AST 301Introduction to AstronomyJohn LacyRLM [email protected] LiRLM [email protected] JeonRLM [email protected] site: www.as.utexas.eduGo to Department of Astronomy courses,AST 301 (Lacy), course websiteTopics for this weekDescribe how the planets appear to move during a nightand from night to night relative to the stars.How did Aristotle explain the motions of the planets?How did Copernicus explain the motions of the planets?Was his explanation more accurate than Aristotle’s?How did Kepler improve on Copernicus’ model?State (and define the term in) Kepler’s laws.What makes a model a scientific theory?Assignment for this weekRead Chapter 3.The homework due on Friday includes your observations ofVenus, Mars, and Spica. I hope you have been able tosee them on at least two nights. The weather doesn’tlook promising for the rest of this week, and it will behard to see Spica after this week (although Venus andMars will stay around for a while longer).If you haven’t seen these objects, use SkyGazer or anothercomputer model to make simulated observations. Sayon your homework if you made your ‘observations’ thisway.Eclipses and the eclipticWhy isn’t there a lunar eclipse every full Moon?The reason is that the orbit of the Moon isn’t exactly in thesame plane as the orbit of the Earth (the ecliptic).Usually the Moon passes above or below the Earth’sshadow.There are only 1 or 2 lunar eclipses each year.And we only see an eclipse if we are on the side of theEarth facing the Moon (i.e. the Moon is up).Eclipses and the eclipticWhy isn’t there a solar eclipse every new Moon?It’s almost the same reason.Usually the Moon’s shadow passes above or below theEarth.Also, the Moon’s shadow is very small when it hits theEarth, and only those people in the shadow see a solareclipse.Did I tell you?The Earth’s rotation on its axis takes one day.The Moon’s orbital motion takes one month.The Earth’s orbital motion takes one year.All of these motions are counter-clockwise looking fromabove the north pole.The motions during a night are almost entirely due to theEarth’s rotation – all objects in the sky move together,east to west.When we talk about the motion of the Moon and planets wewill talk about how they move relative to the stars on thecelestial sphere. That tells us whether they move acrossthe sky slightly faster or slower than the stars. They stillmove east to west during a night.Motions of the planetsDuring a night (or day) the planets appear to move acrossthe sky along with the stars, due to the rotation of theEarth.But from night to night the planets slowly move relative tothe stars.Usually, they move west to east relative to the stars.That is, they move east to west across the sky slightlyslower than the stars do.This is called prograde motion.Occasionally, they reverse their motion, moving east towest relative to the stars.This is called retrograde motion.The Greek and Arabic modelsAristotle, and later Ptolemy, explained the motions of theplanets with a system of spheres around the Earth, withthe planets on smaller spheres rotating on the biggerspheres.They assumed that when the planets appeared to reversetheir direction of motion they actually did.Prograde and retrograde motionRemember: all objects in the sky move east-to-west duringa night due to the west-to-east rotation of the Earth.Prograde motion is when a planet moves west-to-eastrelative to the stars.The Sun and Moon always move prograde.Retrograde motion is when a planet moves east-to-westrelative to the stars.Looking down on the solar system from the north, progrademotion occurs when the line from the Earth to the objectrotates counterclockwise (in the same way the planetsactually move).Retrograde motion occurs when the line rotates clockwisebecause the Earth passes the
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