sun the stars should exhibit parallax (they did not realize just how far away the stars are). They also pointed out that if the Earth were moving about the Sun once per year and spinning once per day it would mean that the Earth would be moving very fast, and yet we do not feel this motion, nor did they measure any correolis effect in the motion of objects on Earth's surface. Some claimed that a Sun-centered Universe went against Biblical scripture and therefore could not be true.

  5. Near the beginning of the movie E.T. the Extra-terrestrial there are some scenes with the Moon visible in the sky. In the first scene it is shortly after sunset and the Moon is visible as a crescent near the horizon. A few scenes later we are back to the same viewpoint but now it is late at night and again the Moon is visible. The Moon's position in both scenes is illustrated below. What is wrong with this?

    A crescent Moon is always near the Sun in the sky because the perspective that yields a crescent shape is one where the Moon is between us and the Sun. If we see a crescent Moon on the horizon after sunset it must be the western horizon and the Moon is about to set. We also know that it must be a waxing crescent Moon, because the wanning crescent Moon sets before sunset. The view of the Moon in the first scene thus indicates that we are looking West and the Moon is about to set. However, in the movie the Moon does the opposite and rises. This is impossible! The Moon only rises in the East, and crescent Moons only rise somtime near sunrise.