AST101 Chap 2 Summary

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Astronomy & Astrophysics
Michael Reid

AST101 Chapter 2 – Discovering the Universe for Yourself 2.1 – Patterns in the Night Sky  Constellation: a region of the sky with well-defined borders; the familiar patterns of stars merely help us locate the constellations  North celestial pole: the point directly over earth’s North Pole  South celestial pole: the point directly over earth’s South Pole  Celestial equator: a projection of the earth’s equator into spaces, making a complete circle around the celestial sphere  Ecliptic: 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 the Earth’s axis  The Milky Way in the night sky traces our galaxy’s disk of stars- the galactic plane- as it appears from our location in the outskirts of the galaxy  The dark lanes that run down the center 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  Local sky: the sky as seen from wherever you happen to be standing  Horizon: the boundary between earth and the sky  Zenith: the point directly overhead  Meridian: an imaginary half circle stretching from the horizon due south, through the zenith, to the horizon due south  We can pinpoint the position of any object in the local sky by stating its direction along the horizon (aka azimuth- degrees clockwise from due north) and its altitude above the horizon  Angular size: the angle an object appears to span in your field of view  Angular distance: the angle that appears to separate a pair of objects in the sky  For more precise astronomical measurements, we subdivide each degree into 60 arcminutes and subdivide each arcminute into 60 arcseconds  We can picture the movement of the sky by imagining the celestial sphere rotating around earth  Stars relatively near the north celestial pole remain perpetually above the horizon, that never rise or set but instead make daily counterclockwise circles around the north celestial pole = circumpolar  Latitude measures north-south positions; it is define to be 0°N at the NP and 90°S at the SP  lines of latitude are circles running parallel to the equator  Longitude measures east-west position  lines of longitude = semicircles extending from the NP to the SP  by international treaty, longitude is defined to be 0° along the prime median  Latitude affects the constellations we see because it affects the locations of the horizon and zenith relative to the celestial sphere  The altitude of the celestial pole in your sky is equal to your latitude  e.g. if you see the north celestial pole at an altitude of 40° above your north horizon, your latitude is 40°N  similarly, if you see the south celestial pole at an altitude of 34° above your south horizon, your latitude is 34°S  Finding the north celestial pole is easy, because it lies very close to the star Polaris (aka the North Star)  In the southern hemisphere, you can find the south celestial pole with the aid of the southern cross  Zodiac: the constellations along the ecliptic  The sun’s apparent location along the ecliptic determines which constellations we see at night 2.2 – The Reason for Seasons  Seasons occur because the tilt of earth’s axis causes sunlight to fall differently on earth at different times of the year o Step 1: - 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  this is why the two hemispheres experience opposite seasons o Step 2: - the axis tilt in June causes sunlight to strike the Northern Hemisphere at a steeper angle and the Southern Hemisphere at a shallower angle - the steeper sunlight angle makes it summer in the Northern Hemisphere for 2 reasons:  the steeper angle means more concentrated sunlight, which tends to make it warmer  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 - this opposite is true for the Southern Hemisphere at this time: the shallower sunlight angle makes it winter because the sunlight is less concentrated and the sun follows a shorter, lower path through the sky o Step 4: it has become winter for the Northern hemisphere and summer in the Southern Hemisphere o Step 3: in between these two extremes, both hemispheres are illuminated equally in March and September  The seasons on earth are caused only by the axis tilt and not by any change in earth’s distance from the sun Solstices and Equinoxes 
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