Student Guide to the Lunar Phase Explorer Applet

Transcription

1 Nebraska Astronomy Applet Project Student Guide to the Lunar Phase Explorer Applet Pre-test in EDU Before working through the student guide, take the EDU pre-test and record your score at right. Background Material Exercises Directions: Work through the following exercises as you read the background pages. Page 1 Introduction to Moon Phases True False The moon can become darkened by the shadow of the earth. True False This phenomenon causes the phases of the moon. True False There is a dark side of the moon. True False This dark side is permanently in shadow. True False There is a side of the moon we can't from earth. True False This side is permanently in shadow. How long does it take the moon to complete one cycle of phases, in days? If the moon is full today, what phase do you expect it to be at in a week? How about one month later? Many words in astronomy also non-astronomical uses as well. Using your knowledge of how the terms on the left are used in astronomy match them with the nonastronomical uses on the right. waning gibbous waxing convex, rounded -- also hunch-backed, having a hump to increase in size, quantity, volume, intensity, etc. decrease in magnitude, importance, brilliancy, intensity, etc.

2 The following sketches of the moon's appearance were made over about four weeks. Identify the phases and put them in the correct numerical order. The first one is labeled for you. Picture Order Phase Picture Order Phase A D B 1 waning gibbous E C F Page 2 Introduction to Moon Phases From the perspective of an observer above the North Pole, the moon moves clockwise / counter-clockwise (circle) in its orbit around the earth. In the diagram below the sun's light is coming in from the right. The moon's location is marked at several points on its orbit. These are the points the moon was at when the sketches above were drawn. Identify each position with the letter of the corresponding sketch.

3 Page 3 The Time of Day Use the interactive diagram at the bottom of the page to determine the direction of the earth s rotation when viewed from above the North Pole. (Hint: rotate the observer the stickfigure to the noontime position, then sunset position, then midnight position, and finally back to sunrise position. The earth has made one complete rotation and the observer has experience one daily (diurnal) cycle of day and night.) When viewed from above the North Pole, does the earth rotate clockwise or counter-clockwise? Page 4 Rising and Setting When the moon crosses the western side of the horizon plane it is rising / setting (circle). When it crosses the eastern side of the horizon plane it is rising / setting (circle). Page 5 The Horizon Diagram Describe the location of the moon in the sky of the horizon diagram at bottom. Use direction words (like north, west, etc.) and estimate its altitude in degrees. Page 6 The Witness and the Detective If we know the moon's position in the sky and its phase, we can estimate the. In general, knowing any two of the following three things allows us to estimate the third: 1. moon's position in the sky 2. 3.

4 The Applet Exercises Directions: Open the Lunar Phase Simulator applet and read the following directions, then proceed with the following exercises. The main part of the applet shows a simplified earth-moon system viewed from above the earth s North Pole. Sunlight illuminates the earth and moon from the left. During the simulation the moon revolves around the earth and the earth rotates about its axis. There are several ways to adjust the orientations of the earth and moon. By checking the animate checkbox you can stop and start the simulation (use the slider at left to control its speed). You can also advance and rewind the simulation in discrete steps using the increment animation buttons. You can also drag the earth and moon directly. Click on the moon and move the mouse while holding the mouse button down to move it anywhere on its orbit. Do the same for the earth to rotate it about its axis. A stickfigure represents our North American observer. Practice moving the observer around to different positions, as you will need to do this throughout the lab. When show time tickmarks is checked, tickmarks appear around the earth that indicate the local times for observers around the globe. The upper right part of the applet shows the appearance of the moon from the earth, and identifies both the phase and the percentage of the moon's disk that is illuminated. The phase name drop down list can be used to move the earth. Exercise 1 Check the animate checkbox and watch the moon orbit the earth as the earth rotates. When viewed from this perspective, what direction, clockwise or counterclockwise, does the earth spin? What direction does the moon move in its orbit? Exercise 2 Each row on the following table shows diagram of the earth-moon system. For each diagram, find the age of the moon at that position (that is, the time passed since new moon), its phase, and its percent illumination. Finally, make a sketch of its general appearance. You will need to take into account the orientation of the sunlight it is different in each diagram from the orientation in the applet. The first row is completed for you.

5 Moon Geometry Age Phase Percent Illumination Sketch 11 days, 9 hours Waxing Gibbous 88%

6 Exercise 3 When observing the moon one thing we might like to know in advance is when it is visible what time it sets, rises, and crosses the meridian (or transits). The applet can help find these times. Work through the following two examples, and then complete the rest of the table. Example 1: What is the setting time for a full moon? First we move the moon to the full position by dragging it, or selecting Full Moon in the phase name drop down list. Next, click on and rotate the earth while keeping an eye on the horizon diagram in the lower right corner. Rotate the earth until the moon just disappears below the western horizon. You should verify that this occurs at 6:00 AM. (Note that the animate checkbox should be unchecked, and don t use the increment animation buttons we want the moon to stay stationary while we rotate the earth.) Example 2: What is the meridian crossing (transit) time for a new moon? Move the moon to its new position. Rotate the earth until the moon is centered on the meridian. For finding transit times it helps to change the perspective of the horizon diagram (by clicking and dragging on it) so that we are looking straight down on the diagram. We find that the transit time of the new moon is 12:00 PM (noon). New Waxing Crescent First Quarter Waxing Gibbous Full Waning Gibbous Last Quarter Waning Crescent Rising Meridian Crossing Setting 12:00 PM 6:00 AM Exercise 4 In the background information pages we learned that although the primary phases new, first quarter, full, and last quarter occur instantaneously, we still use these labels to describe a moon that looks like close to a primary phase. In this exercise we estimate how close is close enough for casual observation. With the animation stopped, use the increment buttons or drag the moon around to the point where the illuminated moon first looks round. That is the point where you can barely perceive any shadow on the moon. Bear in mind that the moon is magnified about 5x compared to what you can see with the naked eye, so don t be too finicky.

7 How old is the moon at this time? Now find the point where the illuminated moon stops looking round. How old is the moon at this point? Based on your answers, about how long does the full moon appear to last? Use the method again to determine when the waxing crescent moon starts to look half lit. Moon s age when it first looks like a first quarter: Moon s age when it noticeably is a waxing gibbous: So, how long does a first quarter moon appear to last? Which phase, first quarter or full, appears to last longer? Is it reasonable to expect the moon to look like a first quarter moon for several nights in a row? How about a full moon? Exercise 5 Rotate the observer to the noontime position, and move the moon to 2 days after new. Where is the sun in the observer s sky? (I.e. what is its direction and altitude?) Where is the moon in the sky?

8 Draw the general appearance of the moon in the space at right. What percentage of the moon s disk is illuminated? Several factors determine the visibility of the moon, including the brightness of the sky, glare from the sun, the moon's phase, and of course its rising and setting times. (We ignore additional local factors like trees, streetlights, and weather.) Would noon be a good time to observe a 2 day old moon? Why or why not? Some cultures place special significance on recording the first visibility of the lunar crescent. When is the best time to spot a thin waxing crescent? Where in the sky should an observer look?