AST 301: Planetary Motion & Early Cosmological Models - Syllabus & Class Notes, Study notes of Astronomy

Download AST 301: Planetary Motion & Early Cosmological Models - Syllabus & Class Notes and more Study notes Astronomy in PDF only on Docsity! Wednesday, Sep. 10 Syllabus, class notes, and homeworks are at: www.as.utexas.edu  courses  AST 301, Lacy Reading for this week: chapter 4 (quiz today) The homework handed out last Friday is due Friday of next week. It requires observations of the Moon and planets. We will take last week’s homework this week. Bring it up after class. The Wednesday help session has been moved to GRG 424 at 5:00 (for the entire semester). Note the new time and place. 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. This means that motions during a night are almost entirely due to the Earth’s rotation – all objects in the sky move together, east to west. When we talk about the motion of the Moon and planets we will talk about how they move relative to the stars on the celestial sphere. That tells us whether they move across the sky slightly faster or slower than the stars. They still move east to west during a night. Eclipses and the ecliptic Why isn’t there a lunar eclipse every full Moon? The reason is that the orbit of the Moon isn’t exactly in the same plane as the orbit of the Earth (the ecliptic). Usually the Moon passes above or below the Earth’s shadow. There are only 1 or 2 lunar eclipses each year. And we only see an eclipse if we are on the side of the Earth facing the Moon (i.e. the Moon is up). Plane of moon's orbit Favorable for eclipse ee ma telal-nela=e-lalae—aelt || Unfavorable for eclipse Line of nodes Unfavorable for eclipse Favorable for eclipse © 2002 Brooks Cole Publishing - a division of Thomson Learning Sunlight i Path of total eclipse © 2002 Brooks Cole Publishing - a division of Thomson Learning Motions of the planets During a night (or day) the planets appear to move across the sky along with the stars, due to the rotation of the Earth. But from night to night the planets slowly move relative to the stars. Usually, they move west to east relative to the stars. That is, they move east to west across the sky slightly slower than the stars do. This is called prograde motion. Occasionally, they reverse their motion, moving east to west relative to the stars. This is called retrograde motion. Path of Mars at 4-day intervals © 2002 Brooks Cole Publishing - a division of Thomson Learning The Greek and Arabic models Aristotle, and later Ptolemy, explained the motions of the planets with a system of spheres around the Earth, with the planets on smaller spheres rotating on the bigger spheres. They assumed that when the planets appeared to reverse their direction of motion they actually did. Prograde and retrograde motion Remember: all objects in the sky move east-to-west during a night due to the west-to-east rotation of the Earth. Prograde motion is when a planet moves west-to-east relative to the stars. The Sun and Moon always move prograde. Retrograde motion is when a planet moves east-to-west relative to the stars. Looking down on the solar system from the north, prograde motion occurs when the line from the Earth to the object rotates counterclockwise (in the same way the planets actually move). Retrograde motion occurs when the line rotates clockwise because the Earth passes the planet. Apparent path of Mars as seen from Earth a y 3 a 8 @ 3 8 = 5 Q a ® zy e c = = S eo o 2 < m, o 3 g 4 > a a B 3 3 = ® a at, 5 oe Phet.colorado.edu Bin oe eo eee ee ee aes Soe dos v system centered Show Traces Le aciite] | BET ec) | 10 Ma | 1 ! th!) | accurate Ici Help Initial Settings time = 11.3 number @ 2 of bodies $k) Les