The Cosmic Perspective - Ch 2 notes

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Astronomy & Astrophysics
Michael Reid

Chapter 2: Discovering the Universe for Yourself 2.1 Patterns in the Night Sky -On a clear, moonless nights far from city lights, more than 2000 stars may be visible to your naked eye, along with the Milky Way -Constellation: A region of the sky with well-defined borders; the familiar patterns of stars merely help us locate the constellations. -The names and borders of the 88 official constellations were chosen in 1928 by members of the International Astronomical Union (IAU). -The stars in a particular constellation appear to lie close to one another but may be quite far apart in reality, because they lie at very different distances from Earth.  Illusion occurs because we lack depth perception when we look into space, a consequence of the fact that the stars are so far away. The ancient Greeks mistook this illusion for reality, imagining the stars and constellation to lie on a great celestial sphere that surrounds the Earth. -The north celestial pole is the point directly over Earth’s North Pole. -The south celestial pole is the point directly over Earth’s South Pole. -The celestial equator, which is a projection of Earth’s equator into space, makes a complete circle around the celestial sphere. -The ecliptic is the path the Sun follows as it appears to circle around the celestial sphere once each year. It crosses the celestial equator at a 23.5’ angle, because that is the tilt of Earth’s axis. -The Milky Way circles all the way around the celestial sphere, passing through more than a dozen constellations. The widest and brightest parts of the Milky Way are most easily seen from the Southern Hemisphere.  It appears somewhat wider in the direction of the constellation Sagittarius, because that is the direction in which we are looking toward the galaxy’s central bulge. The dark lanes that run down the centre of the Milky Way contain the densest clouds and they appear dark because these clouds obscure our view of stars behind them. These clouds generally prevent us from seeing more than a few thousand light-years into our galaxy’s disk. -Half of the celestial sphere that you see at any time represents what we call your local sky. -The boundary between Earth and sky defines the horizon. -The point directly overhead is the zenith. The meridian is an imaginary half circle stretching from the horizon due south, through the zenith, to the horizon due north. -The angular size of an object is the angle it appears to span in your field of view. -The angular distance between a pair of objects in the sky is the angle that appears to separate them. -For more precise astronomical measurements, we subdivide each degree into 60 arcminutes and subdivide each arcminute into 60 arcseconds. They never rise or set but instead make daily counterclockwise circles around the north celestial pole. We say that such stars are circumpolar. -Stars relatively near the south celestial pole never rise above the horizon at all. -All other stars have daily circles that are partly above horizon and partly below it. Because Earth rotates from west to east, the stars appear to rise in the east and set in the west. -Lattitude: measures north-south position. -Longitute: measures east-west position -Longitute measuring 0 degrees is the prime meridian. -Latitude affects the constellations we see because it affects the location of the horizon and cenith relative to the celestial sphere. -The altitude of the celestial pole in your sky is equal to your latitude. 2.2 The reason for seasons -The tilt of the Earth’s axis causes sunlight to fall differently on Earth at different times of the year. -The Northern Hemisphere is tipped towards the Sun in June and away from the Sun in December, while the reverse is true for Southern Hemisphere.  Why the two hemispheres experience opposite seasons Solstices and Equinoxes -The summer (June) solstice, which occurs around June 21, is the moment when the Northern Hemisphere is tipped most directly towards the Sun (and the Southern is tipped almost directly away). -The winter (December) solstice, which occurs around December 21, is the moment when the Northern Hemisphere is tipped mostly directly away from the Sun (and Southern is tipped most directly toward it). -The spring (March) equinox, which occurs around March 21, is the moment when the Northern Hemisphere goes from being tipped slightly away from the Sun to being tipped slightly towards the Sun. -The fall (September) equinox, which occurs around September 22, is the moment when the Northern Hemisphere first starts to be tipped away from the Sun. 2.3 The Moon, Our Constant Companion -The Moon is our constant companion in space, orbiting Earth about once every 27 1/3 days. -The Moon appears to reside on the celestial sphere. -Earth’s daily rotation makes the Moon appear to rise in the east and set in the west each day, -Because of its orbit around Earth, the Moon appears to move eastward from night to night through the constellations of the zodiac. -The moon moves relative to the stars by about ½ degree – its own angular size – each hour. -As it moves through the sky, both
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