ASTR 101 Lecture 2

ASTR 101 Lecture 2 - the observer at the equator will see...

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Lunar Phases and Seasons The moon moves in front of the sun, so it must be closer The sun emits its own light , but the moon only reflects, allowing us to understand lunar phases from the relative positions of the sun and moon
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n.b. stars are on front side of sphere, sun and moon on back Note: the pictures in this animation are all an integer number of sidereal days apart, as seen by the 3 stars in identical positions New Moon First Quarter Full Moon Third Quarter
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Lunar Phases and Sunrise These diagrams also allow us to see that the relationship between moonrise and sunrise is dependent on phase, e.g. at first quarter, the moon rises about 6 hours after the sun. (examples)
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Seasons Because the ecliptic and celestial equator are not the same, the sun is sometimes higher in the sky at noon, sometimes lower We can see this in the same diagram by considering the following two different cases for an observer at the equator, remembering that the celestial equator (define) goes through zenith (define) for this observer
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When the sun's motion along the ecliptic brings it near the celestial equator,
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Unformatted text preview: the observer at the equator will see the sun rise in the East and pass directly overhead at noon. (The situation for observers at other latitudes is somewhat harder to visualize. All will see sunrise due East, but the sun will not be directly overhead at noon. It helps here to draw a version with the round earth replaced by a flat plane inclined as the observers horizon, then mark zenith for the observer). When the sun's motion brings it far to the north, it rises North of East, and does not pass through zenith for the observer on the equator (although it might for other observers in the tropics) Seasons • The points farthest North and South in the Sun's travels along the ecliptic are called solstices, and the Sun's crossings of the celestial equator are called equinoxes Seasons • The change in weather with season is a consequence of the changing angle with which sunlight hits the Earth, and summer in the Northern Hemisphere is winter in the Southern...
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