The Night Sky - Week 4 - ASTR 1345
The Night Sky - Week 4 - ASTR 1345 ASTR 1345-004
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This 2 page Class Notes was uploaded by Sera (Notetaker) on Sunday September 25, 2016. The Class Notes belongs to ASTR 1345-004 at University of Texas at Arlington taught by James J Davis in Fall 2016. Since its upload, it has received 15 views.
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Date Created: 09/25/16
Note taker: Sera ASTR 1345 The Celestial Sphere Greek concept Assumed an unmoving central Earth o Sun and planets moved around Earth An outer spherical shell of stars surrounded the Earth Universe was contained in a sphere Earth’s North pole connects to the sky’s North pole The Ecliptic The imaginary path that the Sun takes across the sky over the course of one year o This path is created by the revolution of the Earth around the Sun o Sometimes referred to as the “Zodiac” The Sun appears in a different position along the ecliptic each day This was interpreted back then as the sun orbiting between Earth and the background celestial sphere It has a consistent repeating pattern of solar positions After the summer equinox, the sun gets lower and lower in the sky and day get shorter, until the winter equinox is reached and the sun begins to get higher and higher in the sky each day (while days get longer) North Celestial Pole A straight line running through the axis of the Earth which connections to the Celestial Sphere Points almost directly at Polaris (the North pole) All other stars appear to rotate around the North celestial pole Celestial Equator Earth’s equator projected out into space (even though space doesn’t really have an equator) Divides the sky into Northern and Southern hemispheres Southern hemisphere wasn’t seen until the time of exploration o Constellations there were named after new animals and new technology because that’s what was important during this time Declination – latitude of the sky; how high or below the celestial equator something is o Lines of declination match lines of latitude on Earth o +90 declination is the North celestial pole o 0 declination is the Celestial equator o o -90 is the South Celestial pole Zenith – the point 90 above the horizon Star Trails Diurnal (“daily”) motion – the apparent motion of the stars around the North celestial pole Note taker: Sera ASTR 1345 o This is due to Earth’s rotation on its axis o Stars complete one circle every 24 hours Where’s Polaris? o Depends on your latitude o At the North pole, Polaris is directly up o At the Equator, Polaris is on the horizon Precession Earth has a gradual wobble due to its molten core which causes the direction of its axis to shift every 26,000 years Today: Axis points at Polaris – Is Pole Star 2700 BC: Thuban (in Draco) – was Pole Star 12000 AD: Vega (in Lyra) – Will be Pole Star Zodiac signs of today are all wrong because of precession The moon keeps our precession under control, keeps us from wobbling around really crazy The Equinoxes Analemma – the curving figure 8 path the sun makes in the sky from day to day as it goes along the ecliptic The spot on the celestial sphere where the celestial equator and the Ecliptic intersect o On these days, the Earth’s equator falls directly in line with Earth’s orbit and the Sun falls directly on the equator Equal length day and night The Seasons Caused by the tilt of the Earth on its axis Summer – North celestial pole points towards the Sun Winter – North Celestial pole points away from the Sun