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Astronomy & Astrophysics
Michael Reid

September 27, 2011 Chapter Two: Discovering the Universe for Yourself 2.1 Patterns in the Night Sky - A constellation is 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 celestial Sphere - The stars in a particular constellation appear to lie close to one another but may be quite far apart in reality, because they may lie at very different distances from Earth - Ancient Greeks imagined the stars and constellations to lie on a great celestial sphere that surrounds Earth - Earth seems to be in the centre of the celestial sphere only because it is where we are located as we look into space - Four special points & circles in the celestial sphere: 1. The north celestial pole is the point directly over Earth‟s North Pole 2. The south celestial pole is the point directly over Earth‟s South Pole 3. The celestial equator, which is a projection of Earth‟s equator into space, makes a complete circle around the celestial sphere 4. 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 23½ angle, because that is the tilt of Earth‟s axis The Milky Way - The band of light that we call 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 - We can observe the distant universe only when we look in directions away from the galactic disk - The darkest lanes that run down the center of the Milky Way contain the densest clouds, and these clouds generally prevent us from seeing more than a few thousand light-years into our galaxy‟s disk The Local Sky - We see only half of the celestial sphere at any particular moment from an particular location, which the other half is blocked from view by the ground - The half of the celestial sphere that we see at any time represents the local sky – the sky as seen from wherever you happen to be standing - 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 September 27, 2011 - We can pinpoint the position of any object in the local sky by stating its direction along the horizon and its altitude above the horizon Angular Sizes and Distances - The angular size of an object is the angle it appears to span in your field of view - The angular sizes of the sun and moon are each about ½ degree - For example, the sun is about 400 times larger in diameter than the moon, but it has the same angular size in our sky because it is also about 400 times farther away - The angular distance between a pair of objects in the sky is the angle that appears to separate them - For more precise astronomical measures, each degree is subdivided into 60 arcminutes (symbol = „) and each arcminute into 60 arcseconds (symbol = ‟‟) - Earth rotates from west to east, making the celestial sphere appear to rotate around us from east to west - Stars relatively near the north celestial pole remain perpetually above the horizon. They never rise or set but instead make daily counterclockwise circles around the north celestial pole. We say that such stars are circumpolar - All other stars have daily circles that are partly above the 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 Variation with Latitude - Latitude measures north-south position; it is defined to be 0 degree at the equator, increasing to 90 degree north at the North Pole and 90 degree south at the South Pole - Longitude measures east-west position, so “lines of longitude” are semicircles extending from the North Pole to the South Pole - Longitude is defined to be 0 degree along the prime meridian, which passes through Greenwich, England - Latitude affects the constellations we see because it affects the locations of the horizon and zenith relative to the celestial sphere - Although the sky varies with latitude, it does not vary with longitude - The altitude of the celestial pole in your sky is equal to your latitude - North celestial pole lies very close to the star Polaris (North Star) - The south celestial pole lies close to the Southern Cross Variation with Time of Year - The night sky changes throughout the year because of Earth‟s changing position in its orbit around the Sun - As we orbit the Sun over the course of a year, the sun appears to move against the background of the distant stars in the constellations - The constellations along the ecliptic are called the constellations of the zodiac September 27, 2011 2.2 The Reason for Seasons - The tilt of Earth‟s axis causes sunlight to fall differently on Earth at different times of year - Earth‟s axis remains pointed in the same direction in space (toward Polaris) throughout the year. As a result, the orientation of the axis relative to the sun changes over the course of each orbit: The Northern Hemisphere is tipped toward the Sun in June and away from the Sun in December, while the reverse is true for the Southern Hemisphere (that is why the two hemispheres experience opposite seasons) - In June, when axis tilt causes sunlight to strike the Northern Hemisphere at a steeper angle and the Southern Hemisphere at a shallower angle, it is summer in the Northern Hemisphere because: 1. the steeper angle gives more concentrated sunlight to the Northern Hemisphere which makes it warmer 2. the steeper angle also means the sun follows a longer and higher path through the sky, giving the Northern Hemisphere more hours of daylight during which it is warmed by the Sun - Earth is only about 3% farther from the Sun at its farthest point than at its nearest, therefore any change in Earth‟s distance from the sun does not cause seasonal changes Solstices and Equinoxes - The summer (June) solstice occurs around June 21, is the moment when the Northern Hemisphere is tipped most directly toward the Sun - The winter (December) solstice, which occurs around December 21, is the moment when the Northern Hemisphere is tipped most directly away from the Sun - 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 toward 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 - The equinoxes occur on the only two days of the year which the Sun rises precisely due east and sets precisely due west. These are also the only two days when sunlight falls equally on both hemispheres First Days of Seasons - Each equinox and solstice marks the first day of a season - The summer solstice occurs when the Northern Hemisphere has its maximum tilt toward the Sun - Although the sun‟s path through the Northern Hemisphere sky is longest and highest around the time of th
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