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Astronomy Lecture 05 - Oct 12.docx

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Astronomy 1021
Jan Cami

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th ASTRONOMY 1021 October 12 , 2010 Astronomy – Lecture 5 Path of the planets, Timekeeping, Some history Astronomy… Goals… o Describe motions of the planets in Earth’s sky o Difference between motions of inner and outer o Reproduce how the geometry of the solar system results in planetary motions Dance of the Planets… o Five planets visible: Mercury, Venus, Mars, Jupiter, Saturn o These objects slowly wander through constellations of the zodiac o In ancient civilizations, their long-term motions with respect to the stars was highly puzzling… Mercury and Venus… o The inner planets are never far away from the Sun o We see them shortly before sunrise, shortly after sunset, and they have phases o Sometimes sun seems to be chasing planets, other times it’s the other way around o See animation/Starry Night o Greatest elongation: Mercury = 28 degrees, Venus = 46 degrees, so at greatest eastern elongation, Venus appears 46 degrees east of the Sun Mars, Jupiter and Saturn… o Outer planets sometimes move backwards and make a loop in the sky over periods of weeks or months. This is called retrograde motion. o Occurs when planet is at opposition when the planet is on the opposite side of the sky from the Sun. o Planet is then visible entire night. o See animation/Starry Night Mysterious Wanderers… o Inner planets can never be at opposition Planet terminology… o Iconjunction: planet lined up with Sun o Inner planets: inferior and superior conjunction o Opposition: outer planet appears in opp. Sky to the sun. (missed slides, see power point) Planetary Transits… o Plane in which planets orbit sun is slightly tilted th ASTRONOMY 1021 October 12 , 2010 o When mercury or Venus I directly in front of the solar disk, there is a planetary transit o These are fairly rare: about 12 Mercury transits per century. Next one May 9 2016. Venus transits in pairs 8 years apart: pairs are separated in 105 years. Next one June 6 2012 Timekeeping Goals… o Describe astronomical roots of days, months, years o Explain leap years o Determine whether or not any given year is a leap year using mod
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