INVESTIGATING LUNAR PHASES AST MESA COMMUNITY COLLEGE

Transcription

1 NAME: DATE: INTRODUCTION This lab examines the Moon s motion in the sky and the cause of the Moon s phases. LEARNING GOALS Analyze measurements of the Moon s motion to find repeating patterns over time. Predict the relationship between the Moon s apparent position and its phases. Use planetarium software to collect measurements of the Moon s motion and phases. Test predictions of the Moon s motion and phases as seen from Earth. Draw conclusions on competing models for the cause of the Moon s phases. EQUIPMENT scientific calculator computer with Starry Night software and web browser PREDICTING THE MOON S MOTION AND PHASES In this lab, you will investigate the Moon s motion in the sky and its phases over time. You will begin by making predictions about the Moon s motion and the Moon s changing appearance, known as the lunar phases. There is no penalty for incorrect predictions. Make predictions that make sense to you. 1) Complete each sentence by filling in the blank with a word, a few words, or a phrase. The Moon is up in the daytime sky % of the time. The Moon is up in the nighttime sky % of the time. The Moon completes one cycle of phases in days. The Moon crosses the Sun s ecliptic path once every days. When the Moon is near the Sun in the sky, we see the Moon in its phase. When the Moon is 90 o from the Sun in the sky, we see the Moon s phase. When the Moon is exactly opposite the Sun, we see the Moon s phase. The phases of the Moon are caused by. 1

2 USING THE STARRY NIGHT PROGRAM The planetarium program Starry Night can display the appearance of the sky for any time of day, date of the year, and any location on Earth or in space. The appearance of the sky is simulated to show daytime and nighttime, the positions of the Sun, Moon, planets, and bright stars. An object of interest can be selected to provide additional information. FIGURE 1: Menu bar and information bar in Starry Night Figure 1 shows the time, date, and location displayed in an information bar at the top of the Starry Night window. The program allows the user to change the time (hours, minutes, seconds) and date (month, day, year) of the display by clicking on and typing each value. Alternately, the user can click on a value and press the up and down arrow keys to change the value. Clicking the Now button selects the current real time and date. The user can stop time within the program by clicking the middle Stop button in the Time Flow Rate controls. The arrow buttons on the far left and right of the control step time forward by an amount set by the Time Flow Rate selection. The inner arrow buttons cause time to proceed at that rate per second. The Viewing Location field near the middle of the information bar shows the observer s position. The viewing location can be changed from the menu item Options > Viewing Location or by pressing CTRL + L. You can return to Mesa at any time by pressing the Home button. A series of information tabs are shown along the left side of the Starry Night program window. Double-clicking any of the tabs will display the information panels. For this lab, you will be collecting information about the Moon. Open the Find tab and click the check-box next to Moon. The other tabs will now display information on the Moon. Click the Info tab to display a variety of information about the selected object for the time and date in the Time and Date window. Clicking the + buttons will open or close each panel. Info, Position in Sky, and Other Data will be most useful for this investigation. FIGURE 2: An Info Tab showing Moon Info 2

3 THE MOON IN THE SKY Question #1: Is the Moon above the horizon during daytime? The Full Moon is an obvious feature of the night sky. But is the Moon up during the day? To examine this question, use Starry Night to find the time of moonrise and moonset over the course of a month. Click on the Find tab on the left side of the main window, then click the small down arrow to the left of Moon. From the menu, click Generate Ephemerides and set Start to 12:00 PM (noon) today and End to 12:00 PM two months from today. Choose 3 days as the Time Step and select Altitude, Rise Time, and Set Time. Record 10 observations. Then report whether the Moon was visible at noon on each date. The ephemeris table will have a bold text entry for those dates. TABLE 1: Observations of the Moon at Noon Date Rise Time Set Time Moon up at Noon? 1) Examine the data you collected and identify patterns you find in the position of the Moon at noontime. For what fraction of your observations was the Moon above the horizon at noon? 2) Return to the question: Is the Moon above the horizon during daytime? Answer: If you carefully searched the daytime sky, you would find the Moon on many dates. The answer is yes. The Moon is above the horizon at noon for about half of each month. 3

4 THE LENGTH OF THE CYCLE OF LUNAR PHASES Question #2: How long does it take the Moon to complete its cycle of phases? From night to night, the Moon s appearance changes slightly. Over many weeks, the Moon completes a whole cycle of phases. This interval is known as the synodic month. Use Generate Ephemerides and set Start to 12:00 PM (noon) today and End to 12:00 PM two months from today. Choose 1 day as the Time Step and select Local Time and Disk Illumination from the Show table. Then click Generate Values. Look for the first date of the Full Moon (when Illumination first reaches 100%). Record the first date in the first row of the table below. Then look for the second date of the Full Moon in the ephemeris. Count the number of days between the two Full Moons. Record the Time Interval below. Return to Generate Ephemerides and change the Start and End dates to one year ago. Look for the first and second dates of the Full Moon in this time range and record them in the second row of the table. Count the number of days between these two Full Moons. Record the Time Interval. Return to Generate Ephemerides once more, choosing Start and End dates one year from now. Record the first and second dates of the Full Moon for this time range in the third row of the table. Then count and record the Time Interval in Days. TABLE 2: Observations of the Moon s Cycle of Phases Date of first Full Moon Date of second Full Moon Time Interval in Days Average length of cycle: 3) Finally, calculate the average length of the lunar cycle using the 3 time intervals you found. Record your average in the bottom cell of Table 2. Round to the nearest decimal place. 4) Return to the question: How long does it take the Moon to complete its cycle of phases? 4

5 LUNAR PHASES AND SUN ANGLE Question #3: Are lunar phases related to the angle between the Sun and Moon in the sky? In Starry Night, select from the menu bar Favorites > A Earth, Moon, and Sun > Moon > Phases from Space. This view displays Earth at the center, the Sun on the ecliptic (outer ring), and the Moon s appearance as viewed from Earth. 5) Use this display to collect phase observations. Record your observations (date of observation, phase name, sketch of Moon phase, and angle between Sun and Moon) in the table below. Be sure to clearly label the column titles. Choose any date to begin observations and collect data every 3 days. TABLE 3: Lunar Phase versus Sun-Moon angle 6) Describe patterns you find in the data, and then answer Question #3. Support your conclusion with observations you collected. 5

6 THE MOON S PATH THROUGH THE STARRY SKY Question #4: How often does the Moon cross the ecliptic path of the Sun? You can go to the starry sky view in Starry Night by clicking the Home button. You can also turn off the horizon and daylight by pressing CTRL+H and CTRL+D. You can display the Sun s ecliptic path from the menu bar View > Ecliptic Guides > The Ecliptic. You can also lock North to be the top of the display by choosing Options > Orientation > Equatorial. 7) Use the space below to describe your data collection method, record your measurements, and describe patterns you find. 8) Answer Question #4. Support your conclusion with measurements you collected. 6

7 EXAMINE THE MOON S BEHAVIOR 9) Devise a question of your own regarding the Moon s motion or appearance in the sky. Investigate this question using the research skills you have developed in this lab. Use the space below to state (i) your research question, (ii) outline a data collection method, (iii) record measurements, (iv) analyze patterns, and (v) answer your research question. Question #5: Method and Measurements: Observed Patterns: Answer (with supporting measurements): 7

8 TESTING PREDICTIONS At the beginning of the lab, you made several predictions about the Moon s motion and its appearance in the sky. Re-examine your predictions and then judge whether each prediction is supported or falsified by measurements. Cite specific measurements for each prediction. The appearance of the Moon in the daytime and nighttime sky: The duration of the Moon s cycle of phases: The crossing of the ecliptic by the Moon: The relationship between the Moon s phases and the Sun-Moon angle: The cause of the Moon s phases: 8