ASTRO 2.1, 2.2, 2.3, 2.4.odt

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Astronomy & Astrophysics
Michael Reid

2.1 Patterns in the Night Sky Constellations:Aregion of the sky with well-defined borders – Every point in the sky belongs to some constellation – The IAU (InternationalAstronomical Unit) divided the sky into 88 constellations – We lack depth perception in space therefore near stars are in reality very far apart – more than 2000 stars can be seen with the naked eye & the milky way – We refer to patterns of stars as constellations – From pluto the constellations would be very similar, differences too small to be visible Polaris = the N star and is fixed throughout the whole night, aligns most closely with the rotation North Celestial Pole: Point directly over earth's north pole South Celestial pole: Point directly over earth's south pole Celestial Equator: Projection of earth's equator into space, makes 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 – The Milky way passes through more than a dozen constellations Local sky: the sky seen from wherever you happen to be standing Zenith: point directly over head Meridian:An imaginary circle stretching from the horizon due south through the zenith to the horizon due north – We can pinpoint any object's position by stating its direction along the horizon and its altitude Angular size: The angle it appears to span in your field of view (also depends on distance) ex: The sun is about 400X larger than the moon but has the same angular size because its 400X farther Angular distance: The angle that appears to separate a pair of objects – moving an object farther away reduced it's angular size – Angular size depends on physical size and distance Circumpolar: Star that remains above the horizon for a particular latitude – Stars near the south celestial pole never rise above the horizon – Because earth turns from west – east the stars appear to rise in the east / set in the west Latitude: North/ south position. 0° at the equator then increases southward or northward Longitude: measures east- west position. Semicircles extending from north pole to south pole – The altitude of the celestial pole in your sky = your latitude – You can determine your latitude by finding the celestial pole in your sky 2.2 Reasons for Seasons – The tilt of the earth's axis causes sunlight to fall differently on earth at different times of the year – The sun strikes the summer hemisphere at a steeper angle which makes it more concentrated and therefore warmer than winter, the sun also follows a longer/ higher path = long days – the N celestial pole tilts the earth causing seasons (closer in summer farther in winter) – seasons are caused ONLY by the axis tilt NOT the distance away from the sun To mark changing season we define four special moments in the year: Summer Solstice: Northern hemisphere receives most direct sunlight (south hem = least) (june) Winter Solstice: Northern hemisphere receives least amount of light (dec) Spring Equinox: Northern hemisphere tips towards the sun
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