astronomy lecture 7

what were the 5 main planets in ancient greece

mercury, venus, mars, jupiter, saturn

how do the planets affect modern life

the 7 day week, 12 months, 12 zodiac

retrograde motion

a little backwards loop

diurnal motion

apparent daily motion with the stars

normal pregrade motion

regular forward motion

bounded elongation

always on the same side of sky as the sun 

bounded elongation planets

mercury and venus

ptolemy developed

earth is fixed at the center, all celestial bodies move in perfect circles with epicycles for retrograde

what system is ptolemy

geocentric

copernicus theory of earth

earth rotates on its axis once per day, orbiting in perfect circles

what system is copernicus

heliocentric

what does copernican theory explain

bounded elongation- planets have smaller orbits around the sun than the earth 

how does copernican theory explain retrograde

occurs when one planet passes another

what did copernicus theory explain about a planet’s appearance during retrograde

it would look bigger and brighter as its closer to earth