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Objectives
Lesson Overview
Materials/Resources
Background Information
Developing the Lesson
Closure
Evaluation


Module 1 - Lesson 4

Locating Celestial Objects:
Altitude and Azimuth vs. Right Ascension and Declination

Module 1: Introduction to the Day and Night Sky

 

Timeframe:

This lesson will require approximately 2 class periods (~ 50 minutes each)

 

Objectives:

Pan-Canadian Curriculum Objectives:

111-5
describe the science underlying particular technologies designed to explore natural phenomena, extend human capabilities, or solve practical problems (e.g., describe how optical principles are demonstrated in a telescope, and aerodynamic principles are applied in rocket and spacecraft engineering)

208-5
state a prediction and a hypothesis based on background information or an observed pattern of events (e.g., predict the next visit of a comet based on past observations)

209-4
organize data using a format that is appropriate to the task or experiment (e.g., maintain a log of their observations of changes in the night sky; prepare a comparative data table on various stars)

210-3
identify strengths and weaknesses of different methods of collecting and displaying data (e.g., compare Earth-based observations to those made from spacecraft; explain why the precise observation of stars is limited by their distance)

210-9
calculate theoretical values of a variable (e.g., calculate the travel time to a distant star at a given speed)

312-4
describe and explain the apparent motion of celestial bodies

 

Lesson Overview:

This lesson builds upon concepts and knowledge detailed in the Apparent Motions in the Night Sky lesson.

 

Materials and Resources:

  1. Computer
  2. LCD Projector
  3. Planetarium software program such as Starry Night
  4. The Celestial Sphere Applet
  5. Circumpolar Motion Applet
  6. Altitude and Azimuth Applet

 

Vocabulary:

    • altitude
    • azimuth
    • celestial pole
    • celestial sphere
    • circumpolar motion
    • declination
    • diurnal motion
    • latitude
    • longitude
    • Polaris
    • precession
    • retical
    • right ascension

 

Background Information:

Celestial Sphere and Celestial Coordinates

Right Ascension and Declination

The stars in our night sky have remained virtually unchanged for hundreds of years. Although stars have their own intrinsic motions in space, they are so far away that we cannot detect their movement unless the same star is observed over a time span of many years. Therefore, the relative positions of the stars will not change noticeably within our lifetime; the patterns we see in the sky are virtually no different than the patterns our ancestors saw.

The first astronomers believed that the stars were fixed on a celestial sphere surrounding the Earth. Although we now know this to be untrue (the universe is three-dimensional and stars are located at various distances from the Earth), it still helps to use this illustration to better understand the night sky. Using this representation, we imagine the stars fixed to the inside of a large sphere encircling Earth, unmoving with respect to each other, but appearing to move as an single entity in our night sky due to diurnal motion, the Earth’s rotation on its axis. As the Earth rotates within the celestial sphere, stars appear to rise in the east, travel across the sky and set in the west. Astronomers use a coordinate system for the celestial sphere much like the coordinate system on Earth. Right Ascension is analogous to longitude, as declination is to latitude. Right ascension is broken into 24 hours, with smaller divisions of minutes and seconds. While not a measurement of time, right ascension is related to time because the entire sky rotates once in a sidereal day , or about 24 hours. Declination is measured in degrees as on the Earth, but is generally listed as positive and negative degrees instead of degrees north and south.

The right ascension and declination coordinate system references the location of an object on the celestial sphere; there is, however, another system that references the location of an object in the sky in relation to the horizon. This is the Altitude-Azimuth coordinate system, and it consists of the apparent altitude of an object in the sky, given in degrees from zero to 90, and the compass direction of the object, given in degrees between zero and 360 (zero being due north, 90 due east, 180 due south, and 270 due west).

Although the coordinates of an object using the right ascension and declination remain virtually unchanged for years, diurnal motion will cause the altitude and azimuth coordinates to change continually over the course of a night. The altitude and azimuth coordinates for an object also differ between locations, while a coordinate in right ascension and declination is a universal system and is the same from any location on the Earth. Although the Altitude-Azimuth coordinate system is useful to determine an object's position in one's local sky, astronomers favour the right ascension and declination system.

The stars of the northern celestial hemisphere appear to circle the north celestial pole, which happens to be near the star Polaris (also known as the North Star). This phenomenon is clearly visible in time-lapse photography of the night sky over a few hours; stars will leave light trails that circle Polaris. One can observe this motion simply by looking at a recognizable or prominent star early in the evening and returning a few hours later to see how the star’s position has shifted in the sky. The further north we are from the equator, the higher the North Star will be in the sky -- at the North Pole, Polaris is directly overhead, at the zenith. Circumpolar stars are near enough to the north celestial pole to never set below the horizon through the course of a year.

 

Developing the Lesson:

Activity 1 (In the computer lab)

Anticipatory Set

Using the planetarium software, focus students' attention on various stars and other celestial bodies. If the software permits, use the retical or cross hair indicator to assist.

Introduction

Begin asking students to identify "exact" locations for any given star. Most students will suggest a cardinal direction or a position relative to the horizon. Have the software play an animation of the sky in hours and then in days. Have students identify difficulties with earlier assumptions of location. It should be clear to students that such a simplistic direction will not be effective in describing the location of any particular star at any given time. Students should then be prompted to think of a alternative way to describe a star's position.

In their notebooks, students should write a brief paragraph hypothesizing an alternative way to describe a star's position at any time.

Once students have had adequate time to compose their paragraphs, begin a demonstration of the Altitude and Azimuth Applet . Ensure all elements of the applet are introduced, including the following:

    1. Angle increment selector
    2. Increasing and decreasing altitude buttons
    3. Altitude diagram and numerical value
    4. Altitude input box and set button
    5. Azimuth diagram and numerical value
    6. Azimuth input box and set button
    7. Constellations (hide/show) button
    8. Retical or Visual guide (hide/show) button
    9. Clock (draggable)
    10. Clock advance forward and back buttons
    11. Stars circled and numbered within the field of view
    12. Text field (you can type here)
    13. Print button
    14. Reset button

Show students how to center the retical or guide on any given star and record (in the applet's text field) an altitude and azimuth. Then have students turn their attention on the clock. Point out to students that this is where their prior assumptions were wrong. Advance the clock slowly for about 2 to 4 hours. Have students notice that the star they just recorded now has a much different altitude and azimuth. Using the back button, return the clock to the original time and record (in the applet's text field) the time of observation.

Draw the students' attention to the questions in the text field and explain how to enter the information they will record into the text field. Make clear that the students are to record the altitude and azimuth for 5 of the 10 identified stars. Also remind students that the observations for the stars must be done according to the specified time of day.

Once the students have completed recording the altitudes and azimuth for the 5 stars, have them continue to answer the questions in the text field.

Hands-On Activity

Once an adequate introduction to the applet has been given, assign one or two students to each computer, and have them begin their task as described above. Instruct the students to use the print feature to print a copy of their observations and their answers to the questions.

Check for Understanding

As the students are working, check to ensure that students are not having difficulties with the applet. At this time, probing questions can be used to check individual student's understandings of the concepts. Ensure students retrieve their printouts.

This should mark the end of the first activity.

Activity 2 (in computer room)

Anticipatory Set

Using the planetarium software, focus students' attention on various stars and other celestial bodies. If the software permits, use the retical or cross hair indicator to assist (same as activity 1; this time students should be able to see the importance of locating celestial objects).

Introduction

Ask several of the students to report their answers to Question 3. Have the other students in the class comment on the suggestions provided. After two or three alternatives have been presented, open the Circumpolar Motion Applet and direct the students' attention to the Right Ascension and Declination buttons.

Click the Right Ascension button to display the right ascension markings on the sky field. Have students think about what the right ascension might be analogous to (recall the Celestial Sphere Applet). Then have students click the Declination button to display the declination markings. Again, have students try to think of an analogous scenario.

Once the students have started to draw correlations on their own, open the Celestial Sphere Applet and draw the students' attention to the red grid on the celestial sphere. By this time, it should be clear to students that right ascension and declination are a much more effective means of describing a celestial body's location in the night sky.

Return to the Circumpolar Motion Applet and again display the right ascension markings. This time add the asterism lines to help students clearly see the progression of the stars on an hourly basis. Have students draw their attention to the clock and then to the right ascension lines on the sky. Point out to the students that the right ascension markings are in units of time (hours, minutes and seconds) corresponding to the elapsed time on the clock. (Note that the clock and the right ascension lines do not have to be the exact same time, as time is relative to the location on the Earth. The right ascension is based on Universal, or Greenwich Mean, time.

Now add the declination markings and have students note that they are in degrees 0 to 90 and 0 to -90, with zero corresponding to the celestial equator (show Celestial Sphere Applet).

Run the Circumpolar Motion Applet with the asterisms shown and have the students note how the right ascension and declination markings appear to "rotate" with the sky; therefore, any given star will have its own unique right ascension and declination that will withstand time (almost).

Independent Practice

Have the students return to the planetarium software to locate and track the progression of the Moon and the planets with respect to the background stars. Have students reflect upon the following questions:

  1. Does the Moon drift at the same rate as the stars? (if not, does it drift slower or faster than the stars?)
  2. Do the planets drift at the same rate as the stars? (if not, do they drift slower or faster than the stars?)
  3. What would be the best way to report the location of the Moon or the planets in the night sky?

As an additional homework activity, students could be asked to write two to three paragraphs clearly explaining the difficulties with the Altitude and Azimuth system and therefore the reasons the Right Ascension and Declination system is superior. Students will need to include a detailed description of the Celestial Sphere and how the Right Ascension and Declination system works. Students should also be asked to expand upon their answers to the 3 questions above.

As a final question, students should be asked if the right ascension and declination for any given object will always be the same? (NO; over thousands of years, the celestial sphere will itself appear to drift as a result of the Earth's precession - this can nicely be shown in a planetarium software advancing the animation in hundreds of years.)

 

 

Closure:

Using the planetarium software, the teacher should find several deep sky objects (galaxies, nebula or star clusters) and help students to comprehend how small these objects appear in our sky, and therefore how difficult locating them can be.

 

Extension:

 

Evaluation:

Students should be evaluated on their homework assignment at the end of activity 2 as well as in their participation and behaviour in the computer lab.

 

Teacher Reflections:

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