Astro 201 Aug 26 2010 Turn in HW 1 in front of room Pick up Telescope Lab Handout Reading Hester et al Chapter 2 Gleiser Chapters 1 6 Wait List students have been added class limit will be increased so everyone can be enrolled Today Discuss Telescope Lab On Line quiz History of cosmology and the scientific method On line Quiz go to d2l Open book 10 questions available after class today do it before class next Thursday Which of the following is the largest a the diameter of the Moon b the diameter of Earth c the diameter of the Milky Way d the diameter of the Sun e 1 astronomical unit Cosmology Some History Lascaux Cave Paintings Cro Magnons recorded the lunar cycle 15 000 years ago http news bbc co uk 2 hi science nature 975360 stm PATTERNS IN THE SKY Ancient peoples observed the Sun and stars and created calendars that marked the yearly passage of time Earth rotates on its axis once every 24 hours Earth revolves around the Sun once every 365 days The stars appear to rise in the east and set in the west every night on a big dome called the celestial sphere The Earth s rotation is counter clockwise looking down on the N pole and results in the apparent motion of the celestial sphere everything rises in the East and sets in the west 1 solar day time from one noon to the next 24 hours Circumpolar Stars Always above Horizon In the Northern Hemisphere they Appear to rotate around The North Star Polaris In the Southern Hemisphere No bright star at rotation center Celestial Sphere and Constellations The sky is divided into 88 official constellations although in reality the stars in any particular constellation are at different distances from Earth Figure 2 3 The celestial sphere as a useful fiction Figure 2 4 As viewed from the Earth s North Pole Figure 2 5 Our perspective on the sky depends on our location Figure 2 9 Right Celestial sphere at different latitudes Figure 2 10 Earth s orbit and constellations along the ecliptic The Zodiac Rise and set of the Sun and Stars has 2 possible explanations GEOCENTRIC Earth at the center Sun and Stars go around the Earth once per day HELIOCENTRIC Earth rotates on its axis and so the Sun and stars appear to go around the Earth Ancient Egyptian celestial sphere the goddess Nut noot c 3000 BC Nut gave birth to the Sun god RA every morning Hindu Rig Veda 1500 BC Written cosmology Sanskrit Cyclical or recursive universe Universe originates from the Hiranyagarbha or golden egg Timescales eerily coincident with modern results e g age of the Earth Hebrew Cosmology And God said Let there be a vault in the midst of the waters and let it divide water from water And God made the vault and it divided the water beneath the vault from the water above the vault and so it was Genesis 1 6 Michaelangelo s depiction of the creation Of the planets and stars God had divided the waters above from the waters below by constructing an immense dome that held open the space for dry land In the Hebrew Bible the dome is called raqi a meaning a firm substance and rendered in the King James translation as the firmament The firmament in Biblical times was understood to be firm only by the will of God If God were angered the windows of heaven and the fountains of the deep could burst open once again and those lovely blue waters would destroy the Earth A painting by the American Edward Hicks 1780 1849 showing the animals boarding Noah s Ark two by two Aristotle 4th century BC Greece First to give reasons why the Earth is spherical Aristotle s 1 reason st Gravity pulls matter to center of Earth compressing the Earth into as compact a shape as possible a sphere Thanks to Barbara Ryden for this discussion 2 reason nd You see different stars from the south than from the north Big Dipper Southern Cross If the Earth were flat Big Dipper Southern Cross 3 reason rd The shape of the Earth s shadow During a lunar eclipse Earth s shadow is always circular Only object whose shadow is always circular is a sphere Another Greek Aristarchus of Samos 3rd century BC Proposed that the Earth rotates on its axis goes around the Sun Rejected by contemporaries He used geometry to estimate the size of the Sun and Moon and their relative distances What was his reasoning The Sun is farther away than the Moon because the Moon comes between the Sun and the Earth during a solar eclipse ANIMATION The phases of the Moon result from the Moon orbiting the Earth ANOTHER ANIMATION Aristarchus measured the angle between the Moon and the Sun when the Moon was half lit and deduced the ratio of the Sun Earth distance to the Moon Earth distance Here s a picture more to scale M E 90 87 S 3 Earth Sun distance E S is 19 times Earth Moon distance E M actual value 400x Thus Aristarchus correctly discovered that the Sun is much farther away from the Earth than the Moon He then concluded that the Sun was MUCH bigger than the Earth or Moon He reasoned that it was more plausible that the smaller body would orbit the bigger body thus the Earth orbits the Sun Despite Aristarchus for 2000 years a GEOCENTRIC model of the Universe was favored EARTH at the center STARS affixed to the celestial sphere Moon Planets and the Sun are between the EARTH and STARS PTOLEMY 2nd century AD Ptolemy s ALMAGEST was translated into Latin in 1496 GEOCENTRIC Earth at center Copernicus 1473 1543 Heliocentric Model Ptolemy s Epicycles In order to understand the motion of the planets in the sky Ptolemy s model had to be modified In terms of the heliocentric model the planets orbit the Sun in ellipses not circles Retrograde motion of the planets with respect to the stars required EPICYCLES Ecliptic the fact that the planets orbit the Sun in a plane means that they always appear to lie on a great circle on the sky called the ecliptic Retrograde motion The planets sometimes appear to be traveling west to east unlike the stars and Sun which always go east to west Next slide animation Astronomy Picture of the Day for Dec 20 2001 Jupiter and Saturn showing retrograde motion Combining 23 pictures taken at 2 week intervals from June 2000 May 2001 Planet wanderer Ptolemy s explanation for retrograde motion of the planets Earth Off Center Earth Equant Epicycle Deferent The planet moves along its epicycle as the epicycle moves along the deferent around the Earth To make the observations as accurate as possible it was necessary to place the Earth slightly off center of the orbits but to preserve symmetry that meant that there was an equal place Equant opposite the