science of astronomy

apparent retrograde motion

occurs when we “lap” another planet or it laps us, appears to move other way in sky for a few weeks

rejection of real explanation for planetary motion by ancient Greeks because they couldn’t observe

stellar parallax - didn’t think stars were that far away

ancient astronomy applications

could predict seasons from orientations of crescent Moon, used obelisks as sundials

measurement of Earth’s circumference

Ptolemaic model

explains retrograde motion with concentric circles - planets orbit around a point that orbits the Sun and do actually move backwards

sidereal day calculation

Tsidereal = Tyear/Nsidereal

Copernicus

proposed Sun-centric model and used it to determine layout of solar system, but still overcomplicated system because it used circle

Brahe

most accurate naked eye measurements ever made of planetary positions, still couldn’t detect stellar parallax, thought that earth was at center but other planets went around Sun

Kepler

first tried to match Brahe’s observations with circular orbits but an eight arcminute discrepancy led him to ellipses

kepler’s first law

the orbit of each planet around the Sun is an ellipse with the Sun as one focus

perihelion

point in orbit closest to Sun

aphelion

point in orbit furthest from Sun

Kepler’s second law

as a planet moves around its orbit, it sweeps out equal area in equal times - planet travels faster when nearer to the Sun

Kepler’s third law

more distant planets orbit Sun at slower average speeds

p² =

a³ ; p is orbital period (y) and a is avg distance from Sun (AU)

kepler’s laws are phenomenological because

they described observations but didn’t explain them because concept and equations of gravity didn’t exist

Galileo’s evidence of heliocentricity and other challenges to “heavenly perfection”

objects with inertia so Earth could be moving, sunspots and Moon’s craters, telescope showed stars were much further away so lack of parallax wasn’t a problem,Venus has phases like the Moon, Jupiter has moons

Occam’s razor

assume the simplest explanation

scientific model

explain observed phenomena solely with natural causes, make testable predictions that would force us to revise/abanson if they don’t agree with observations

theory

explains a variety of observations with a few simple principles, supported by a large body of evidence and not failing any crucial validity test