Observing the Constellations of the Zodiac

Activity UCIObs – 3 Grade Level: 8 – 12

Source: Copyright (2009) by Tammy Smecker­Hane. Contact [email protected] with any questions.

Standards: This activity addresses these California Science Content Standards. Gr 8: Earth Sciences 4b. Students know that the Sun is one of many stars in the Milky Way galaxy and that stars may differ in size, temperature, and color. 4d. Students know that stars are the source of light for all bright objects in outer space and that the Moon and planets shine by reflected sunlight, not by their own light. 4e. Students know the appearance, general composition, relative position and size, and motion of objects in the solar system, including planets, planetary satellites, comets, and asteroids. Gr 9–12 Earth Sciences: 1d. Students know the evidence indicating that the planets are much closer to Earth than the stars are. 2d. Students know that stars differ in their life cycles and that visual, radio, and X­ray telescopes may be used to collect data that reveal those differences.

Gr 9–12: Investigation & Experimentation 1d. Formulate explanations by using logic and evidence. 1g. Recognize the usefulness and limitations of models and theories as scientific representations of reality. 1i. Analyze the locations, sequences, or time intervals that are characteristic of natural phenomena (e.g., relative ages of rocks, locations of planets over time, and succession of species in an ecosystem).

4A­1 What's This Activity About? Tips and Suggestions:

This activity teaches students about the You can either construct the Zodiac demos constellations of the Zodiac and when they yourself beforehand or have the students can be observed as a function of the month make it themselves. and time of day/night. After this activity, students would enjoy What Will Students Do? observing the night sky and verifying their Groups of 3 to 4 students will construct predictions. Free night sky maps for given and use a simple hand­held model to months can be obtained on the web at explore which Zodiac constellations are http://skymaps.com. visible at a given time and month.

What Will Students Learn?

Concepts: Motion of Objects in the Night Sky (Daily and Yearly) Inquiry Skills: Visualizing, Inferring Big Ideas: Spatial Reasoning, Patterns of Change

What you will need:

1 photocopy of Zodiac.ppt on cardstock 1 medium­sized ball to represent the Sun 1 photocopy of the exercise (pages 12­13) for each group 1 photocopy of Figure 1 (page 3) per group; if possible use card stock rather than normal paper for rigidity 1 photocopy of Figures 2 and 3 (pages 4­5) per group on sheets of transparency “paper” Hole Punch One brass fastener per group Scotch Tape

Teacher Preparation:

1. If you are unfamiliar with basic astronomy please read Sections I­VI in the accompanying document entitled Night_Sky_Lectures.pdf by T. Smecker­Hane. This will teach you all you need to know to do this experiment.

4A­2 2. Make one copy of the Zodiac Demo beforehand if you are going to have the students construct their own or make one Zodiac Demo per group beforehand if you do not have time for students to construct their own in class. If students will make the Demos in class then we recommend you punch the holes in the Figures beforehand. Note that uou can reuse these demos from year to year if you use cardstock paper, which is thicker than normal paper for Figure 1.

Demo Assembly Instructions: Step 1: Punch a hole through the Earth on Figure 1 (normal paper or card stock) and a hole through the Earth on Figures 2 and Figure 3 (transparency papers). Put Figure 2 on top of Figure 1 and Figure 3 on top of Figure 2 and fasten all three together with the brass fastener. For added durability, tape down the ends of the fastener to the backside of the cardstock.

Note that you could make one Demo entirely out of transparencies (substituting transparency paper for cardstock for Figure 1) if you want the class to use it collectively by showing it on an overhead projector. This would save you from making additional Demos, but students get a lot more out of using them individually and hence that is what we recommend.

3. In the Zodiac.ppt that you printed out, page 2 and 3 are the front and back sides of the NSEW orientation card. Tape the two together with the printed sides facing outward.

Teacher’s Classroom Instructions:

1. Pass out the Zodiac demos or have students put them together. Assembly Instructions: Step 1: Punch a hole through the Earth on Figure 1 (normal paper or card stock) and a hole through the Earth on Figures 2 and Figure 3 (transparency papers). Put Figure 2 on top of Figure 1 and Figure 3 on top of Figure 2 and fasten all three together with the brass fastener.

2. Give a lecture to the class that goes over the material in Sections I­VI of Night_Sky_Lectures.pdf. The most important lesson is that an object's location in an observer's sky varies daily and monthly because: (1) the Earth rotates on its axis once per day, and (2) the Earth revolves around the Sun once per year.

3. Now do a three dimension simulation of the ecliptic plane by asking the students to form a wide circle in the classroom then hand out the 12 constellations of the Zodiac, each photocopies on its own piece of cardstock (see Zodiac.ppt), to students evenly spaced

4A­3 around the circle. Be sure to get the order of constellations/RA/month correct; refer to Figure 1. Choose one student to be the observer on the Earth and give them the NSEW orientation card to hold in front of them and place them in the center of the circle. Note that one side is designated to face them and the other side is designated to face the class.

4. Lead the class through the following examples taken from Section VI of Night_Sky_Lectures.pdf. Tell the class the following:

The 12 Constellations of the Zodiac are the constellations that the Sun appears to pass through during one year, as the Earth orbits the Sun.Viewed from above looking down on the North pole of the Earth, the Earth rotates counter­clockwise once a day, and the Sun appears to move counter­clockwise through the constellations of the ecliptic plane at a rate of 1 constellation per month, or 2 hrs of Right Ascension (RA) per month. (Remember, however, that in reality it is the Earth that orbits the Sun!) On each constellation card is marked the RA and month in which the Sun, as viewed from Earth, lies in the direction of that constellation.

Tell students to image that the student who is the observer on the Earth is located somewhere near the equator and is lying on his back looking straight up at the zenith. Using the student who is the observer, show that an observer on Earth can only see one half the celestial sphere at any given time. The other half is hidden by the Earth.

Give the Sun (the ball) to one of the students holding a constellation card. What month are you simulating? Give the observer the NSWE orientation card and have him/her turn counterclockwise (as viewed from above) to mimic the Earth turning on its axis. Note that objects rise on his/her Eastern horizon and set on his/her Western horizon. Next demonstrate that the time of day for the observer on the Earth depends on where the Sun is in his/her sky. At noon, the observer is pointed directly at the Sun (Sun is closest to the zenith) and when the observer is pointed directly away from the Sun it is midnight. When the Sun is on the observer's Eastern horizon it is 6 am (sunrise), and when the Sun in on the observer's Western horizon it is 6 pm (sunset).

Examples: Orient the Earth & Sun as in Figure 4, which shows an observer who sees the constellation Virgo closest to zenith, Sagittarius is rising on the eastern horizon, and Gemini is setting. The Sun is located in Gemini and the month of the year is July. From the relative positions of the Sun, Earth and observer, we know that the observer's time of day is approximately 6 pm because the Sun is setting on the observer's Western horizon. Do the other examples in Figures 5 and 6. 4A­4 This teaches you an important lesson in observational astronomy: an object in the sky that you may want to study usually is available in the night sky for only a certain period of time during the night, and may only be accessible during certain months of the year.

5. Now divide the students into groups with 3 to 4 students per group and let them use the Zodiac Demos to do the same 3 examples.

Tell the students that when its not practical to recreate a three dimensional ecliptic plane, we can use these simple Zodiac Demo to see what constellations of the ecliptic are visible on a given date and time. The half of the sky that the observer can see is represented by the semicircle with the middle of the semicircle centered on the observer. As the Earth rotates counter­clockwise, the observer and the semicircle also rotate which means different constellations are brought into view (different constellations lie inside the semicircle). The eastern horizon (E) lies at the leading edge of the semicircle, the western horizon (W) lies at the trailing edge, and the observer's zenith, the point in sky most directly overhead, lies in the middle of the arc of the semicircle. The Zodiac demo shows these constellations seen as viewed from above looking down on the North pole of the Earth. From this vantage point, the Earth rotates counter­clockwise once a day, and the Sun appears to move counter­ clockwise through the constellations of the ecliptic plane at a rate of 1 constellation per month, or 2 hrs of Right Ascension (RA) per month. (Remember, however, that in reality it is the Earth that orbits the Sun!)

Remember that, for an observer, noon is defined to be the time when the Sun is most directly overhead, which would be when the Sun lies in the middle of the arc of the semicircle. Midnight is 12 hrs later when the Earth has rotated 180 and the observer is facing in the opposite direction. Sunset is approximately at 6 pm when the Sun is setting on the Western horizon, and sunrise is approximately at 6 am when the Sun is rising on the Eastern horizon.

6. Next assign the exercise (pages 15­16) to the students to do themselves.

Exercise Answers:

1. In September at 6 pm, the Sun is in the constellation Leo. Scorpio is highest overhead (nearest the zenith), Aquarius is rising, and Leo is setting.

4A­5 2. To show September at 8 pm, we rotate the observer's semicircle for the problem above by (10 – 6) hrs x 360 degrees / 24 hrs = 4 x 360 / 24 degrees = 60 degrees (2 constellations) in the clockwise direction. Capricornus is at the zenith, Aries is rising and Libra is setting.

3. When Virgo is on the observer's Eastern horizon and the time of day is 6 pm, the Sun must be setting on the Western horizon. Thus the Sun lies in Pisces and the month is April. Therefore, April is the best time to view Virgo because it is visible in the nighttime sky for 12 hours, i.e., all night long.

4. When the Sun is in Sagittarius in January, Aquarius is on the Eastern horizon and rising when the Zenith points to Scorpio. This is one constellation before the Sun reaches the zenith so it is (1 constellation x 24 hrs/12 constellations) = 2 hrs before noon, which is 10 am. Aquarius rises during daytime, and it sets at 2 hours before midnight (10 pm). Aquarius is visible in the nighttime sky from approximately 6 pm (when the Sun sets) until 10 pm. Therefore, Aquarius is visible in the nighttime sky for only 4 hours in January.

Optional Add­Ons

I. Nighttime Observations

You can have the class predict which constellation is highest in the night sky at a given time, say 8 pm, and have them go out and verify their prediction. You also can ask them to go out at 10 pm and verify that this constellation has moved approximately 2 hr x 360 degrees / 24 hrs = 30 degrees further to the west. Which constellation is highest overheat at 10 pm?

II. What is your Zodiac Sign?

Some inquisitive students might notice that on Figure 1 the Sun is not in the constellation of their “Zodiac Sign” in the month that they were born. Thousands of years ago, when the Greeks defined these constellations about the 6th or 5th century BC, the Sun indeed appeared in their “Zodiac Sign” constellation in the month when they were born. However, the axis around which the Earth rotates has slowly been changing its orientation in space (like a spinning top will do, especially as it slows down; this is called nutation) . The change has been appreciable over time, resulting in the Sun now being nearly one constellation removed from the original definition.

4A­6 Figure 1. This figure shows the Earth and the 12 Constellations of the Zodiac as seen by an outside observer looking down on the Earth's North Pole. The names of the constellations are marked as well as their Right Ascension, which is given in hours (0 to 24 hrs), and the month of the year in which the Sun appears in that constellation.

4A­7 Figure 2. Imagine an observer at the Earth's equator. This figure shows the view from an outside observer looking down on the Earth's North pole. The semi­circle represents the half of the night sky that the observer on Earth can see, and the semi­circle rotates with the Earth in the counter­clockwise direction, as shown by the arrow. The zenith is the direction directly over the observer's head, perpendicular to the Earth's surface. Objects will come into view (rise) on the observer's Eastern horizon (marked E) and objects will disappear from view (set) on the observer's western horizon (marked W).

4A­8 Figure 3. This figure represents the Sun and its location with respect to the 12 Constellations of the Zodiac. When put the Zodiac demo together be sure to center the circle below on the Earth in the other figures.

4A­9 Figure 4 ­ In July, the Sun lies in the constellation of Gemini. At sunset, approximately 6 pm, Virgo is directly overhead, Sagittarius is rising on the Easter horizon and Gemini is setting on the Western horizon.

4A­10 Figure 5 ­ At midnight in July, Sagittarius is most directly overhead, Pisces is rising on the Eastern horizon and Virgo is setting on the Western horizon.

4A­11 Figure 6 ­ If you wanted to observe star­forming clouds near the constellation Taurus with a telescope, what would be the best time of year to do it? You would want to be able to see Taurus for the longest amount of time during the night. That means you want Taurus to be rising at sunset, overhead at midnight, and setting at sunrise. If we draw the semicircle centered on Taurus at midnight then the Sun must lie in the constellation directly opposite to Taurus, which is Scorpio. Thus December would be the optimal month to observer the star­forming clouds in Taurus.

4A­12 Date: Per: Constellations of the Zodiac Exercise

Write the names of the students in your group below:

Draw a sketch to illustrate your answers in order to receive full credit for each of the following questions. Use the back sides of the paper, if necessary. Use your Zodiac demo to answer the questions, keeping in mind these important facts: l The Earth orbits the Sun in 1 year, which is 12 months, so the Sun appears to move through the12 Zodiac Constellations at a rate of 1 constellation per month. l The Earth rotates around its axis once per day, which is 24 hours. In this demo, the Earth will rotate in the counter­clockwise direction. l On the transparency paper, the large semi­circle represents what half of the sky the observer can see. The other half of the sky is hidden by the Earth. l The time of day for an observer on Earth depends on the relative location of the Sun. Noon is when the Sun is most directly overhead or nearest the zenith. At dawn, approximately 6am, is when the Sun is on the observer's Eastern horizon (E) and is rising. At sunset, approximately 6pm, is when the Sun is on the observer's Western horizon (W) and is setting. l Assume the Sun rises at 6 am and sets at 6 pm (ignore daylight savings time).

Questions:

1. In September at 6pm, which Zodiac constellation is highest overhead at 6 pm? Which constellation is rising? Which constellation is setting?

4A­13 2. In September, what Zodiac constellation is highest overhead at 10 pm? (Hint: the Earth completes one full rotation around its axis in 24 hrs.) Which constellations are rising and setting?

3. If you want to observe galaxies in the Virgo cluster of galaxies, which lies in the constellation Virgo, in what month could you start observing at 6 pm when Virgo is just beginning to rise?

4. In January, when does the Zodiac Constellation Aquarius rise? For how many hours is it visible in the nighttime sky in January? (Hint: When does the Sun rise/set and when does Aquarius rise/set?)

4A­14