Questions

Review Questions

Question 1.1

What is the difference between a hypothesis and a theory?

Question 1.2

How are scientific theories tested?

Question 1.3

How are constellations useful to astronomers? How many stars are not part of any constellation?

Question 1.4

A fellow student tells you that only those stars in Figure 1-3b that are connected by blue lines are part of the constellation Orion. How would you respond?

Question 1.5

Why are different stars overhead at 10:00 p.m. on a given night than two hours later at midnight? Why are different stars overhead at midnight on June 1 than at midnight on December 1?

Question 1.6

What is the celestial equator? How is it related to Earth’s equator? How are the north and south celestial poles related to Earth’s axis of rotation?

Question 1.7

Where would you have to look to see your zenith?

Question 1.8

How do the stars appear to move over the course of the night as seen from the north pole? As seen from the equator? Why are these two motions different?

Question 1.9

Using a diagram, explain why the tilt of Earth’s axis relative to Earth’s orbit causes the seasons as we orbit the Sun.

Question 1.10

Give two reasons why it is warmer in summer than in winter.

Question 1.11

What are the March and September equinoxes? What are the northern and southern solstices? How are these four points related to the ecliptic and the celestial equator?

Question 1.12

How does the daily path of the Sun across the sky change with the seasons? Why does it change?

Question 1.13

Describe how the seasons would be different if Earth’s axis of rotation, rather than having its present 23½° tilt, were tilted (a) by 0° or (b) by 90°.

Question 1.14

Explain the difference between sunlight and moonlight.

Question 1.15

Explain why the Moon exhibits phases.

Question 1.16

At approximately what time does the Moon rise when it is (a) a new moon; (b) a first quarter moon; (c) a full moon; and (d) a third quarter moon?

Question 1.17

If you lived on the Moon, would you see Earth go through phases? If so, would the sequence of phases be the same as those of the Moon as seen from Earth, or would the sequence be reversed? Explain using Figure 1-21.

Question 1.18

What is the difference between a sidereal month and a synodic month? Which is longer? Why?

Question 1.19

What is the difference between the umbra and the penumbra of a shadow?

Question 1.20

Why doesn’t a lunar eclipse occur at every full moon and a solar eclipse at every new moon?

Question 1.21

Which type of eclipse—lunar or solar—do you think more people on Earth have seen? Why?

Question 1.22

How is an annular eclipse of the Sun different from a total eclipse of the Sun? What causes this difference?

Web Chat Questions

Question 1.1

Scientists assume that “reality is rational.” Discuss what this means and the thinking behind it.

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Question 1.2

All scientific knowledge is inherently provisional. Discuss whether this is a weakness or a strength of the scientific method.

Question 1.3

Examine a list of the 88 constellations. Are there any constellations whose names obviously date from modern times? Where are these constellations located? Why do you suppose they do not have archaic names?

Question 1.4

In William Shakespeare’s Julius Caesar (act 3, scene 1), Caesar says:

But I am constant as the northern star,
Of whose true-fix’d and resting quality
There is no fellow in the firmament.

Translate Caesar’s statement about the “northern star” into modern astronomical language. Is the northern star truly “constant”? Was the northern star the same in Shakespeare’s time (1564–1616) as it is today?

Collaborative Exercises

Question 1.1

A scientific theory is fundamentally different from the everyday use of the word “theory.” List and describe any three scientific theories of your choice and creatively imagine an additional three hypothetical theories that are not scientific. Briefly describe what is scientific and what is nonscientific about each of these theories.

Question 1.2

Using a bright light source at the center of a darkened room or a flashlight, use your fist held at arm’s length to demonstrate the difference between a full moon and a lunar eclipse. (Use yourself or a classmate as Earth.) How must your fist “orbit” Earth so that lunar eclipses do not happen at every full moon? Create a simple sketch to illustrate your answers.

Question 1.3

Imagine you are planning a trip to see a solar eclipse in the future. Using Table 1-2 showing when and where solar eclipses are visible, which solar eclipse would you most want to go see and why?

Observing Questions

Question 1.1

If you have access to the Starry Night™ planetarium software, install it on your computer. There are several guides to the use of this software. As an initial introduction, you can run through the step-by-step basics of the program by clicking the Sky Guide tab to the left of the main screen and then clicking the Starry Night basics hyperlink at the bottom of the Sky Guide pane. A more comprehensive guide is available by choosing the Student Exercises hyperlink and then the Tutorial hyperlink. A User’s Guide to this software is available under the Help menu. As a start, you can use this program to determine when the Moon is visible today from your location. If the viewing location in the Starry Night control panel is not set to your location, select Set Home Location in the File menu (on a Mac, this command is found under the Starry Night menu). Click the List tab in the Home Location dialog box; then select the name of your city or town and click the Save As Home Location button. Next, use the hand tool to explore the sky and search for the Moon by moving your viewpoint around the sky. (Click and drag the mouse to achieve this motion.) If the Moon is not easily seen in your sky at this time, click the Find tab at the top left of the main view. The Find pane that opens should contain a list of solar system objects. Ensure that there is no text in the edit box at the top of the Find pane. If the message “Search all Databases” is not displayed below this edit box, then click the magnifying glass icon in the edit box and select Search All from the dropdown menu that appears. Click the + symbol to the left of the listing for Earth to display The Moon and double-click on this entry in the list in order to center the view upon the Moon. (If a message is displayed indicating that “the Moon is not currently visible from your location,” click on the Best Time button to advance to a more suitable time.) You will see that the Moon can be seen in the daytime as well as at night. Note that the Time Flow Rate is set to 1x, indicating that time is running forward at the normal rate. Note also the phase of the Moon.

  • a) Estimate how long it will take before the Moon reaches its full phase. Set the Time Flow Rate to 1 minute.
  • b) Find the time of moonset at your location.
  • c) Determine which, if any, of the following planets are visible tonight: Mercury, Venus, Mars, Jupiter, and Saturn. (Hint: Use the Find pane and click on each planet in turn to explore the positions of these objects.) Feel free to experiment with the many features of Starry Night™.

Question 1.2

Use Starry Night™ to observe the diurnal motion of the sky. First, set Starry Night™ to display the sky as seen from where you live, if you have not already done so. To do this, select File > Set Home Location… (on a Mac, Starry Night > Set Home Location) and click on the List tab to find the name of your city or town. Highlight the name and note the latitude of your location as given in the list and click the Save As Home Location button. Select Options > Other Options > Local Horizon… from the menu. In the Local Horizon Options dialog box, click on the radio button labeled Flat in the Horizon style section and click OK. For viewers in the northern hemisphere, press the “N” key (or click the N button in the Gaze section of the toolbar) to set the gaze direction to the northern sky. If your location is in the southern hemisphere, press the “S” key (or click the S button in the Gaze section of the toolbar) to set the gaze direction to the south. Select Hide Daylight under the View menu to view the present sky without daylight. Select View > Constellations > Astronomical and View > Constellations > Labels to display the constellation patterns on the sky. In the toolbar, click on the Time Flow Rate control and set the time step to 1 minute. Then click the Play button to run time forward. (The rapid motions of artificial Earth-orbiting satellites can prove irritating in this view. You can remove these satellites by clicking on View > Solar System and turning off Satellites).

  • a) Do the stars appear to rotate clockwise or counterclockwise? Explain this observation in terms of Earth’s rotation.
  • b) Are any of the stars circumpolar, that is, do they stay above your horizon for the full 24 hours of a day? If some stars at your location are circumpolar, adjust time and locate a star that moves very close to the horizon during its diurnal motion. Click the Stop button and right-click (Ctrl-click on a Mac) on the star and then select Show Info from the contextual menu to open the Info pane. Expand the Position in Sky layer and note the star’s declination (its N-S position on the sky with reference to a coordinate system whose zero value is the projection of Earth’s equator).
  • c) How is this limiting declination, above which stars are circumpolar, related to your latitude, noted above?
    • i) The limiting declination is equal to the latitude of the observer’s location.
    • ii) The limiting declination is equal to (90 − latitude).
    • iii) There is no relationship between this limiting declination and the observer’s latitude.
  • d) Now center your field of view on the southern horizon (if you live in the northern hemisphere) or the northern horizon (if you live in the southern hemisphere) and click Play to resume time flow. Describe what you see. Are any of these stars circumpolar?

Question 1.3

Use Starry Night™ to explore the concept of the celestial sphere. Open Favourites > Explorations > Celestial Sphere. The view shows Earth as it might look from space with time flowing at 300x. In the background are the stars of the Milky Way. Superimposed on Earth is a grid of latitude and longitude with the equator shown in red. Earth’s poles are also shown, the aqua line indicating the north pole and yellow line indicating the south pole. Superimposed on the background sky is a spherical grid pattern in red, which represents the celestial sphere. Zoom out as far as possible and use the location scroller tool to examine the celestial sphere. Now open Favourites > Explorations > Surface View of CS to examine the celestial sphere from the north pole of Earth. Notice that unlike the red gridlines on the celestial sphere, the gray meridian is fixed to the observing location on Earth since it does not move with respect to the horizon. Use the hand tool and Zoom controls to examine the celestial sphere from this perspective. Switch between these two perspectives as necessary to answer the questions that follow.

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  • a) What is the relationship between the poles of Earth and the celestial poles?
  • b) What is the relationship between Earth’s equator and the celestial equator?

Question 1.4

Use the Starry Night™ program to measure angular spacing between stars in the sky. Open Favourites > Explorations > N Pole to display the northern sky from Calgary, Canada, at a latitude of 51°. This view shows several asterisms, or groups of stars, outlined and labeled with their common names. The stars in the Big Dipper asterism outline the shape of a “dipper,” used for scooping water from a barrel. The two stars in the Big Dipper on the opposite side of the scoop from the handle, Merak and Dubhe, can be seen to point to the brightest star in the Little Dipper, the Pole Star. The Pole Star is close to the north celestial pole, the point in the sky directly above the north pole of Earth. It is thus a handy aid to navigation for northern hemisphere observers because it indicates the approximate direction of true north. Measure the spacing between the two “pointer stars” in the Big Dipper and then the spacing between the Pole Star and the closest of the pointer stars, Dubhe. (Hint: These measurements are best made by activating the angular separation tool from the cursor selection tool on the left side of the toolbar.)

  • a) What is the angular distance between the pointer stars Merak and Dubhe?
  • b) What is the angular spacing, or separation, between Dubhe (the pointer star at the end of the Big Dipper) and the Pole Star?
  • c) Approximately how many pointer-star spacings are there between Dubhe and the Pole Star?

Click the Play button in the toolbar. Notice that the Pole Star will appear to remain fixed in the sky as time progresses because it lies very close to the north celestial pole. Select Edit > Undo Time Flow or File > Revert from the menu to return to the initial view. Select View > Celestial Guides > Celestial Poles from the menu to indicate the position of the north celestial pole on the screen. Right-click on the Pole Star (Ctrl-click on a Mac) and select Centre from the dropdown menu to center the view on the Pole Star. Zoom in and use the angular separation tool to measure the angular spacing between the Pole Star and the north celestial pole.

  • d) What is the angular separation between the Pole Star and the north celestial pole?

Open Favourites > Explorations > S Pole to view the southern sky from Brisbane, Australia, at a southern latitude of 27½°. For observers in the southern hemisphere, there is no prominent star at or near to the position of the south celestial pole so its position is displayed on the view. The key asterism for estimating the position of the south celestial pole is the Southern Cross.

  • e) Use the angular separation tool to measure the angle between the fainter of the two stars making up the major axis of the cross, Gacrux, and the brighter star, Acrux. Extend this line toward the pole to demonstrate that this alignment is not exact.
  • f) Measure the angle between Acrux and the south celestial pole.
  • g) How many “pointer-star” spacings does this angle represent?
  • h) Measure the angle between the pole and the horizon at Brisbane. What is the relationship between this angle and the latitude of the observer? Click the Play button to demonstrate that the sky appears to rotate about this south celestial pole because of the rotation of Earth.

Question 1.5

Use Starry Night™ to demonstrate the reason for seasonal variations on Earth at midlatitudes. A common misconception is that summertime is warmer because Earth is closer to the Sun in the summer. The real reason is that the tilt of the spin axis of Earth to its orbital plane places the Sun at a higher angle in the sky in the summer than in the winter. Thus, sunlight hits Earth’s surface at a less oblique angle in summertime than in winter, thereby depositing greater heat. Open Favourites > Explorations > Seasonal Variations to view the southern sky in daylight from Calgary, Canada, at a latitude of 51°N, on December 21, 2013, at 12:38 p.m., local standard time, when the Sun is at its highest angle on that day. Form a table of values of Sun altitude and Sun-Earth distance as a function of date in the year. To find these values, move the cursor over the Sun to reveal the Info panel. (Ensure that the relevant information is displayed by opening File > Preferences > Cursor Tracking (HUD) and clicking on Altitude and Distance from Observer.) Note the Date, Sun Altitude, and Distance in your table. Advance the Date to March 21, 2014, and then to June 21, 2014, noting the values of these parameters.

  • a) On which of the three dates, in winter, spring and summer, respectively, is the Sun at the highest altitude in the Calgary sky?
  • b) From the values in your table, what is the Sun altitude at midday in December in Calgary?
  • c) How does the Sun’s altitude at midday on March 21 relate to the latitude of Calgary?
    • i) The Sun’s altitude is equal to the latitude of Calgary.
    • ii) The Sun’s altitude is 90° minus the latitude of Calgary.
    • iii) The Sun’s altitude is not related to the latitude of the location of the observer.
  • d) On which of these three observing dates is Earth closest to the Sun?
  • e) On which of these three dates is Earth farthest from the Sun?

Question 1.6

Use the Starry Night™ program to observe the Sun’s motion on the celestial sphere. Select Favourites > Explorations > Sun from the menu. The view shows the entire celestial sphere as if you were at the center of a transparent Earth on January 1, 2010. The view is centered upon the Sun and shows the ecliptic, the celestial equator, and the boundary and name of the constellation in which the Sun is located. With the Time Flow Rate set to 8 hours, click the Play button. Observe the Sun for a full year of simulated time. The motion of Earth in its orbit causes this apparent motion.

  • a) How does the Sun appear to move against the background stars?
  • b) What path does the Sun follow and does it ever change direction?
  • c) Through which constellations does the Sun appear to move over the course of a full year? In the toolbar, click the Now button to return to the current date and time.
  • d) In which constellation is the Sun located today? The Sun (and therefore this constellation) is high in the sky at midday.
  • e) Approximately how long do you think it will take for this constellation to be high in the sky at midnight?

Question 1.7

Use Starry Night™ to demonstrate the phases of the Moon. From the menu, select Favourites > Explorations > Moon Phases. In this view, you are looking down upon the southern hemisphere of Earth from a location 69,300 kilometers above Earth’s surface, with Earth centered in the view. The size of the Moon is greatly exaggerated in this view. The face of the Moon in this diagrammatic representation is configured to show its phase as seen from Earth. The inner green circle represents the Moon’s orbit around Earth. The position of the Sun with respect to Earth is shown on the outer green circle, the ecliptic, near to the top of the view. In reality, the Sun would be in this direction from Earth but at a far larger distance. Click the Play button and observe the relative positions of the Sun, Earth, and Moon and the associated phases of the Moon.

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  • a) Describe the geometry of the Sun, Moon, and Earth when the Moon is new.
  • b) Describe the geometry of the Sun, Moon, and Earth when the phase of the Moon is full.
  • c) When the phase of the Moon is at first or third quarter, what is the approximate angle at the Moon between the Moon-Sun and Moon-Earth lines?
  • d) What general term describes the phase of the Moon when the angle created by the Moon-Sun and Moon-Earth lines at the Moon is less than 90°?
  • e) What general term describes the phase of the Moon when the angle created by the Moon-Sun and Moon-Earth lines at the Moon is greater than 90°?

Question 1.8

Use Starry Night™ to observe a lunar eclipse from a position in space near the Sun. From the main menu select Favourites > Explorations > Lunar Eclipse. This view from a position near to the Sun shows Earth and two tan-colored circles. The inner circle represents the umbra of Earth’s shadow and the outer circle represents the boundary of the penumbra of Earth’s shadow. (If these circles do not appear, click on Options > Solar System > Planets-Moons… and click on Shadow Colour to adjust the brightness of the umbral and penumbral outlines.)

  • a) What is the Moon’s phase? Click the Play button and watch as time flows forward and the Moon passes through Earth’s shadow. Select File > Revert from the menu, move the cursor over the Moon, right-click the mouse, and click on Centre to center the view on the Moon. Zoom in to almost fill the field of view with the Moon and click the Play button again to watch the lunar eclipse in detail. Use the time controls to determine the duration of totality to the nearest minute.
  • b) What is the duration of totality for this eclipse?