Why we see the Moon go through phasesWhy we always see the same side of the MoonThe differences between lunar and solar

Slide 2:Why we see the Moon go through phases Why we always see the same side of the Moon The differences between lunar and solar eclipses Why not all lunar eclipses are total eclipses

Why solar eclipses are visible only from certain special locations on Earth How ancient Greek astronomers deduced the size of the Earth In this lecture, you will learn

Slide 3:Last time we talked about the illumination of the Earth by the Sun and how the Earth orbits the Sun. As everyone probably knows the Moon orbits the Earth, as the Earth orbits the Sun. Just as for the Earth, at any given time the Moon is half illuminated � the side facing the Sun. This picture shows the Earth and Moon as photographed by a space probe on it�s way to Jupiter � The Sun is far to the left in the picture and we see the Earth and Moon �hanging out in space� and basking in the Sun�s rays. Figure 3-1 The Earth and the Moon This picture of the Earth and the Moon was taken in 1992 by the Galileo spacecraft on its way toward Jupiter. The Sun, which provides the illumination for both the Earth and the Moon, was far to the right and out of the camera�s field of view when this photograph was taken. (NASA/JPL)? Last time we talked about the illumination of the Earth by the Sun and how the Earth orbits the Sun. As everyone probably knows the Moon orbits the Earth, as the Earth orbits the Sun. Just as for the Earth, at any given time the Moon is half illuminated � the side facing the Sun. This picture shows the Earth and Moon as photographed by a space probe on it�s way to Jupiter � The Sun is far to the left in the picture and we see the Earth and Moon �hanging out in space� and basking in the Sun�s rays. Figure 3-1 The Earth and the Moon This picture of the Earth and the Moon was taken in 1992 by the Galileo spacecraft on its way toward Jupiter. The Sun, which provides the illumination for both the Earth and the Moon, was far to the right and out of the camera�s field of view when this photograph was taken. (NASA/JPL)?

Slide 4:Remember from last time that the �plane� of Earth�s orbit around the Sun is called the Ecliptic plane. The orbit of the Moon around the Earth is nearly � but not exactly � in the ecliptic plane. The Moon takes about a month to orbit the Earth so the appearance of the Moon goes through a month long cycle � the �phases of the Moon�. When the moon is �new� it�s on the Sun side of the Earth so we are looking at the back non-illuminated side � the Moon appears dark. When the Moon is on the opposite side of the Earth to the Sun we see the full illuminated face � the Moon is �full�. When the Moon is off to the side of the Earth we can see half of the illuminated face, and it�s called first quarter or third quarter � where quarter refers to the fraction of the way through the cycle, and not the illumination fraction. Figure 3-2 Why the Moon Goes Through Phases This figure shows the Moon at eight positions on its orbit, along with photographs of what the Moon looks like at each position as seen from Earth. The changes in phase occur because light from the Sun illuminates one half of the Moon, and as the Moon orbits the Earth we see varying amounts of the Moon�s illuminated half. It takes about 291/2 days for the Moon to go through a complete cycle of phases. (Photographs from Yerkes Observatory and Lick Observatory)? Teaching Note: Point out the inner circle shows the Moon�s motion around the Earth. The photographic images show the Moon as seen by the observer standing on the Earth. Remember from last time that the �plane� of Earth�s orbit around the Sun is called the Ecliptic plane. The orbit of the Moon around the Earth is nearly � but not exactly � in the ecliptic plane. The Moon takes about a month to orbit the Earth so the appearance of the Moon goes through a month long cycle � the �phases of the Moon�. When the moon is �new� it�s on the Sun side of the Earth so we are looking at the back non-illuminated side � the Moon appears dark. When the Moon is on the opposite side of the Earth to the Sun we see the full illuminated face � the Moon is �full�. When the Moon is off to the side of the Earth we can see half of the illuminated face, and it�s called first quarter or third quarter � where quarter refers to the fraction of the way through the cycle, and not the illumination fraction. Figure 3-2 Why the Moon Goes Through Phases This figure shows the Moon at eight positions on its orbit, along with photographs of what the Moon looks like at each position as seen from Earth. The changes in phase occur because light from the Sun illuminates one half of the Moon, and as the Moon orbits the Earth we see varying amounts of the Moon�s illuminated half. It takes about 291/2 days for the Moon to go through a complete cycle of phases. (Photographs from Yerkes Observatory and Lick Observatory)? Teaching Note: Point out the inner circle shows the Moon�s motion around the Earth. The photographic images show the Moon as seen by the observer standing on the Earth.

Slide 5:As described in the book you can visualize the illumination of the Moon and how it appears as viewed from the Earth. Imagine your head is the Earth. Hold out a baseball or similar. When you hold it towards a bright light source you're seeing the dark backside. When you hold it away from the light source you see the illuminated front side. When you hold it off to the side you see half of the illuminated side and half of the dark side � the disk of the Moon appears half illuminated. Box 3-1 Phases and Shadows Figure 3-2 shows how the relative positions of the Earth, Moon, and Sun explain the phases of the Moon. You can visualize lunar phases more clearly by doing a simple experiment here on Earth. All you need are a small round object, such as an orange or a baseball, and a bright source of light, such as a street lamp or the Sun. In this experiment, you play the role of an observer on the Earth looking at the Moon, and the round object plays the role of the Moon. The light source plays the role of the Sun. Hold the object in your right hand with your right arm stretched straight out in front of you, with the object directly between you and the light source (position A in the accompanying illustration). In this orientation the illuminated half of the object faces away from you, like the Moon when it is in its new phase (position A in Figure 3-2). Now, slowly turn your body to the left so that the object in your hand �orbits� around you (toward positions C, E, and G in the illustration). As you turn, more and more of the illuminated side of the �moon� in your hand becomes visible, and it goes through the same cycle of phases�waxing crescent, first quarter, and waxing gibbous�as does the real Moon. When you have rotated through half a turn so that the light source is directly behind you, you will be looking face on at the illuminated side of the object in your hand. This corresponds to a full moon (position E in Figure 3-2). Make sure your body does not cast a shadow on the �moon� in your hand�that would correspond to a lunar eclipse! As you continue turning to the left, more of the unilluminated half of the object becomes visible as its phase moves through waning gibbous, third quarter, and waning crescent. When your body has rotated back to the same orientation that you were in originally, the unilluminated half of your handheld �moon� is again facing toward you, and its phase is again new. If you continue to rotate, the object in your hand repeats the cycle of �phases,� just as the Moon does as it orbits around the Earth. The experiment works best when there is just one light source around. If there are several light sources, such as in a room with several lamps turned on, the different sources will create multiple shadows and it will be difficult to see the phases of your hand-held �moon.� If you do the experiment outdoors using sunlight, you may find that it is best to perform it in the early morning or late afternoon when shadows are most pronounced and the Sun�s rays are nearly horizontal. As described in the book you can visualize the illumination of the Moon and how it appears as viewed from the Earth. Imagine your head is the Earth. Hold out a baseball or similar. When you hold it towards a bright light source you're seeing the dark backside. When you hold it away from the light source you see the illuminated front side. When you hold it off to the side you see half of the illuminated side and half of the dark side � the disk of the Moon appears half illuminated. Box 3-1 Phases and Shadows Figure 3-2 shows how the relative positions of the Earth, Moon, and Sun explain the phases of the Moon. You can visualize lunar phases more clearly by doing a simple experiment here on Earth. All you need are a small round object, such as an orange or a baseball, and a bright source of light, such as a street lamp or the Sun. In this experiment, you play the role of an observer on the Earth looking at the Moon, and the round object plays the role of the Moon. The light source plays the role of the Sun. Hold the object in your right hand with your right arm stretched straight out in front of you, with the object directly between you and the light source (position A in the accompanying illustration). In this orientation the illuminated half of the object faces away from you, like the Moon when it is in its new phase (position A in Figure 3-2). Now, slowly turn your body to the left so that the object in your hand �orbits� around you (toward positions C, E, and G in the illustration). As you turn, more and more of the illuminated side of the �moon� in your hand becomes visible, and it goes through the same cycle of phases�waxing crescent, first quarter, and waxing gibbous�as does the real Moon. When you have rotated through half a turn so that the light source is directly behind you, you will be looking face on at the illuminated side of the object in your hand. This corresponds to a full moon (position E in Figure 3-2). Make sure your body does not cast a shadow on the �moon� in your hand�that would correspond to a lunar eclipse! As you continue turning to the left, more of the unilluminated half of the object becomes visible as its phase moves through waning gibbous, third quarter, and waning crescent. When your body has rotated back to the same orientation that you were in originally, the unilluminated half of your handheld �moon� is again facing toward you, and its phase is again new. If you continue to rotate, the object in your hand repeats the cycle of �phases,� just as the Moon does as it orbits around the Earth. The experiment works best when there is just one light source around. If there are several light sources, such as in a room with several lamps turned on, the different sources will create multiple shadows and it will be difficult to see the phases of your hand-held �moon.� If you do the experiment outdoors using sunlight, you may find that it is best to perform it in the early morning or late afternoon when shadows are most pronounced and the Sun�s rays are nearly horizontal.

Slide 6:The reflected sunlight from the Moon is bright enough that it can be seen during the day. The angle at which we see it at a given time of day depends on the phase of the Moon. The picture of the Moon above shows a full moon at a low angle in the sky during daytime. Can you see why this makes sense? Think about the previous diagram � we are standing on the Sun side of the Earth (daytime) and looking low on the horizon back �behind the Earth� where we can see the fully illuminated �sun-ward� side of the Moon. Figure 3-3 The Moon During the Day The Moon can be seen during the daytime as well as at night. The time of day or night when it is visible depends on its phase. [Karl Beath/Gallo Images/Getty Images] The reflected sunlight from the Moon is bright enough that it can be seen during the day. The angle at which we see it at a given time of day depends on the phase of the Moon. The picture of the Moon above shows a full moon at a low angle in the sky during daytime. Can you see why this makes sense? Think about the previous diagram � we are standing on the Sun side of the Earth (daytime) and looking low on the horizon back �behind the Earth� where we can see the fully illuminated �sun-ward� side of the Moon. Figure 3-3 The Moon During the Day The Moon can be seen during the daytime as well as at night. The time of day or night when it is visible depends on its phase. [Karl Beath/Gallo Images/Getty Images]

Slide 7:You may have noticed that although the illuminated fraction of the Moon varies over its cycle the features (craters etc) which we can see don't � it's as if the Moon keeps always the same face towards us. But how can that be? The Moon is orbiting us. In fact the Moon turns at just exactly the right rate so that it presents always the same side towards the Earth. This is not coincidence � it's due to a phenomenon called �tidal locking� which happens when objects orbit close to one another. The closest planet to the Sun � Mercury � is tidally locked. On Mercury one half of the planet continually faces the Sun and is baking hot day, while the other half is permanent frigid night. Figure 3-4 The Moon�s Rotation These diagrams show the Moon at four points in its orbit as viewed from high above the Earth�s north pole. (a) If the Moon did not rotate, then at various times the red crater would be visible from Earth while at other times the blue crater would be visible. Over a complete orbit, the entire surface of the Moon would be visible. (b) In reality, the Moon rotates on its north-south axis. Because the Moon makes one rotation in exactly the same time that it makes one orbit around the Earth, we see only one face of the Moon. You may have noticed that although the illuminated fraction of the Moon varies over its cycle the features (craters etc) which we can see don't � it's as if the Moon keeps always the same face towards us. But how can that be? The Moon is orbiting us. In fact the Moon turns at just exactly the right rate so that it presents always the same side towards the Earth. This is not coincidence � it's due to a phenomenon called �tidal locking� which happens when objects orbit close to one another. The closest planet to the Sun � Mercury � is tidally locked. On Mercury one half of the planet continually faces the Sun and is baking hot day, while the other half is permanent frigid night. Figure 3-4 The Moon�s Rotation These diagrams show the Moon at four points in its orbit as viewed from high above the Earth�s north pole. (a) If the Moon did not rotate, then at various times the red crater would be visible from Earth while at other times the blue crater would be visible. Over a complete orbit, the entire surface of the Moon would be visible. (b) In reality, the Moon rotates on its north-south axis. Because the Moon makes one rotation in exactly the same time that it makes one orbit around the Earth, we see only one face of the Moon.

Slide 8:So there is no �Dark Side of the Moon� - there is just the side we can never see from Earth. Until the Apollo astronauts flew around the back side no human being had ever seen the other side of the Moon � there could have been alien cities there for all we knew. So there is no �Dark Side of the Moon� - there is just the side we can never see from Earth. Until the Apollo astronauts flew around the back side no human being had ever seen the other side of the Moon � there could have been alien cities there for all we knew.

Slide 9:Now we come to eclipses. There are two kinds: 1) Lunar eclipses where the Full Moon or some portion of it rapidly goes dark and then reappears again. 2) Solar eclipses � by far the most spectacular astronomical events which ever occur � the Sun �goes out�. The Sun in total eclipse, March 29, 2006. (Stefan Seip)? Now we come to eclipses. There are two kinds: 1) Lunar eclipses where the Full Moon or some portion of it rapidly goes dark and then reappears again. 2) Solar eclipses � by far the most spectacular astronomical events which ever occur � the Sun �goes out�. The Sun in total eclipse, March 29, 2006. (Stefan Seip)?

Slide 10:A lunar eclipse occurs when the �shadow of the Earth� strikes the otherwise illuminated full moon. Because the Sun is not a point source of light � it's a disk 0.5 deg across in the sky � the edge of its shadow is not sharp. This means that one can get partial or total eclipses as shown in the diagram. Figure 3-8 Three Types of Lunar Eclipse People on the nighttime side of the Earth see a lunar eclipse when the Moon moves through the Earth�s shadow. In the umbra, the darkest part of the shadow, the Sun is completely covered by the Earth. The penumbra is less dark because only part of the Sun is covered by the Earth. The three paths show the motion of the Moon if the lunar eclipse is penumbral (Path 1, in yellow), total (Path 2, in red), or partial (Path 3, in blue). The inset shows these same paths, along with the umbra and penumbra, as viewed from the Earth. A lunar eclipse occurs when the �shadow of the Earth� strikes the otherwise illuminated full moon. Because the Sun is not a point source of light � it's a disk 0.5 deg across in the sky � the edge of its shadow is not sharp. This means that one can get partial or total eclipses as shown in the diagram. Figure 3-8 Three Types of Lunar Eclipse People on the nighttime side of the Earth see a lunar eclipse when the Moon moves through the Earth�s shadow. In the umbra, the darkest part of the shadow, the Sun is completely covered by the Earth. The penumbra is less dark because only part of the Sun is covered by the Earth. The three paths show the motion of the Moon if the lunar eclipse is penumbral (Path 1, in yellow), total (Path 2, in red), or partial (Path 3, in blue). The inset shows these same paths, along with the umbra and penumbra, as viewed from the Earth.

Slide 11:A solar eclipse occurs when the Moon gets between the Sun and Earth and blocks the incoming light. Because the Moon is much smaller than the Earth solar eclipses are much rarer � compare this slide and the last. The full shadow of the Moon only just reaches Earth and makes a small spot � if the Moon was a bit further (or a bit smaller) away there would be no such thing as a total solar eclipse. The very cool inset photo shows the spot of a total solar eclipse moving across the Earth's surface as seen from the Mir space station in orbit around the Earth. Figure 3-11 The Geometry of a Total Solar Eclipse During a total solar eclipse, the tip of the Moon�s umbra reaches the Earth�s surface. As the Earth and Moon move along their orbits, this tip traces an eclipse path across the Earth�s surface. People within the eclipse path see a total solar eclipse as the tip moves over them. Anyone within the penumbra sees only a partial eclipse. The inset photograph was taken from the Mir space station during the August 11, 1999, total solar eclipse (the same eclipse shown in Figure 3-10). The tip of the umbra appears as a black spot on the Earth�s surface. At the time the photograph was taken, this spot was 105 km (65 mi) wide and was crossing the English Channel at 3000 km/h (1900 mi/h). (Photograph by Jean-Pierre Haigner�, Centre National d�Etudes Spatiales, France/GSFS)? A solar eclipse occurs when the Moon gets between the Sun and Earth and blocks the incoming light. Because the Moon is much smaller than the Earth solar eclipses are much rarer � compare this slide and the last. The full shadow of the Moon only just reaches Earth and makes a small spot � if the Moon was a bit further (or a bit smaller) away there would be no such thing as a total solar eclipse. The very cool inset photo shows the spot of a total solar eclipse moving across the Earth's surface as seen from the Mir space station in orbit around the Earth. Figure 3-11 The Geometry of a Total Solar Eclipse During a total solar eclipse, the tip of the Moon�s umbra reaches the Earth�s surface. As the Earth and Moon move along their orbits, this tip traces an eclipse path across the Earth�s surface. People within the eclipse path see a total solar eclipse as the tip moves over them. Anyone within the penumbra sees only a partial eclipse. The inset photograph was taken from the Mir space station during the August 11, 1999, total solar eclipse (the same eclipse shown in Figure 3-10). The tip of the umbra appears as a black spot on the Earth�s surface. At the time the photograph was taken, this spot was 105 km (65 mi) wide and was crossing the English Channel at 3000 km/h (1900 mi/h). (Photograph by Jean-Pierre Haigner�, Centre National d�Etudes Spatiales, France/GSFS)?

Slide 12:So why aren't there solar and eclipses every month? As mentioned earlier the plane of the Moon's orbit around the Earth � the �flat sheet� which it orbits in - is not quite the same as the plane of Earth's orbit around the Sun � it's slightly tilted. The intersection of 2 planes is a line as we can see in the diagram above. Figure 3-6 The Inclination of the Moon�s Orbit This drawing shows the Moon�s orbit around the Earth (in yellow) and part of the Earth�s orbit around the Sun (in red). The plane of the Moon�s orbit (shown in brown) is tilted by about 5� with respect to the plane of the Earth�s orbit, also called the plane of the ecliptic (shown in blue). These two planes intersect along a line called the line of nodes. So why aren't there solar and eclipses every month? As mentioned earlier the plane of the Moon's orbit around the Earth � the �flat sheet� which it orbits in - is not quite the same as the plane of Earth's orbit around the Sun � it's slightly tilted. The intersection of 2 planes is a line as we can see in the diagram above. Figure 3-6 The Inclination of the Moon�s Orbit This drawing shows the Moon�s orbit around the Earth (in yellow) and part of the Earth�s orbit around the Sun (in red). The plane of the Moon�s orbit (shown in brown) is tilted by about 5� with respect to the plane of the Earth�s orbit, also called the plane of the ecliptic (shown in blue). These two planes intersect along a line called the line of nodes.

Slide 13:As we heard last time the tilt direction of the Earth's axis of rotation remains fixed as the Earth's orbits the Sun. In the same way the tilt of the Moon's orbital plane remains fixed as the Earth-Moon system orbits the Sun. (Actually it's not completely fixed but don't worry about that.)? An eclipse is only possible when the Sun, Earth and Moon line up as shown in the diagram. Figure 3-7 Conditions for Eclipses Eclipses can take place only if the Sun and Moon are both very near to or on the line of nodes. Only then can the Sun, Earth, and Moon all lie along a straight line. A solar eclipse occurs only if the Moon is very near the line of nodes at new moon; a lunar eclipse occurs only if the Moon is very near the line of nodes at full moon. If the Sun and Moon are not near the line of nodes, the Moon�s shadow cannot fall on the Earth and the Earth�s shadow cannot fall on the Moon. As we heard last time the tilt direction of the Earth's axis of rotation remains fixed as the Earth's orbits the Sun. In the same way the tilt of the Moon's orbital plane remains fixed as the Earth-Moon system orbits the Sun. (Actually it's not completely fixed but don't worry about that.)? An eclipse is only possible when the Sun, Earth and Moon line up as shown in the diagram. Figure 3-7 Conditions for Eclipses Eclipses can take place only if the Sun and Moon are both very near to or on the line of nodes. Only then can the Sun, Earth, and Moon all lie along a straight line. A solar eclipse occurs only if the Moon is very near the line of nodes at new moon; a lunar eclipse occurs only if the Moon is very near the line of nodes at full moon. If the Sun and Moon are not near the line of nodes, the Moon�s shadow cannot fall on the Earth and the Earth�s shadow cannot fall on the Moon.

Slide 14:Lunar eclipse takes hours. When in total eclipse the Moon is still faintly visible due to scattered light from Earth. (The same reason you can see the dark part of the Moon's disk during its regular cycle.)? Figure 3-9 Total Lunar Eclipse This sequence of nine photographs was taken over a 3-hour period during the lunar eclipse of January 20, 2000. The sequence, which runs from right to left, shows the Moon moving through the Earth�s umbra. During the total phase of the eclipse (shown in the center), the Moon has a distinct reddish color. (Fred Espenak, NASA/Goddard Space Flight Center; �2000 Fred Espenak, MrEclipse.com)? Lunar eclipse takes hours. When in total eclipse the Moon is still faintly visible due to scattered light from Earth. (The same reason you can see the dark part of the Moon's disk during its regular cycle.)? Figure 3-9 Total Lunar Eclipse This sequence of nine photographs was taken over a 3-hour period during the lunar eclipse of January 20, 2000. The sequence, which runs from right to left, shows the Moon moving through the Earth�s umbra. During the total phase of the eclipse (shown in the center), the Moon has a distinct reddish color. (Fred Espenak, NASA/Goddard Space Flight Center; �2000 Fred Espenak, MrEclipse.com)?

Slide 15:There were total lunar eclipses visible from North America in 2007 and 2008. There won't be one this year but there will be one in late 2010. On average, two or three lunar eclipses occur in a year. Table 3-1 lists all 12 lunar eclipses from 2007 to 2011. Of all lunar eclipses, roughly one-third are total, one-third are partial, and one-third are penumbral. There were total lunar eclipses visible from North America in 2007 and 2008. There won't be one this year but there will be one in late 2010. On average, two or three lunar eclipses occur in a year. Table 3-1 lists all 12 lunar eclipses from 2007 to 2011. Of all lunar eclipses, roughly one-third are total, one-third are partial, and one-third are penumbral.

Slide 16:Lunar eclipses are perhaps not that exciting for most people. But solar eclipse is another matter � there is something visceral about the �Sun going out�. In ancient times it was a terrifying event � more so because it was unpredictable. Astronomers who could predict eclipses appeared to have great knowledge. The sky goes dark and the stars come out as seen in the picture above. Figure 3-10 A Total Solar Eclipse (a) This photograph shows the total solar eclipse of August 11, 1999, as seen from El�zig?, Turkey. The sky is so dark that the planet Venus can be seen to the left of the eclipsed Sun. (Fred Espenak, MrEclipse.com)? Lunar eclipses are perhaps not that exciting for most people. But solar eclipse is another matter � there is something visceral about the �Sun going out�. In ancient times it was a terrifying event � more so because it was unpredictable. Astronomers who could predict eclipses appeared to have great knowledge. The sky goes dark and the stars come out as seen in the picture above. Figure 3-10 A Total Solar Eclipse (a) This photograph shows the total solar eclipse of August 11, 1999, as seen from El�zig?, Turkey. The sky is so dark that the planet Venus can be seen to the left of the eclipsed Sun. (Fred Espenak, MrEclipse.com)?

Slide 17:When the disk of the Sun is fully blocked � the eclipse reaches �totality� - the Sun's corona can be seen. No it's not a type of beer � it's super heated material streaming off the surface of the Sun and out into space as the solar wind. Figure 3-10 A Total Solar Eclipse (b) When the Moon completely covers the Sun�s disk during a total eclipse, the faint solar corona is revealed. (Fred Espenak, MrEclipse.com)? When the disk of the Sun is fully blocked � the eclipse reaches �totality� - the Sun's corona can be seen. No it's not a type of beer � it's super heated material streaming off the surface of the Sun and out into space as the solar wind. Figure 3-10 A Total Solar Eclipse (b) When the Moon completely covers the Sun�s disk during a total eclipse, the faint solar corona is revealed. (Fred Espenak, MrEclipse.com)?

Slide 18:As we saw in the diagram earlier the �footprint� of a total solar eclipse is a small rapidly moving patch on the Earth's surface. At any location on the surface totality last for only a few minutes and the whole event is over in a few hours. Note the track which starts in India and tracks across China and into the Pacific � this eclipse will occur in a July of this year. Figure 3-13 Eclipse Paths for Total Eclipses, 1997�2020 This map shows the eclipse paths for all 18 total solar eclipses occurring from 1997 through 2020. In each eclipse, the Moon�s shadow travels along the eclipse path in a generally eastward direction across the Earth�s surface. (Courtesy of Fred Espenak, NASA/Goddard Space Flight Center)? As we saw in the diagram earlier the �footprint� of a total solar eclipse is a small rapidly moving patch on the Earth's surface. At any location on the surface totality last for only a few minutes and the whole event is over in a few hours. Note the track which starts in India and tracks across China and into the Pacific � this eclipse will occur in a July of this year. Figure 3-13 Eclipse Paths for Total Eclipses, 1997�2020 This map shows the eclipse paths for all 18 total solar eclipses occurring from 1997 through 2020. In each eclipse, the Moon�s shadow travels along the eclipse path in a generally eastward direction across the Earth�s surface. (Courtesy of Fred Espenak, NASA/Goddard Space Flight Center)?

Slide 19:The eclipse this year will be exceptionally long and pass over well populated areas � it will be the longest solar eclipse in the 21st century. The one next year will hit only ocean and the very south tip of south America. The details of solar eclipses are calculated well in advance. They are published in such reference books as the Astronomical Almanac and are available on the World Wide Web: http://sunearth.gsfc.nasa.gov/eclipse/eclipse.html Figure 3-13 shows the eclipse paths for all total solar eclipses from 1997 to 2020. Table 3-2 lists all the total, annular, and partial eclipses from 2007 to 2011, including the maximum duration of totality for total eclipses. The eclipse this year will be exceptionally long and pass over well populated areas � it will be the longest solar eclipse in the 21st century. The one next year will hit only ocean and the very south tip of south America. The details of solar eclipses are calculated well in advance. They are published in such reference books as the Astronomical Almanac and are available on the World Wide Web: http://sunearth.gsfc.nasa.gov/eclipse/eclipse.html Figure 3-13 shows the eclipse paths for all total solar eclipses from 1997 to 2020. Table 3-2 lists all the total, annular, and partial eclipses from 2007 to 2011, including the maximum duration of totality for total eclipses.

Slide 20:This animation shows the path of the July 2009 eclipse across the globe � best viewing from China or islands to the South of Japan. This animation shows the path of the July 2009 eclipse across the globe � best viewing from China or islands to the South of Japan.

Slide 21:The ancient Greeks used observations of eclipses to conclude that the Earth is spherical. They noted that the shadow that the Earth casts on the Moon always has a circular edge. Only a sphere casts a circular shadow when illuminated from any angle. Armed with this knowledge in 200 B.C. Eratosthenes went on to calculate the circumference of the Earth. In the diagram above it is midday and the Sun's rays are falling directly down at Syrene in Africa � a man at the bottom of a well can see the Sun. Meanwhile the Sun is 7 degrees from zenith as seen further north in Alexandria. 7 degrees is a fiftieth of a circle. So Eratosthenes reasoned that the circumference of the Earth is fifty times the distance from Alexandria to Syrene. All he had to do was then measure that distance and multiply. We're not sure how accurately he did that because we don't don't know how long the Greek unit of distance the stadium was but he got pretty darn close Figure 3-14 Eratosthenes�s Method of Determining the Diameter of the Earth Around 200 B.C., Eratosthenes used observations of the Sun�s position at noon on the summer solstice to show that Alexandria and Syene were about 7� apart on the surface of the Earth. This angle is about one-fiftieth of a circle, so the distance between Alexandria and Syene must be about one-fiftieth of the Earth�s circumference. The ancient Greeks used observations of eclipses to conclude that the Earth is spherical. They noted that the shadow that the Earth casts on the Moon always has a circular edge. Only a sphere casts a circular shadow when illuminated from any angle. Armed with this knowledge in 200 B.C. Eratosthenes went on to calculate the circumference of the Earth. In the diagram above it is midday and the Sun's rays are falling directly down at Syrene in Africa � a man at the bottom of a well can see the Sun. Meanwhile the Sun is 7 degrees from zenith as seen further north in Alexandria. 7 degrees is a fiftieth of a circle. So Eratosthenes reasoned that the circumference of the Earth is fifty times the distance from Alexandria to Syrene. All he had to do was then measure that distance and multiply. We're not sure how accurately he did that because we don't don't know how long the Greek unit of distance the stadium was but he got pretty darn close Figure 3-14 Eratosthenes�s Method of Determining the Diameter of the Earth Around 200 B.C., Eratosthenes used observations of the Sun�s position at noon on the summer solstice to show that Alexandria and Syene were about 7� apart on the surface of the Earth. This angle is about one-fiftieth of a circle, so the distance between Alexandria and Syene must be about one-fiftieth of the Earth�s circumference.

Slide 22:Another clever Greek called Aristarchus used observations of the Moon to determine the relative distance of the Sun/Earth and Earth/Moon. The points in the cycle when we see the disk of the Moon half illuminated are not exactly when it is 90 degrees away from the sun direction. As we see in the diagram above the closer the Sun is the smaller the Sun-Moon angle when the disk is half illuminated. Simple trigonometry then gives the ratio of the distances. Unfortunately the angle is close to 90 degrees and hard to measure. Aristarchus got 87 degrees making the Sun-Earth distance 20 times the Earth-Moon distance � and given their similar angular size in the sky making the diameter of the sun 20 times bigger than that of the Earth. In fact the true angle is much closer to 90 degrees and the Sun-Earth distance is actually 390 times the Earth-Moon distance. Still it's impressive that people were even trying to figure such things out more than 2000 years ago. Figure 3-15 Aristarchus�s Method of Determining Distances to the Sun and Moon Aristarchus knew that the Sun, Moon, and Earth form a right triangle at first and third quarter phases. Using geometrical arguments, he calculated the relative lengths of the sides of these triangles, thereby obtaining the distances to the Sun and Moon. Another clever Greek called Aristarchus used observations of the Moon to determine the relative distance of the Sun/Earth and Earth/Moon. The points in the cycle when we see the disk of the Moon half illuminated are not exactly when it is 90 degrees away from the sun direction. As we see in the diagram above the closer the Sun is the smaller the Sun-Moon angle when the disk is half illuminated. Simple trigonometry then gives the ratio of the distances. Unfortunately the angle is close to 90 degrees and hard to measure. Aristarchus got 87 degrees making the Sun-Earth distance 20 times the Earth-Moon distance � and given their similar angular size in the sky making the diameter of the sun 20 times bigger than that of the Earth. In fact the true angle is much closer to 90 degrees and the Sun-Earth distance is actually 390 times the Earth-Moon distance. Still it's impressive that people were even trying to figure such things out more than 2000 years ago. Figure 3-15 Aristarchus�s Method of Determining Distances to the Sun and Moon Aristarchus knew that the Sun, Moon, and Earth form a right triangle at first and third quarter phases. Using geometrical arguments, he calculated the relative lengths of the sides of these triangles, thereby obtaining the distances to the Sun and Moon.

Slide 23:Got the ones to do with the Moon right but the Sun was off by large factor due to Aristachus� inaccurate measurement. Table 3-3 summarizes some ancient and modern measurements of the sizes of Earth, the Moon, and the Sun and the distances between them. Some of these ancient measurements are far from the modern values. Yet the achievements of our ancestors still stand as impressive applications of observation and reasoning and important steps toward the development of the scientific method. Got the ones to do with the Moon right but the Sun was off by large factor due to Aristachus� inaccurate measurement. Table 3-3 summarizes some ancient and modern measurements of the sizes of Earth, the Moon, and the Sun and the distances between them. Some of these ancient measurements are far from the modern values. Yet the achievements of our ancestors still stand as impressive applications of observation and reasoning and important steps toward the development of the scientific method.

Slide 24:Lunar Phases: The phases of the Moon occur because light from the Moon is actually reflected sunlight. As the relative positions of the Earth, the Moon, and the Sun change, we see more or less of the illuminated half of the Moon. The Moon�s Orbit: The plane of the Moon�s orbit is tilted by about 5� from the plane of the Earth�s orbit, or ecliptic.

Key Ideas

Slide 25:Conditions for Eclipses: During a lunar eclipse, the Moon passes through the Earth�s shadow. During a solar eclipse, the Earth passes through the Moon�s shadow. Lunar eclipses occur at full moon, while solar eclipses occur at new moon. Either type of eclipse can occur only when the Sun, Earth and Moon are almost in a line. If this condition is not met, the Earth�s shadow cannot fall on the Moon and the Moon�s shadow cannot fall on the Earth.

Key Ideas

Slide 26:The Moon and Ancient Astronomers: Ancient astronomers such as Aristarchus and Eratosthenes made great progress in determining the sizes and relative distances of the Earth, the Moon, and the Sun.

Key Ideas

Slide 27:The ancient Greeks were not right about everything. They believed based on no evidence that the Earth was stationary and at the center of the Universe. The Celestial Sphere was not just an idea � the stars really were mounted on a vast sphere and the reason they moved across the sky was because the sphere rotated about the Earth. To explain the motion of the Sun and the Earth they posited that these revolved around the Earth at a slightly slower speeds.The ancient Greeks were not right about everything. They believed based on no evidence that the Earth was stationary and at the center of the Universe. The Celestial Sphere was not just an idea � the stars really were mounted on a vast sphere and the reason they moved across the sky was because the sphere rotated about the Earth. To explain the motion of the Sun and the Earth they posited that these revolved around the Earth at a slightly slower speeds.

Slide 28:One way to imagine this model is that the entire Universe rotates around the Earth like the platform of a merry-go-round, while the Sun and Moon �walk� on the platform slowly in the opposite direction. Figure 4-1 A Merry-Go-Round Analogy (a) Two children walk at different speeds around a rotating merrygo- round with its wooden horses. (b) In an analogous way, the ancient Greeks imagined that the Sun and Moon move around the rotating celestial sphere with its fixed stars. Thus, the Sun and Moon move from east to west across the sky every day and also move slowly eastward from one night to the next relative to the background of stars. One way to imagine this model is that the entire Universe rotates around the Earth like the platform of a merry-go-round, while the Sun and Moon �walk� on the platform slowly in the opposite direction. Figure 4-1 A Merry-Go-Round Analogy (a) Two children walk at different speeds around a rotating merrygo- round with its wooden horses. (b) In an analogous way, the ancient Greeks imagined that the Sun and Moon move around the rotating celestial sphere with its fixed stars. Thus, the Sun and Moon move from east to west across the sky every day and also move slowly eastward from one night to the next relative to the background of stars.

Slide 29:This model works for the stars, Sun and Moon. But the motion of the planets is more complex � sometimes they appear to move forward with respect to the stars but sometimes backwards � dubbed �retrograde motion�. Studying and explaining the motion of the planets was a major preoccupation for the ancient Greeks � In Greek the word �planet� means �wanderer�. Figure 4-2 The Path of Mars in 2011�2012 From October 2011 through August 2012, Mars will move across the zodiacal constellations Cancer, Leo, and Virgo. Mars�s motion will be direct (from west to east, or from right to left in this figure) most of the time but will be retrograde (from east to west, or from left to right in this figure)? during February and March 2011. Notice that the speed of Mars relative to the stars is not constant: The planet travels farther across the sky from October 1 to December 1 than it does from December 1 to February 1. This model works for the stars, Sun and Moon. But the motion of the planets is more complex � sometimes they appear to move forward with respect to the stars but sometimes backwards � dubbed �retrograde motion�. Studying and explaining the motion of the planets was a major preoccupation for the ancient Greeks � In Greek the word �planet� means �wanderer�. Figure 4-2 The Path of Mars in 2011�2012 From October 2011 through August 2012, Mars will move across the zodiacal constellations Cancer, Leo, and Virgo. Mars�s motion will be direct (from west to east, or from right to left in this figure) most of the time but will be retrograde (from east to west, or from left to right in this figure)? during February and March 2011. Notice that the speed of Mars relative to the stars is not constant: The planet travels farther across the sky from October 1 to December 1 than it does from December 1 to February 1.

Slide 30:To generate such motion the Greeks introduced �circles-on-circles� or epicycles. The idea was that the planets revolved around a point, and that point revolved around the Earth. When the big and little circles were headed in the same direction as shown on this slide the direction of motion of the planet on the sky was normal or �direct�. Figure 4-3 A Geocentric Explanation of Retrograde Motion (a) The ancient Greeks imagined that each planet moves along an epicycle, which in turn moves along a deferent centered approximately on the Earth. The planet moves along the epicycle more rapidly than the epicycle moves along the deferent. (b) At most times the eastward motion of the planet on the epicycle adds to the eastward motion of the epicycle on the deferent. Then the planet moves eastward in direct motion as seen from Earth. (c) When the planet is on the inside of the deferent, its motion along the epicycle is westward. Because this motion is faster than the eastward motion of the epicycle on the deferent, the planet appears from Earth to be moving westward in retrograde motion. To generate such motion the Greeks introduced �circles-on-circles� or epicycles. The idea was that the planets revolved around a point, and that point revolved around the Earth. When the big and little circles were headed in the same direction as shown on this slide the direction of motion of the planet on the sky was normal or �direct�. Figure 4-3 A Geocentric Explanation of Retrograde Motion (a) The ancient Greeks imagined that each planet moves along an epicycle, which in turn moves along a deferent centered approximately on the Earth. The planet moves along the epicycle more rapidly than the epicycle moves along the deferent. (b) At most times the eastward motion of the planet on the epicycle adds to the eastward motion of the epicycle on the deferent. Then the planet moves eastward in direct motion as seen from Earth. (c) When the planet is on the inside of the deferent, its motion along the epicycle is westward. Because this motion is faster than the eastward motion of the epicycle on the deferent, the planet appears from Earth to be moving westward in retrograde motion.

Slide 31:When the small circle motion was in the opposite direction to the large circle motion it could cancel it out and generate �retrograde� or reverse of normal motion. Using detailed measurements made over 100�s of years and recorded in the great library of Alexandria Ptolemy constructed a very elaborate geocentric model which could make accurate predictions of future astronomical events � something which was greatly valued in the ancient world. Aristarchus had suggested that in fact everything was simpler assuming that the Earth and other planets orbited the Sun. But this suggestion simply seemed absurd � it�s obvious that the Earth is still! Next lecture we see that the Greek Sun centered Universe became an almost religious tenant and persisted well into medieval Europe. Figure 4-3 A Geocentric Explanation of Retrograde Motion (a) The ancient Greeks imagined that each planet moves along an epicycle, which in turn moves along a deferent centered approximately on the Earth. The planet moves along the epicycle more rapidly than the epicycle moves along the deferent. (b) At most times the eastward motion of the planet on the epicycle adds to the eastward motion of the epicycle on the deferent. Then the planet moves eastward in direct motion as seen from Earth. (c) When the planet is on the inside of the deferent, its motion along the epicycle is westward. Because this motion is faster than the eastward motion of the epicycle on the deferent, the planet appears from Earth to be moving westward in retrograde motion. When the small circle motion was in the opposite direction to the large circle motion it could cancel it out and generate �retrograde� or reverse of normal motion. Using detailed measurements made over 100�s of years and recorded in the great library of Alexandria Ptolemy constructed a very elaborate geocentric model which could make accurate predictions of future astronomical events � something which was greatly valued in the ancient world. Aristarchus had suggested that in fact everything was simpler assuming that the Earth and other planets orbited the Sun. But this suggestion simply seemed absurd � it�s obvious that the Earth is still! Next lecture we see that the Greek Sun centered Universe became an almost religious tenant and persisted well into medieval Europe. Figure 4-3 A Geocentric Explanation of Retrograde Motion (a) The ancient Greeks imagined that each planet moves along an epicycle, which in turn moves along a deferent centered approximately on the Earth. The planet moves along the epicycle more rapidly than the epicycle moves along the deferent. (b) At most times the eastward motion of the planet on the epicycle adds to the eastward motion of the epicycle on the deferent. Then the planet moves eastward in direct motion as seen from Earth. (c) When the planet is on the inside of the deferent, its motion along the epicycle is westward. Because this motion is faster than the eastward motion of the epicycle on the deferent, the planet appears from Earth to be moving westward in retrograde motion.