Moon Rotation spinning motion about an axis Earth every 24 hrs o From some locations circumpolar stars circle celestial pole in 24 hours Revolution orbital motion of one body around another o Earth revolves around Sun in 365 25 days o Different stars are visible at different times of the year o Sun appears to move with respect to the stars over 1 year through the constellations of the o Today in Leo the Sun rises and sets with stars in Leo At night we easily see Pisces Capricorn zodiac and Aquarius Sun spends 1 month in each constellation We see different constellations at night over the year Earth s axis is tilted at 23 5 degrees Seasonal changes are more extreme at high latitudes o Ex Path of the Sun on the summer solstice at the Arctic Circle Summer Solstice o At NP sunlight for 24 hours above 66 5 degrees north 1 day of entire midnight sun o SP has total night for 24 hours and below 66 5 degrees south 1 day of total night Winter Solstice o Exact opposite of SS For College Park o Summer sun is high in the sky days are longer and rays are concentrated o Winter sun is low in sky days are shorter and rays are spread out Planets and moons shine by reflected sunlight Refraction by Earth s atmosphere allows us to see Moon in shadow o Refraction when light bends as it encounters and moves trough a different medium o Dust scatters blue light creating reddish appearance of eclipsed moon Harvest Moon o Full moon closest to autumnal equinox o Instead of 50 min later near AE it is only 35 min later for a few nights o facing east moon s orbit close to ecliptic since opposite the Sun full moon is close to vern a equinox where cel eq something cross The equinox just occurred but there wasn t exactly 12 hours of sunlight darkness o The Sun seemed to stay above the horizon slightly longer than 12 hours Due to atmospheric refraction curving the sun s rays so that even when the sun was exactly on the horizon the sun seemed slightly higher to us Falsification of geocentric theory by Galileo s observations of phases of Venus Why not 2 eclipses each month o Moon and Earth cast long skinny shadows o Moon s orbit is inclined at 5 degrees to Earth s orbital plane so shadow usually misses o Eclipses appear at points where orbits cross called nodes Eclipses Solar 2013 Annular 5 9 Australia S Pacific o Lunar 2013 Total 11 3 Africa Partial 4 25 Asia Africa Europe Penumbral 5 24 only 2 in shadow Penumbral 10 18 N America o Great total solar eclipse for US coming 8 21 17 Solstice and Equinox markers Standing stones o Stonehenge England Lunar cycle Eclipse prediction o Nabta England 6 000 years old o Medecine Wheels N America o Incas S America Buildings China o Newgrange Tomb Ireland o Mayas Caracol at Chichen Itza Mexico Interest in Venus o Pyramids Egypt o Kept great astronomical records New stars novae Comets Sunspots Eclipse prediction 2700 years ago Greece India 2600 ya Maya Aztec 1500 ya Birth of Greek Science 600 BC o Anaximander humans descended from simple lifeforms o Democritus everything made of tiny units o Hippocrates diseases NOT caused by gods or demons o Aristarchus Earth moves around the Sun Aristotle 350 BC Geocentric theory Everything in sky circles around the stationary Earth o Sky appears to rotate in 24 hrs o Doesn t feel like we re moving o No wind o Don t get thrown off 1000 miles hr o No stellar parallax observed Prediction If Earth moves around the Sun then stellar positions should shift o They weren t able to observe stellar parallax because stars are too far and they don t have telescopes so they thought stars weren t moving and so Greeks dismissed the heliocentric theory Ptolemy 140 AD Retrograde motion of planets observed o Usually W E prograde o E W retrograde o Used epicycles Model predicts planetary positions o Accuracy within 1 to 2 degrees after 1 century Hipparchus 130 BC Compared earlier star maps to his o Discovered that NCP isn t fixed Earth s axis wobbles Precession o 26 000 year cycle o Affects positions of NCP north star and equinoxes Thuban was north star when Egyptian pyramids were built 5000 ya o 3000 years ago the Vernal Equinox was in Aries and it will be in Aquarius at approximately 2700 AD Aristotle Ptolemy Consistent observations Geocentric hypothesis No stellar parallax Accuracy 1 to 2 degrees per century for planetary positions Retrograde motion Copernicus 1473 1543 Reintroduced heliocentric theory Sun centered hypothesis Simpler model but with circular orbits still needed epicycles No stellar parallax ok stars are distant Accuracy 1 to 2 degrees per century for planets Tycho Brahe 1546 1601 1577 demonstrated that comets are outside Earth s atmosphere King liked that Funded his observatory At his observatory measured precise planetary positions to a few arcminutes for 20 years no telescopes yet Blended geo helio models Accuracy horrible Needed revision Hired Kepler but Tycho died in 1601 Kepler 1571 1630 Used Tycho s data to revise heliocentric model o After 70 trials over 4 years he tried ellipse for circle books 1609 1619 It matched with improved accuracy 100 times better than geocentric or Copernicus Planet within 1 2 arcminute per century Kepler s work is summarized as 3 Laws of Planetary Motion p 40 o Elliptical orbits o Equal areas in equal times o P 2 a 3 with P orbital period in years a semimajor axis in astronomical units Galileo 1564 1630 Built the first astronomical telescope in 1609 Galileo saw sunspots proposed that the Sun rotated He saw lunar features like those on Earth seas mts Galileo observed orbital potion of Jupiter s moons Galileo observed 5 phases of Venus o And it changed angular size o Predicted by heliocentric inconsistent with geocentric Venus observations falsified Venus observations Sir Isaac Newton 1642 1727 Expressed gravity as an attractive force Developed 3 laws of motion Derived Kepler s laws mathematically Newton s 3 Laws 1 Inertia 2 F m x a o A body stays at rest or in motion unless acted upon by a force o m mass measures amount of material o a acceleration change in speed or direction o 1 and 2 combine to produce orbital motion Why are astronauts aboard space shuttle weightless o Falling toward Earth at same rate gravity is pulling Acceleration due to gravity is independent of a body s mass 3 Action and Reaction o Each body exerts an equal and opposite force on each other o Fire extinguisher trike o Water rocket Moon is constantly falling toward Earth but the forward inertia of the moon keep it moving in an orbital