Phases of The Moon and Eclipses Introduction - Lab 7 | AST 112, Lab Reports of Astronomy

Download Phases of The Moon and Eclipses Introduction - Lab 7 | AST 112 and more Lab Reports Astronomy in PDF only on Docsity! LAB #7: PHASES OF THE MOON & ECLIPSES INTRODUCTION: The moon orbits the earth once each month, and this orbital motion leads to two well known effects: the cycle of lunar phases and eclipses. Understanding the causes of these phenomena is the subject of this exercise. The material is divided into two parts. The first part treats lunar phases while the second deals with eclipses. TYPE: Indoor MATERIALS: Notebook, pencil, straightedge OBJECTIVES:  Be able to identify a given phase of the moon, tell the order in which the phases occur, and describe the cause of the phases.  Be able to estimate the approximate rising and setting times of the moon given the moon’s phase.  Understand why the moon always keeps one face toward the earth.  Be able to name the types of lunar and solar eclipses and know the phase of the moon at which they can occur.  Be able to make a drawing that illustrates how any given type of eclipse occurs.  Know why eclipses do not occur each month. BEFORE THE LAB: A. Phases of the Moon The earth, moon, and planets in our solar system are spherically shaped objects that orbit the source of light, the sun. Obviously, only half of each of these spherical objects can experience sunlight at any moment. In the case of the moon, one hemisphere is always illuminated. However, the entire illuminated face moon cannot usually be seen from the earth. The relative positions of the moon, earth, and sun at a given time determine what fraction of the illuminated face is visible. As examples, when the moon is in that part of its orbit that is opposite in the sky 1 from the sun, then the fully illuminated face can be seen (see the first illustration), but when moon in a position such as shown in the second illustrations then only part of the illuminated face would be seen from earth. 2 to its orbit. These two things cause the moon to appear to "rock" a little as it orbits, both from side to side and also a bit up and down. These apparent rocking motions are called librations. Because the earth is moving in its orbit about the sun, the cycle of phases, or synodic period, of the moon is slightly longer (29.5 days) than the sidereal period (the true orbital period of the moon). Study the following diagram/ The figure above illustrates that in the 27.3 days it takes the moon to make one complete revolution around the earth relative to the stars, the earth has moved in its orbit about the sun. Because the earth’s motion, the moon must travel a little farther around its orbit to return to full moon phase. Therefore, it takes a about 2.2 days longer than the orbital period for the moon to complete its phase cycle (for example, from one full moon to the next). 5 Earth Earth 2.2 more days 27.3 days for one complete revolution relative to the stars Full moon Full moon Waxing Gibbous B. Lunar Eclipses Eclipses occur when the moon and the earth are so aligned with the sun that the shadow of one of these falls on the other. If the earth is between the sun and the moon, then we have a lunar eclipse (when the earth’s shadow falls on the moon). The figure above demonstrates three possible cases of lunar eclipses, which occur when the moon enters the earth's shadow. If the moon is in the penumbra of the earth's shadow, there is a penumbral eclipse. In this case, some direct sunlight reaches all of the moon facing the earth, and the entire moon merely diminishes in brightness. Most penumbral eclipses pass by unnoticed and are therefore usually not counted as real lunar eclipses. When the moon is partially immersed in the earth's umbra, part of the moon is removed from direct sunlight, and we have a partial lunar eclipse. Finally, if the moon is completely within the umbra, we have a total lunar eclipse. During a total lunar eclipse, the part of the moon that should be completely dark often has a faint red glow to it. The moon then glows because the earth's atmosphere refracts some sunlight into the umbra of the earth. But in passing through the atmosphere, the blue light from the sun is scattered much more than red light and so is removed. The light finally entering the umbra has a definite reddish hue to it. Thus, even at midtotality the moon remains visible as a copper-red disc in the sky. From the diagram it can be seen that a lunar eclipse can be witnessed by anyone on the night side of the earth. A lunar eclipses is therefore visible over about half the earth’s surface. 6 SUN Earth Penumbra Umbra Sun Earth C. Solar Eclipses A less common occurrence is a solar eclipse. In a solar eclipse the moon is aligned between the sun and the earth. Because the moon is smaller than the earth, the lunar shadow never covers the earth completely. This figure illustrates the geometry of a solar eclipse. For somebody standing within the umbra, the moon blocks the entire sun and a total solar eclipse is viewed.. In the penumbra surrounding the umbral area only part of the sun’s direct rays are blocked and somebody standing within this region views a partial solar eclipse. The lunar orbit is somewhat elliptical, and the mean distance between the earth and the moon varies from about 363,000 to 406,000 km (226,000 to 252,000 miles). Because the moon's orbit is not perfectly circular, some solar eclipses occur when the moon at the more distand parts of its orbit and thus appears smaller. Solar eclipses that occur at times when the moon appears smaller than the sun are called annular solar eclipses. A diagram of an annular solar eclipse looks similar to that of a total solar eclipse; however, since the moon is somewhat further away from the earth, the umbra converges before it reaches the earth's surface. 7 Penumbra Umbra (1) (2) S U N Penumbra Umbra (1) (2) S U N 10 CUT ALONG THE DOTTED LINES 11 In co m in g s un lig h t Mid- night 6 PM 6 AM Noon 9 PM 3 AM 9 AM 3 PM 1st QTR Waning Gibbous Full Moon Waxing Gibbous Waxing Crescent New Moon 3rd QTR Waning Crescent The non-shaded regions of the moon are the illuminated regions E W 12