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General Astronomy Physics 103 Dr. Tyler E. Nordgren Dr. Julie A. Rathbun 1 Topics: The Night Sky … 1 … from the Solar System… 1 … to the Stars… 1 … to the Milky Way … 1 … and to the Universe Beyond. 1 Our Goal • Understand: – The sky above • “What’s that bright yellow star over head?” • How can I tell what time it is by where the Sun is? – News reports • “Scientists discover giant black hole. Should I be afraid?” – Popular entertainment • “Armageddon”, “Enterprise”, etc • Learn how science works and how we know what we know. 1 Organization • Lecture two days a week (Tues. and Thurs.): – 1 hour and 20 minutes • Lab one day a week (Mon. or Wed.): – 2 hours and 50 minutes in Hornby 2 • Evening observing (4 times): 7:30 pm – 9:30 pm • Check the syllabus and web site for exact dates! 1 Grading • • • • Homework: 20% Quizzes: 10% Labs: 20% Exams: 30% (15% each) – 3 exams, lowest grade dropped • Final: 20% 1 Naked-Eye Astronomy The second oldest profession 1 Goals • • • • How does astronomy affect your daily life? The Sun: Time without clocks The Seasons: Dates without calendars The ancients did it, so can you! 1 The Sun • • • • The Sun is a star. We live by its warmth. We die by its extreme heat or frigid absence. It is the source of nearly all our energy. – How? • It is the source of all our food. – How? 1 The Day • The Earth is a sphere that turns on its axis. – How long does it take to turn once on its axis? • Light from the sun lights only one half of Earth. • As the Earth turns the sun “rises” and “sets.” • So does the moon. • So do the stars. 1 The Year • The Earth orbits the Sun. – How long does it take to go once around the Sun? • The orbit is nearly circular, but is actually an ellipse. An exaggeration. 1 Group Question • Why are there seasons? Hint: It has nothing to do with the distance from the Sun. 1 The Seasons • The Earth is tilted on its axis. • During half the year, the northern hemisphere is tilted more towards the Sun. – Summer in California. – Winter in Australia • During the other half of the year the southern hemisphere is tilted towards the Sun. – Winter in California – Summer in Australia http://kids.msfc.nasa.gov/News/2000/News-VernalEquinox.asp 1 Summer • During summer: – – – – Hemisphere is tilted towards Sun. Sun is high overhead. Sunlight is more direct. Daytime is longer. Why? S N 1 Winter • During winter: N S – – – – Hemisphere is tilted away from Sun. Sun is low on horizon. Sunlight is less direct. Daytime is shorter. Why? 1 Time • Q: So, how can we use the Sun to tell time? • A: Sundials 1 Time • Sun rises in the East. – If the Sun is towards the East, it is before noon. • Sun sets in the West. – If the Sun is towards the West, it is after noon. • At noon? – For most of northern hemisphere: Sun is due south. – For most of northern hemisphere: Sun is due north. – Near equator, Sun is overhead. 1 Date • During northern summer, Sun high over head. – When Sun casts the smallest shadow at noon Summer solstice. • During northern winter, Sun low to the south. – When Sun casts the longest shadow at noon Winter solstice. • During northern spring and fall, Sun in between. • Sun also rises due east, sets due west. – When Sun casts shadow due west at sunrise spring or fall equinox. 1 • In Summer, the sun is high. • In winter, the sun is low. • The height of the sun (at a given time) can tell you the date. 1 The Constellations • We may also use the stars to tel time. • We see stars projected on the sphere of the sky. • They make patterns: constellations. – No physical association • We see those constellations opposite the Sun from Earth. 1 Seasonal Constellations • As the Earth orbits the Sun, different stars are up at night. – There are summer and winter constellations! • “The Dog Days of Summer” 1 Ancient Astronomy • Stonehenge (~3000 BC): 1 Ancient Astronomy • Chaco Canyon (“Anasazi” ~1200 AD): 1 Fajada Butte 1