Observing Projects
Observe the Moon on each clear night over the course of a month. On each night, note the Moon’s location among the constellations and record that location on a star chart that also shows the ecliptic. After a few weeks, your observations will begin to trace the Moon’s orbit. Identify the orientation of the line of nodes by marking the points where the Moon’s orbit and the ecliptic intersect. On what dates is the Sun near the nodes marked on your star chart? Compare these dates with the dates of the next solar and lunar eclipses.
It is quite possible that a lunar eclipse will occur while you are taking this course. Look up the date of the next lunar eclipse in Table 3-1, on the World Wide Web, or in the current issue of a reference such as the Astronomical Almanac or Astronomical Phenomena. Then make arrangements to observe this lunar eclipse. You can observe the eclipse with the naked eye, but binoculars or a small telescope will enhance your viewing experience. If the eclipse is partial or total, note the times at which the Moon enters and exits Earth’s umbra. If the eclipse is penumbral, can you see any changes in the Moon’s brightness as the eclipse progresses?
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. (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 between the Moon-Sun and Moon-Earth lines at the Moon is greater than 90°?
Use Starry Night™ to study the Moon’s path in the sky and the phenomenon of eclipses, both lunar and solar. Select Favourites > Explorations > Moon Motion to display the sky as seen from the center of a transparent Earth on August 30, 2010, with daylight removed and the Moon centered in the view. As a guide to the positions and motions in the sky, the celestial equator and the associated equatorial grid are shown. The ecliptic, representing the plane of Earth’s orbit extended onto the sky, is also shown. This line also represents the apparent path of the Sun across our sky in the course of a year. The Time Flow Rate is set to 1 hour but this can be adjusted if necessary to follow the Moon across the sky. (a) Is the Moon on the ecliptic at this time? (b) With the view locked onto the Moon, click the Play button. In which direction does the Moon move against the background stars? Keep in mind that the celestial equator runs in the east-west direction with east on the left. Note also that the ecliptic intersects this equator at an angle of 23½°. Does the Moon ever change its direction of motion relative to the stars? Why or why not? Describe its path relative to the ecliptic. (c) Determine how many days elapse between successive times that the Moon is on the ecliptic. (d) Set the Time and Date to 11 p.m. on July 21, 2009, and set the Time Flow Rate to 1 minute. Note that the Moon is almost on the ecliptic and that it is close to the position of the Sun in the sky. Run time forward at this 1-minute rate and stop time when the Moon is closest to the Sun. Zoom in to a field of view of about 3° and adjust the time in single steps to move the Moon to the position of maximum eclipse. What is the time of maximum eclipse? (e) Sketch the Sun and Moon on a piece of paper. Even though the Moon may not be directly over the Sun on the screen, an eclipse is occurring. What type of eclipse is it? Why is the Moon not directly over the Sun at this time? (f) Click on the Events tab, expand the Event Filters layer, and deselect all but the Lunar and Solar Eclipse Events. Expand the Events Browser layer, set the Start and End dates to cover the period from January 21, 2009, to January 21, 2010, and click Find Events. Right-click on each solar eclipse in turn, choose View Event, Zoom out to about 5°, and Run time forward and backward at a rate of 1 minute until the Moon is again closest to the Sun. Draw the Sun and Moon. Are they separated by the same amount as in part (d)? If not, why not? If there is a difference, what effect does it have on the eclipse?
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? (c) Is it possible for a lunar eclipse to have a longer duration of totality than the one depicted in this simulation? Explain your answer.
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Collaborative Exercise
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.