lecture4 - Discovering the Universe for Yourself Lecture 4...

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Unformatted text preview: Discovering the Universe for Yourself Lecture 4 Quick Review of Celestial Sphere 1. Phases of the Moon 2. Eclipses 3. Retrograde motion of the planets A planet has 45˚ axis tilt and rotates once every 48 hours. 3 minute paper: Name and MSU NetID at the top. 8.5”x11” (no partial sheets). Write complete sentences describing what this planet would be like. Be as thorough as you can. Grade: 1 to 3 points depending on quality. (0 points if nothing is turned in.) When time is called, pass your paper to the right. The Moon passes through one cycle of phases in about 29.5 days Why do we see phases of the Moon? Phases occur because our view of the side of the Moon lit by the Sun changes as the Moon orbits Earth 1 Waxing = growing Waning = shrinking Phases of the moon First or Third Quarter = “Half Moon” Gibbous = not quite full Figure 2.19 Waxing = growing Waning = shrinking First or Third Quarter = “Half Moon” Moon rise and set Gibbous = not quite full Notice that we always see the same side of the Moon What is often called the “dark side” of the Moon should be called the “far side” of the Moon The Moon always shows the same face to us because it rotates once each time it orbits once 2 What causes eclipses? A solar eclipse happens when the moon blocks our view of the Sun Solar eclipse: Moonʼs shadow on Earth Lunar eclipse: Earthʼs shadow on Moon Moonʼs angular size in sky is almost identical to Sunʼs angular size: about half a degree Cause of eclipse Angular size of moon Both Sun and Earth lie in the ecliptic plane Moonʼs orbit is not in the ecliptic plane Ecliptic plane Moon orbit ecliptic 3 Moonʼs orbit crosses the ecliptic plane at two points called nodes Moon orbit with nodes Eclipses occur when Moon is new or full near the nodes You see a total solar eclipse if you are in the umbra You see a partial solar eclipse if you are in the penumbra If your view of the Sun is completely blocked, you are in the umbra If the Moon is too far for the umbra to reach Earth, an annular solar eclipse can occur If your view of the Sun is only partially blocked, you are in the penumbra Figure 2.23 (Interactive) Both Moon and Sun move appear to move from east to west with celestial sphere because of Earthʼs rotation Evolution of total solar eclipse However, Moonʼs orbit carries it from west to east on the celestial sphere by about 1/2 degree per hour 4 Checking out the eclipse progress Hereʼs the one I saw in March 2006 Partial sun images through the key card totality 360-degree sunset Paths of total solar eclipses for the next 2 decades: the MSU Planetarium is advertising a trip to China for next summer. (See folder on Angel: same lead organizer.) 5 Because the planets orbit the Sun, their positions on the celestial sphere change with time What was mysterious about planetary motion in our sky? Usually they appear to drift from west to east on the celestial sphere 2002 planetary alignment East West Occasionally, a planet will appear to drift from east to west on the celestial sphere -- a behavior known as apparent retrograde motion Our changing perspective when we pass another planet causes its apparent path on the celestial sphere to change directions We see apparent retrograde motion when Earthʼs orbit carries it past another planet Why did the ancient Greeks reject the real explanation for planetary motion? (Animation 2.32) 6 Finger/eye test question: Which finger appears to shift more against the wall (background) when you alternately open and close each eye? A. Finger close to my eyes B. Finger far from my eyes C. It doesn’t matter. Parallax is the apparent shift in position of a nearby object as our point of view changes. The Greeks believed that Earth was standing still because the parallax of stars is undetectable with our eyes. 7 ...
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This note was uploaded on 11/13/2011 for the course ISP 205 taught by Professor Donahue during the Fall '08 term at Michigan State University.

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